What is Feed Conversion Efficiency?

Feed Conversion Efficiency (FCE) in beef production can be described as how well an animal converts feed into meat. This is found by calculating how many kilograms of live weight gain can be achieved from a tonne of dry matter (DM) feed. This can range from 60–300 kg of growth per tonne of DM.

Feed is the most significant cost of producing beef and, as such, is an important bearing on total feed cost/kg gain. When determining the cost of home-grown feeds, such as silage and cereals, input costs, such as fuel and fertiliser, should be considered. Other cost contributors to overall yield, utilisation and the avoidance of waste should also be taken into account.

For purchased feeds, the cost is largely dependent upon buying decisions, although it is essential to minimise/avoid wastage — particularly with moist feeds. Out of these feed input costs, the most important factor affecting overall feed cost/kg gain is FCE.

How to achieve reductions in feed costs?

By targeting, estimating and controlling FCE, you can achieve significant reductions in feed cost/kg gain, especially in the finishing period, when FCE is often poorer due to the increased energy costs of fat deposition. It follows that measurement and improvement of FCE should be a major key performance indicator of beef production.

The MechFiber Difference

KEENAN MechFiber diet feeders are unique in the proven nutritional benefits they deliver. Each KEENAN is engineered to use a gentle mixing action to produce an evenly and thoroughly mixed, light and fluffy ration that is never over- or under-processed. We call this optimal mix ‘MechFiber’. Independent trials have consistently shown that MechFiber retains the fibre structure to stimulate rumination, allowing greater absorption of energy and maximised FCE*. The composition of the MechFiber ration is such that it also slows the passage of feed through the digestive tract allowing more time for digestion. Synced to the InTouch feed management platform, the KEENAN controller ensures all feed ingredients are consistently added in the correct order and ratio.

By improving FCE through the provision of a well-mixed ration, rumen function can be optimised, even in situations where diets show high levels of starch.

Take control of your feed costs

The impact of the KEENAN system in terms of loading and mixing accuracy applies equally to beef systems as it does dairy. However, assessing FCE within the context of a beef system is not as immediately apparent as with a dairy system. This is because performance, in terms of weight gain, cannot be recorded daily, as would be the case with milk yield. At best, an assessment would have to be made based on weights taken at 3–4-week intervals. However, many farmers still do not weigh their cattle regularly.

Using well-developed models to predict animal growth from several easily defined inputs, the InTouch system can monitor the weekly performance of beef cattle, predict FCE, measure total feed costs and provide an estimation of total feed cost/kg gain. This data informs regular decisions regarding the most appropriate feeding strategy.

* University of Illinois 2008, University of Reading 2008 & Coleman et al, 2011. Professional Animal Scientist, 27, 505-17

MechFiber Mix – The KEENAN MechFiber ration retains the fibre structure to stimulate rumination, allows a greater absorption of energy and maximises feed conversion efficiency.

Beef FCE – Feed Conversion Efficiency in beef production can range from 60kgs to 300kgs of growth per tonne of DM.

KEENAN - Synced to the InTouch feed management platform, the KEENAN controller ensures that all feed ingredients are added in the correct order and ratio. The resulting mix is consistent and maximises the intake and passage of food through the digestive system.

The periods that require the most balanced nutritional intake are the beginning and end of lactation when sheep are in the barn. TMR can therefore be considered beneficial for the majority of sheep farming systems.

The benefits of using TMR rations are as follows:

1. Reducing stress at mealtime thus preventing digestive disorders.
2. Reduction of individual space required at the feed face – allowing for greater numbers inside.
3. Better control of nutritional inputs for ewes.
4. Higher lamb birth weight (up to 0.25 Kg larger lambs)
5. Enhanced performance and profitability.

TMR – end of pregnancy

At the end of the gestation period, ewes require increased nutritional intake. In fact, the foetus gains up to 70% of its weight in the last six weeks. These figures highlight the importance of calculating the nutritional requirements of the ewes when they are inside to ensure adequate food consumption.

The final formulation of the TMR ration depends mainly on the quality of available forages, the body condition of the ewe and the expected lambing rate.

TMR – after lambing

The growth curves of the lamb during the first six weeks of his life depend largely on the mother’s milk supply. As a result, the rate of sheep milk production is very important. A ewe who feeds two lambs, with a daily growth of 300 grams, must produce 2.5 Kg of milk per day. Under these conditions, despite a weight loss of 150 grams per day, the ewe has an energy requirement in the range of 24 to 26 MJ.

TMR feeding for sheep is a robust way to go to maintain herd health and stave-off metabolic problems such as prolapses.

Mr Preece from Herefordshire uses a KEENAN MechFiber320 with InTouch to feed his cattle and sheep flock during the winter period, and has seen a reduction in prolapsed ewes, going from 3% to 0. “The KEENAN system makes my everyday winter work simpler and ensures that my animals are making a better use of the feeds. The cattle and the flock are healthier”, says Mr Preece.

Alltech & KEENAN advise a simple but well-calculated and precise TMR diet to ensure a healthy and productive flock, particularly over the winter period. KEENAN have a range of new and approved second-hand machines to suit every system for the coming winter.

For further advice on feeding your flock this coming winter, please KEENAN on 059 977 1200

As a manufacturer of diet feeders, KEENAN has set the standard in machine performance, reliability and service for many years. The old KEENAN adage “You’ll never miss a feed” has become a byword for the excellence of KEENAN service.

However, sometimes our service agents, who are based throughout the UK, operate above and beyond the call of duty, as Yorkshire dairy farmers David and Andrew Pattison discovered last Christmas Eve.

“At four o clock on Christmas Eve, as I was feeding the cows, I heard a knocking noise coming from the machine” David explains “ Everything was going around, apart from the discharge augur, so I decided to shut it down.”

The Pattisons have a KEENAN which was bought second hand seven years ago, and had experienced very few problems with the machine, and have been really impressed with the reliability of the machine.

Now can you imagine how it feels – you have 180 cows to feed, it’s Christmas Eve? It’s bad at the best of times to be in such a situation, but a nightmare on this particular day.

“I honestly did not think for one minute that anyone would turn out, but I remembered that the plate on the back of the machine states “Peter Woodhouse – 365 days a year service”.

David rang Peter Woodhouse, who said to him “Just put the machine in the shed, leave the lights on, go to bed, and it will be repaired by the morning for you.”

Peter left Bootle immediately for Northallerton – a drive of two and a half hours. “I didn’t even have second thoughts about doing this repair, it’s the service we provide for KEENAN 365 days of the year” he says. “Our service vans are fully stocked so we are able to carry out the majority of repairs and service on site, which saves farmers both time and money.”

Co-incidentally, one of Peter’s mechanics, Fred Gear was carrying out a repair at Ken Brough’s KEENAN at Wigton.  “I rang Fred and he met me at the Pattison’s so we worked together on the repair.   It was a big job and we worked until about 2.30 am, finally arriving back home in Bootle at around 5.30 am.”

The Pattison’s were amazed at this service. “We are milking 180 cows, averaging around 8,500 litres. If the KEENAN had not been repaired, we would have had to feed silage using a for-end loader over the holidays.”

“We also rely heavily on InTouch. It’s easy to use, especially now that it’s web-based. It makes it simple to alter a ration and ensure that we are feeding the optimum ration for the right number of cows.”

“Apart from the inconvenience, our milk yields and quality would probably have dropped dramatically, and this would have led to a loss of income.”

The Pattison brothers conclude “This was fantastic service from KEENAN and Peter Woodhouse. There are not many people who would turn out on a Christmas Ever to help you out.”

Through an increased understanding of rumen metabolism, numerous attempts have been made to manipulate rumen function in order to improve animal performance through increased feed intake or an improved supply of end products of digestion to the animal. These aims can clearly be achieved by optimal ration formulation.

A key driver of rumen function is the balance between the supply of ruminal degradable protein and soluble carbohydrates. This has a significant impact on protein supply to the animal through resultant variation in microbial protein production. At the same time, the need to promote rumination to stimulate feed intake and further support efficient rumen function needs to be recognised. Therefore, maintaining sufficient dietary levels of structural fibre is critical to avoid large reductions in rumen pH associated with feeding excess soluble carbohydrates. In such situations, TMR feeding has an important role to play.

Many different feed additives have been proposed and a resume of their effects would not be appropriate within the context of this short review. However, some are worthy of mention. A common example is the dietary addition of yeast (Yea-Sacc) in order to augment the buffering capacity of the rumen. This approach has been shown to be effective in most situations with a correctly balance ration. earch studies have shown that despite similar intakes of the same mixed feed to those being consumed by later-lactation cows producing lower levels of milk, when all cows were challenged with the same amount of an additional high starch feed source, the higher yielding cows were less able to maintain rumen pH levels. This may suggest that the mineral need of the higher yielding cows was compromised and in such situations, use of feed buffers to support rumen buffering capacity is appropriate.

In a similar way, there has been considerable interest over the last few years in the use of yeasts to improve rumen fermentation. Earlier research has shown that despite low levels of addition, yeasts or yeast cultures are effective within the rumen environment. Several effects have been suggested including uptake of stray oxygen, improved fibre digestion through enhanced activity of fibrolytic microbes and control of lactic acid levels on high starch rations. In respect of lactic acid metabolism, research evidence has shown the numbers of lactate utilising bacteria are increased by yeast supplementation yet other studies have reported increased numbers of lactate producers. Overall effects on feed intake, milk yield and milk composition were generally highly positive. One notable effect being an improvement in acid detergent fibre digestibility.

Methane is an unavoidable end-product of enteric fermentation and its production from ruminants has attracted attention from both consumers and political powers. Reducing methane production should, theoretically, increase the total energy available from the ration, as well as reduce the environmental impact.  There has been much research into reducing methane output from ruminants, not least through nutrition with a number of approaches having been considered. Use of halogenated compounds was considered to be effective against methanogenic microbes but most of the effects noted were transient and this approach has not been continued. Recent research in Australia has focussed on the immunisation of animals against methanogenic archaea with original claims of a total elimination of methane production. However, the alternative metabolic fate of both carbon and hydrogen produced from fermentation, and whether there was a subsequent effect on animal performance, was not made clear. More recently the claims for immunisation have been seriously down-sized and latest research claims only a 7.7% reduction in methane production, a response which could be achieved through improved nutritional management.

Other suggested strategies to reduce methane production have included the introduction of methane oxidising bacteria into the rumen, or acetogens, known to increase acetate production at the expense of carbon dioxide disposal via methane production. Defaunation, which involves the removal of protozoa, has also been proposed as protozoa provide an important environmental niche for rumen methanogens. In this respect, studies which have increased the level of free oil in the ration have been shown to reduce protozoal numbers with a concomitant decline in methane production, the effects being particularly pronounced with coconut oil. Defaunation has also been shown to improve microbial protein supply to the intestines due to a significant reduction in protein turnover by the rumen microbial population. However, recent work demonstrates that significant changes to the microbial ecosystem, such as defaunation, are only transient and the rumen will revert to its prior state. Additionally, research has shown that it may also be unwise to focus primarily on reducing methane – a rumen that exhibits reduced methane production isn’t necessarily more efficient. There is, currently, much focus on whole systems and how the rumen microbial ecosystem influences gross parameters, such as methane emission and production efficiency. The Ruminomics EU FP7 project has delivered some excellent work on this topic.

Conclusions

This short report has attempted to present some of the underlying principles of rumen function and the important role it has in determining overall feeding value. The intimacy between energy (carbohydrate) and protein metabolism in the rumen should not be underestimated as this can seriously impact upon the nature as well as the quantity of nutrients absorbed from the GI tract and made available to the host animal. It is evident from this report that the rumen is the major site of digestion and provides justification for efficient feeding of the rumen ecosystem if overall performance of the animal is to be optimised.

Author: Denis Dreux

Our previous blogs have already discussed some situations where the processes of rumen function are likely to be less than optimal. There are many feeding situations where the efficiency of ruminal utilisation of dietary proteins is compromised, due principally to the nature as well as the amount of protein being consumed. An imbalance between protein supply and the ability of the microbes to utilise that protein can result in significant losses of dietary protein as ammonia prior to the small intestines. High levels of protein in freshly grazed forages is one such situation, especially when levels of water soluble carbohydrates in the forage are compromised, possibly due to low levels of sunshine affecting overall photosynthetic activity of the plant. It is on this basis of providing an improved balance of protein and ruminal degradable carbohydrate that the concept of high sugar grasses was developed through plant breeding. However, the responses in terms of improved animal performance have been unpredictable, possibly due to variable effects on total sugar content being reported.

Excess production of rumen ammonia leads to increased urea production in the liver. It is crucial that the animal removes most of this ammonia to prevent it from entering peripheral circulation where it can have toxic effects. In most situations this can be achieved by the condensation of two molecules of ammonia to form one molecule of urea. When ammonia load in the liver is high however, an alternative pathway will operate where one of the two NH2 groups is provided by the catabolism of specific free amino acids, the net outcome being a reduction in the quantity of amino acids that are available to the animal. At the same time, it is often reported that the production of urea from excess rumen ammonia may have a significant energy cost. In reality this is not likely to be the case, certainly not to the level that is frequently suggested.

It is also pertinent to note that high rumen ammonia levels can arise when high levels of non-protein nitrogen (eg urea) are included in the ration, and in extreme cases may result in animals dying from ammonia toxicity. In such situations, the load of ammonia is too high to allow efficient capture by the rumen microbes, and when liver capacity to remove this excess ammonia is exceeded, a pronounced rise in plasma ammonia level is inevitable. Research has shown that with increasing plasma ammonia levels, animals’ first show reluctance to consume feed. This can be quickly followed by the animals becoming recumbent and if the condition continues, coma followed by death can occur.

It has already been mentioned that when the supply of ruminal degradable carbohydrate is high, especially when a significant amount is being derived from non-fibre carbohydrates (eg starch), there may be a pronounced increase in rumen lactate levels with a concomitant reduction in rumen pH. This can lead to subclinical rumen acidosis, defined as rumen pH levels below 6.0, leading in some cases to acute rumen acidosis when rumen pH levels rumen fall below 5.5. The impact of this on rumen function can be quite severe. At consistently low rumen pH, the functionality of the rumen papillae will be compromised, often noted by thickening or clumping of the papillae (rumen parakeratosis) which inevitably compromises the absorptive capacity of the rumen epithelium. In turn this will exacerbate the already low rumen pH conditions. Further damage of the rumen epithelium may seriously compromise its integrity and allow microbial toxins, produced by the rumen microbes which are now under considerable metabolic stress, to pass through into peripheral circulation. If the condition is not treated rapidly these toxins can ultimately result in the death of the animal.

Sub-clinical or clinical rumen acidosis must be avoided in all situations through appropriate dietary management. Feeding large amounts of starch-rich feeds in single meals is not advisable whilst balancing the nature of the starch in the ration to provide a balanced supply of rapidly and slowly degradable starch is recommended. Contrary to general opinion however, low rumen pH levels are not only caused by high starch feeding, and when formulating rations it is important to take account of the levels of water soluble carbohydrates being consumed by the animal. Whilst sugars tend to be fermented more to acetate and butyrate than to propionate and lactate, they are an almost instantaneously available source of carbohydrate which can seriously impact on rumen pH. This has been demonstrated recently in research from Australia where cattle grazing lush irrigated pastures where found to have rumen pH levels below 6.0 for up to 16hrs during each 24hr period and on occasions, pH levels approached 5.5 where the condition could be considered to be on the border of becoming acute. At the same time, rumen ammonia-N levels exceeded 30mg/100 ml for over 12hrs during the day, adding a further burden to the animal in terms of excreting this as urea, whilst indicating considerable loss of valuable plant protein.

When rumen pH falls below 6.0, one further consequence is the effect on rumen fibre digestion. Fibre degrading microbes are relatively slow growing compared with starch utilising microbes and most importantly are sensitive to rumen pH level. At or below pH 6.0, the fibrolytic microbes are affected, with reduced production of fibrolytic enzymes leading to impaired fibre digestion. This will immediately impact on the overall nutritional value of the feed being consumed, as any reduction in the digestibility of dietary fibre will reduce total dietary ME value. Given that fibre generally constitutes between 30 and 40% of total ration DM intake, this can be quite significant. Compromised digestion of fibre in the rumen will reduce the production of acetate and butyrate which are important precursors of milk fat synthesis and as a consequence milk fat levels and total milk fat production are likely to be reduced. Finally, in response to compromised fibre digestion in the rumen, feed intake will decline, simply due to a reduced rate of clearance of feed by digestion in or passage out of the rumen.

Given carbohydrate and protein account for most of the nutrients that are digested in the rumen, it follows that in those situations where rumen function is optimal, between 85-90% of the total nutrients digested in the rumen should ultimately be either absorbed as VFA across the rumen wall, or transferred as microbial biomass for subsequent digestion in the small intestine.  Where efficiency is compromised, the usual cause is either insufficient or excess protein. In the former case, the extent of carbohydrate digestion in the rumen will be reduced with a concomitant decline in feed intake, whilst in cases of excess nitrogen supply, as much as 30% of the ingested nitrogen may be absorbed as ammonia from the rumen. Effects of this magnitude have been noted with fresh forages containing high levels of protein and provides evidence that in such situations supplementation with a suitable energy source could be beneficial.

The rumen is also a major site of starch digestion but both rate and extent of digestion will be influenced by a number of important factors. These clearly include the chemical characteristics of the starch where two types, amylose and amylopectin are known to have contrasting digestion profiles. As cereal crops mature, generally there is an increase in the level of amylopectin at the expense of amylose, with the former having a lower rate of degradation. With the exception of sheep, cereal grain need to be processed before they are fed in order to disrupt the seed coat and allow access to the rumen microbial enzymes. This can be achieved by grinding or rolling of dry or moist grains or in the case of cereal silages (whole crop or maize) by use of in machine grain crackers or a dedicated mill. However, the process of seed coat disruption does not need to be extensive and very finely ground grains should be avoided as these will accelerate the rate of starch digestion in the rumen – and may predispose the animal to rumen acidosis. In such situations, there is opportunity to treat grains with sodium hydroxide in the preparation of soda grain. This will effectively remove the seed coat and allow total digestion of the starch contained within the treated grain at controlled rates. It is imperative however when considering the inclusion of different starch containing feeds into rations, that their likely degradation characteristics are taken into account, thus allowing balanced rations containing both rapidly and slowly degrading sources of starch to be used.

There has been much conjecture about the value of ruminal protected starch that will by-pass the rumen and contribute directly to small intestinal digestion. Clearly, this can be seen to be advantageous in that it will supply glucose directly to the animal and thus avoid the attendant losses, albeit relatively small, associated with the conversion of starch in the rumen to propionate followed by the subsequent conversion of absorbed propionate in the liver to glucose. However, there are no specific feeds which contain high levels of ruminal protected starch and previous attempts to produce these on a commercial scale have not been successful. Thus in almost all situations, the rumen is likely to remain as the principal site of starch digestion. With the relatively low levels of starch consumed, when compared with a whole tract starch digestibility approaching 100%, ruminal starch digestion will account for over 95% of this achievement.

At higher levels of starch inclusion, provided rumen function is not compromised, ruminal starch digestion will still account for in excess of 90% digestion which occurs in the whole tract, which in turn should be of the order of 97%+. Even at starch intake levels approaching 7kg/day which have been measured for cows consuming in excess of 24kgDM/d of rations containing as much as 28% starch, the amount of starch entering the small intestines is unlikely to exceed 1.2 to 1.5kg/d, indication at least 80% of ingested starch has been digested prior to the small intestine. In this respect, one area of concern has been the effect of maize silage maturity on starch digestion in the rumen. As the maize crop matures, so starch levels will increase to over 30% DM basis and as indicated earlier the nature of that starch will change. This led to the suggestion that more mature maize silage may promote the post-ruminal digestion of starch and a series of in vitro studies carried out in Holland supported this claim. However, studies at CEDAR, which quantified the sites of starch digestion in lactating dairy cows receiving total mixed rations containing equal amounts of maize but provided from harvests of increasing maturity (23 to 38% DM), showed only small increases in the amount of starch which escaped during rumen degradation.

From this, it is concluded that in most situations the post-ruminal digestion of the starch component of maize silage is not increased by crop maturity and should not be considered as important when rations are being formulated. At the same time however, it is highly likely that the ruminal rate of digestion of the starch component of more mature maize silage will be slower than that seen with less mature crops.

Author: Denis Dreux

Protein Digestion

Some rumen microbes will effect considerable degradation of dietary protein, depending upon the susceptibility of that protein to digestion in the rumen. Proteins are initially reduced to constituent free amino acids and limited amounts of peptides of varying chain length, by the action of microbial proteases, followed by the further reduction of a significant proportion of the degraded amino acids to ammonia by microbial deaminases. Both ammonia and amino acids, and in some instances short chain peptides, can be used by the rumen microflora for the synthesis of microbial protein. The most commonly occurring plant protein is ribulose 1.5 bicarboxylase, often referred to as Rubisco or fraction 1 protein. It is the principal photosynthetic enzyme of green plants and is found extensively in grasses and legumes. It has high inherent solubility and as such is readily degraded by rumen microbes, which accounts for the increased levels of rumen ammonia that often accompany large intakes of fresh forage. Other proteins are also found in forages, including membrane proteins but these tend to be less degradable although they can make some contribution to the overall supply of nitrogen to the rumen microbes.

Some forages, especially certain legumes, contain significant amounts of tannins and after ingestion these can react with some of the proteins in those forages to form insoluble complexes which are not capable of being degraded by the rumen microbes. This is a naturally occurring rumen protection and in specific forages such as sain-foin and lotus, research has shown that this may lead to marked increases in the amount of protein which ultimately reaches the small intestine. Sain-foin and lotus are however not widely grown and most forms of ruminal-protected protein that are fed have been produced by pre-treatment of the feed. One obvious example of this is formaldehyde-treated soya bean meal, sold in some markets whilst an alternative approach has relied on the treatment of protein feeds with wood sugars. These contain significant amounts of xylose and xylitol and promote the formation of some products which are equally resistant to rumen degradation. Ruminal-protected protein feeds can be used to enhance total supply of protein to the intestines and to the animal when microbial derived protein is considered to be limiting.

As indicated earlier, ammonia is a key substrate for microbial protein but certain bacteria show considerable propensity to produce ammonia in excess of their own specific requirements. These are known as the high ammonia producing (HAP) bacteria and together with rations which contain high levels of soluble protein, can contribute to high rumen ammonia levels which are in considerable excess of the ability of the microbes to assimilate ammonia into microbial protein. The concept of rumen synchrony was proposed several years ago in which rations would be developed to provide balanced amounts of ruminal degradable carbohydrate and protein. Whilst this approach had sound scientific grounds, subsequent research was unable to establish sizeable benefits and has now generally been abandoned. However, it is pertinent to note that total mixed ration feeding will provide some opportunities in this respect although direct research evidence does not exist.

Author: Denis Dreux

The “rumen”, comprising the reticulum, the dorsal and ventral sacs of the rumen, the omasum and the abomasum, together with the combined contents of digester, accounts for as much as 100-120kg of the total live weight of dairy cows, perhaps 80kg of a finishing beef steer and even 15kg of an adult sheep.

After ingestion (feed which has been mixed to varying degrees with saliva during prehension), enters the rumen where the processes of microbial digestion commence. It joins previously ingested feed which is at different stages of digestion, together with ingested water and saliva. Rumen digester has an average dry matter (DM) content of between 10 and 12% and whilst relatively fluid, some stratification is generally evident, with ventral sac contents usually having a higher DM content. At the interface of the dorsal/ventral sac, the rumen mat normally develops and is important in the initiation of good bouts of rumination. Digestion occurs as a consequence of microbial fermentation, supported by rumination, where feed bols are regurgitated for further physical dissimilation by chewing.

As previously stated, the rumen is highly anaerobic, with low levels of oxygen resulting in a highly reducing environment. The main route of entry for oxygen is with ingested feed but in a healthy rumen this will not have any marked effect on the reducing properties of the rumen. The rumen is dominated by large populations of anaerobic micro-organisms which exist in a symbiotic relationship with the host animal, the nature of the rumen microbial population being significantly influenced by the type of ration being fed. On high fibre rations, fibrolytic bacteria will dominate, and together with limited colonisation of dietary fibre by anaerobic fungi, are responsible for most of the digestion that occurs in the rumen of cattle and sheep fed high forage rations. In contrast, on starch-rich rations, the bacterial population will be dominated by amylolytic species which together with protozoa effect most of the starch digestion which occurs.

Carbohydrate digestion

The principal role of the microbes is to degrade dietary carbohydrates and more specifically the dietary fibre which cannot be digested in the small intestines and to only a limited extent in the hindgut. This does not preclude the digestion of starch and sugars in the rumen which is generally quite extensive. The ruminal dissimilation of dietary carbohydrates comprises initially of the degradation of polysaccharides to simple monomers (hexoses and pentose’s; example below) followed by extensive fermentation of these to yield energy (Adenosine triphosphate; ATP) which the micro-organisms utilise for the purposes of maintenance and growth (example 2-6). As the rumen environment is anaerobic, the yield of ATP per mole of degraded carbohydrate is much lower than that which would be derived from the aerobic oxidation of carbohydrate, and depending upon the type of fermentation is usually between 4 and 5 moles ATP/mole carbohydrate, compared with 38 moles/mole for complete oxidation. Clearly the production of volatile fatty acids (principally acetate, propionate and butyrate) accounts for the major part of the energy contained in ruminal digested carbohydrate, generally of the order of 80-88% on a stoichiometric basis. Not all carbohydrate digested in the rumen however is fermented and a significant but variable proportion of the  degraded  carbohydrate is used to support microbial biomass synthesis, principally microbial polysaccharide, protein, nucleic acids and some lipid.

The principal pathways of carbohydrate dissimilation are provided below;

Polysaccharides = Monosaccharides (Hexose {C6} and Pentose’s {C5})

Hexose = 2 Pyruvate.

Pyruvate = Acetate (CH3COOH) + CO2 +H2 + ATP    Pyruvate + H = Propionate (C2H5COOH)

2 Pyruvate = Butyrate (C3H7COOH) +2CO2 +2H2+ATP

CO2 + 2H2 = CH4 +ATP

Examining these reactions in respect of carbon utilisation provides clear insight of the energetics of carbohydrate fermentation. Reaction 2 shows a carbon input/output ratio of 1.0, as each molecule of pyruvate contains 3 carbon atoms. The production of acetate (2 carbon atoms; reaction 3) however results in the net loss of 1 carbon atom with an associated production of hydrogen. Reaction 5, the production of butyrate has a similar loss of carbon for whilst it takes two molecules of pyruvate (6 carbon atoms), butyrate has four carbon atoms, the other two being converted into carbon dioxide. There is also an associated production of hydrogen.

In contrast, the production of propionate requires 1 molecule of pyruvate and with propionate being a three carbon molecule, there is no net loss of carbon. In addition, the conversion of pyruvate to propionate requires hydrogen which is provided from the rumen environment, and as such propionate production is considered to be a net utiliser of hydrogen, thus competing with other routes for the disposal of rumen hydrogen. From this it follows that propionate production is inherently more efficient than either acetate or butyrate production, especially as these reactions contribute both carbon dioxide and hydrogen unlike propionate production. Finally any carbon dioxide and hydrogen produced during the ruminal fermentation of feed (principally carbohydrates) is converted to methane by methanogenic bacteria, which in turn derive a small amount of ATP from this reaction.

On high forage rations, acetate will be the dominant VFA (>65% of all VFA), usually with modest levels of propionate (approx. 16%) and butyrate (10%), with higher carbon length VFAs as well as branched chain VFA providing the balance. Increasing levels of starch in the ration will increase propionate levels, generally at the expense of acetate but only rarely will propionate account for more than 25% of total VFA, whilst acetate will still be the major VFA (circa 55%). On high sugar rations, especially when significant amounts of molasses or fodder beet are being fed, butyrate levels may increase to approximately 15%, with most notable increases occurring immediately following feed ingestion. In those situations where carbohydrate degradation in the rumen is extensive both in terms of amount and rate of digestion, significant amounts of lactic acid may be produced from the conversion of pyruvate and as lactate is a stronger acid than any of the VFA this can have a more pronounced effect on rumen pH. Lactate is cleared from the rumen either through further metabolism by lactate-utilising bacteria or by absorption across the rumen wall and subsequent metabolism in the liver. In those situations where lactate production significantly exceeds lactate clearance by metabolism or absorption, rumen pH levels will fall quite dramatically, leading ultimately to subclinical rumen acidosis which can have serious adverse effects on both rumen and animal health.

Author: Denis Dreux

Overall digestibility of feed, described as the amount of feed dry matter (DM) consumed less the amount of faecal DM, is a major determinant of overall feeding value. On high quality feeds, overall digestibility can approach 800g/kg DM consumed, whilst on lower quality feeds it may be closer to 500g/kg DM – or possibly even less. This will impact on total energy availability to the animal (metabolisable energy, ME) but also on the overall availability of other nutrients including protein, starch, lipid and some minerals. The ruminant gastrointestinal (GI) tract comprises three major sites of digestion, namely the rumen, the small intestine and the large intestines or hindgut. The rumen is the principal site of digestion and generally accounts for at least 80% of primary digestion occurring within the whole tract. As discussed below, the rumen is the major site of fibre digestion, which relies on an active population of microbes, namely bacteria and protozoa; whilst recent research has established the presence of filamentous fungi in rumen contents.

The major end products of rumen digestion are volatile fatty acids (VFA), which are subsequently absorbed across the rumen wall and after transfer to the liver they may be metabolised or exported to peripheral tissues for subsequent metabolism. Lactic acid is by definition not a volatile fatty acid but on some rations, particularly those containing high levels of starch, levels in the rumen may increase substantially, largely due to the numbers of lactate-producing bacteria exceeding the lactate-utilising bacteria.

An additional end product of rumen metabolism is microbial biomass, which ultimately leaves the rumen via the omasum and abomasum for subsequent digestion within the intestine. It is by this route that ruminant livestock achieve the major part of their protein requirement, given that microbial biomass is rich in protein; although it does contain other nutrients such as lipids and polysaccharides. The rumen is also the site of production for two important waste metabolites. As will be noted later, methane is an obligatory by-product of rumen fermentation and may account for as much as 6-8% of total gross energy intake, depending upon the nature of the ration consumed. The principal route for methane eliminating is via eructation although limited amounts may be absorbed across the rumen wall, later to be excreted via the lungs, after being absorbed into the blood stream. The other waste product is ammonia, resulting from the degradation of dietary protein or non-protein nitrogen (NPN, e.g. urea). Both degraded amino acids and ammonia are important substrates for microbial protein synthesis, but where excessive amounts of protein (or NPN) are fed, or the supply of degraded energy and protein are not synchronised, rumen ammonia levels can increase markedly, often resulting a substantial loss of nitrogen from the GI tract.

Ammonia absorption is pH dependent, according to the relative concentrations of free ammonia (NH3) and the dissociated ammonium (NH4) ion. Absorbed ammonia or ammonium ions enter the ruminal vein and after transfer to the liver are converted into urea, which can either be recirculated via saliva to the rumen or excreted via the kidneys in the urine. The latter is generally the principal route of urea disposal although the partition between excretion and recycling will shift, especially in animals on sub-maintenance regimes where conservation of nitrogen to support rumen function is important to the long-term survival of the animal.

The rumen is also the principal site of starch digestion, for whilst most forms of starch could be digested by mammalian enzymes in the small intestines, both amylolytic (starch loving) ) bacteria and protozoa will ferment starch in the rumen, with the associated production of VFA. Any starch which is not digested in the rumen, may be digested by enzymatic digestion in the small intestines, depending upon the accessibility of the starch, with whole grains not being digested due to an inability to digest the outer seed coat. One exception will be sheep, where due to their ability to chew whole grains, significant amounts of whole grain may be fed which will be digested in the rumen or the intestines without significantly increasing the amount of undigested starch in the faeces. The principal end product of starch digestion in the small intestine is glucose and in cattle, the small intestine has the capacity to digest approximately 2kg starch/day. Above this, the quantity of starch entering the terminal ileum will increase, leading to substantial hindgut fermentation of starch.

Lipid, which may constitute as much as 5% of total ration DM is not fermented in the rumen but does undergo extensive hydrolysis with an associated production of free long chain fatty acids. As outlined below, the rumen is a highly anaerobic environment, with significant levels of hydrogen resulting principally from carbohydrate fermentation and consequently substantial hydrogenation of free long chain fatty acids may occur at this time. This can substantially change the nature of these fatty acids, with a marked reduction in the degree of unsaturation as double bonds between Carbone atoms are transformed into single bonds following the insertion of hydrogen atoms. Ultimately this will impact on the composition of ruminant milk or meat fat, with fat sources containing high levels of saturated fat being suggested to have adverse effects on human health. The small intestine is the major site of lipid absorption, where dietary lipids are absorbed, principally as free fatty acids, along with any microbial lipid (also as free fatty acids) which has been synthesised during the processes of rumen digestion, usually from the direct use of monosaccharides (sugars) derived from carbohydrate digestion (without fermentation) in the rumen.

Undoubtedly the small intestine is a major site for the digestion of microbial protein but in some situations this may be augmented by significant amounts of dietary protein that have not been digested in the rumen. Collectively, microbial protein and undergraded feed protein contribute to the animal’s total protein supply, although the composition of the amino acids which finally become available to the animal will bear limited resemblance to those which were consumed. Protein digestion in the small intestine is effected by mammalian enzymes secreted by cells within the intestinal wall, and the end products of that digestion are subsequently absorbed across the intestines, before entering the portal vein for onward transfer to the liver. Whilst free amino acids are the principal end products of protein digestion in the small intestine, there is evidence of limited amounts of short chain peptides being absorbed as well.

The large intestine, consisting of the caecum and the colon which are located posterior to the terminal ileum, has no pre-defined role and is generally considered to be a site of compensatory digestion in those situations where digestion in anterior parts of the GI tract may have been compromised. In most situations large intestinal digestion does not account for more than 10% of total digestion in the whole GI tract, compared with approximately 60% in the rumen anterior to the duodenum, and 30% between the duodenum and the terminal ileum. These estimates contrast with the earlier suggestion that the rumen is responsible for at least 80% of primary digestion, as the small intestine is a major site for the digestion of protein previously synthesised as microbial protein in the rumen, together with possible contributions of microbial polysaccharides (starch) and lipids also synthesised in the rumen.

Limited digestion of any dietary fibre which has not been digested in the rumen may occur within the large intestines, especially the caecum, which has a small resident population of microbes, similar but not identical to those found in the rumen. Some compensatory starch digestion may also occur here, with the end products (VFA) being absorbed into the blood stream but hindgut digestion of starch is generally indicative of compromised starch digestion in the anterior parts of the GI tract and should be avoided wherever possible. Limited amounts of microbial protein may also be synthesised as a consequence of caecal fermentation, but there is no evidence that this makes any real contribution to the animal’s total protein supply.

Author: Denis Dreux

Good quality colostrum is essential if calves are to survive and thrive. Colostrum contains immunoglobulins (Ig, antibodies) that protects the calf from disease and kick-starts its immune system during the first few weeks of life. Therefore, ensuring calves receive sufficient, quality colostrum as soon as possible after birth is critical to any dairy operation.

Timing

As there are no placental transfer of immunoglobulins (Ig, antibodies) from cow to calf, the calf relies on passive transfer of Ig from colostrum to the calf’s blood until its own immune system starts to produce Ig around one month of age. However, the calf’s ability to absorb Ig from the colostrum decreases by 50% by around six hours of life and is negligible by one day of age.  This highlights the importance of ensuring adequate colostrum as soon as possible after birth and calves fed colostrum soon after birth will have higher blood Ig levels (indicating better passive transfer) from the same volume and quality of colostrum compared with calves fed later (Weaver et al. 2000). There is a threshold for serum Ig level (10mg/ml) below which failure of passive transfer (FPT) is said to have occurred rendering those individual animals at much greater risk of morbidity and mortality.

Quantity

Guidelines for feeding colostrum are based on when and how much to feed. Each calf should receive four litres of good quality colostrum by 4 hours of age, with the majority of that offered within the first two hours of life where possible.

Quality

Colostral quality is often measured on farm using a hydrometer/colostrometer, which determines the specific gravity of the sample and uses that to infer the concentration of IgG (the most prevalent Ig in colostrum), which gives an indication of quality. A threshold of 50g/L is given below which, the colostrum is deemed of low quality with regards to Ig level. This method does have drawbacks with regards to its sensitivity and specificity, as well as being affected by temperature. However, it is a relatively rapid and easy way to estimate colostral quality. Another method used to determine colostrum quality is the Brix refractometer. This device values related to Ig level in the sample and a Brix value of 22% usually corresponds with the 50g/L threshold on a hydrometer.

Failure of passive transfer

Although calves have a function immune system from birth, they appear unable to respond sufficiently to disease challenges until around one month old (Osburn et al., 1982) and thus rely on the colostral Ig until that time. Failure of passive transfer is responsible for a great deal of morbidity and mortality in dairy operations and it’s crucial to identify those animals that have FPT. Refractometry can also be used to assess whether passive transfer has taken place. Serum total protein can be used to estimate whether the calf has received sufficient Ig. Blood samples taken from 2-7 of age can be tested and samples with values below 5.5g serum total protein/dL are deemed to have FPT (Weaver et al, 2000). There are other methods, such as ELISA and zinc sulphate turbidity test, that can be used to assess passive transfer.

Aside from measuring and monitoring colostrum and passive transfer, basic management practices can also help to reduce calf loss, including clean, warm environments, refrigeration of colostrum, feeding colostrum at body temperature and ensuring any buckets containing colostrum (or milk/milk replacer) have lids to prevent contamination.

Author: Helen Warren

References

Osburn et al. (1982), Ontogeny of the immune system, Journal of the American Veterinary Medical Association, 181: 1049-1052

Weaver et al. (2000) Passive transfer of colostral immunoglobulins in calves, Journal of Veterinary Internal Medicine, 14(6): 569-577