The liver of the lactating dairy cow approximates to 8-10kg metabolically active tissue whilst Gibb et al (1992) provided evidence of extensive liver hypertrophy during the periparturient period. The liver is the main site for glucose production and with an estimated glucose requirement of 70g/litre milk produced, principally to support lactose production, a cow yielding 40kg milk/day requires a daily output of almost 3kg glucose. This will be sourced primarily by the hepatic conversion of ruminal-derived propionate to glucose, together with some sacrifice of gluconeogenic amino acids when propionate conversion fails to meet total glucose needs. The liver is also important for the disposal of ruminal produced ammonia plus any ammonia arising from the deamination of absorbed amino acids used by the cow for catabolic purposes. The principal outcome of hepatic ammonia removal is urea, most being excreted in urine. At relatively modest ammonia loads, two moles of ammonia will condense to produce one mole of urea, but where ammonia load is increased, an alternative pathway, involving donation of one NH2 group from an amino acid may be stimulated, the net result being an associated loss of amino acids from the circulatory system.

The liver also plays a key role in fat metabolism. The modern dairy cow is likely to mobilise significant amounts of body fat during early lactation when feed intake (primarily energy supply) fails to meet total requirements and has been estimated to be as much as 60kg in some high yielding cows. A major role of the liver is to process the non-esterified fatty acids (NEFA) derived from body tissue mobilisation into triacyglycerides (TAG). These are then exported as VLDL (very low density lipoproteins) to other metabolically active sites (e.g. udder) or alternatively, the NEFA can be oxidised within the liver. Where oxidation is complete, carbon dioxide is the principal end product, but ketone bodies are produced when oxidation capacity of the liver is exceeded. As NEFA supply to the liver increases further, the ability of the liver to dispose of these either by oxidation or export as VLDL will become limiting, resulting in a significant accumulation of TAG in liver cells which in turn gives rise to fatty liver syndrome. Under extreme conditions, the level of accumulation may approach 500g fat/d, and at this rate of accumulation the liver would become totally saturated with fat in less than 2 weeks. In such situations, it was found cellular TAG concentration to have negative effect on urea production, reporting a 50% decline as cellular TAG concertation increased from less than 3 to 25mg TAG/mg liver DNA. It has been estimated that 60% of UK cows had more than 7.5mg TAG/mg DNA on day 1 post-partum, which would amount to a 20% reduction in the ability of the liver to convert ammonia to urea.

Such effects could partly explain some of the early lactation feed inappetence problems found in many cows. It has been reported that high ammonia levels in cerebrospinal fluid associated with high liver TAG levels and an associated development of hepatic encephalopathy, leading to lost appetite, general depression, recumbence and in extreme cases, coma. The situation is similar with respect to glucose production from ruminal derived propionate, where it was showed high liver fat levels caused a 40% reduction in glucose output. In a similar situation, it showed an even greater reduction in the extent of propionate conversion to glucose when ammonia load was increased.

One major implication of lipid overload on the liver is the production of ketone bodies due to failure of the liver to completely oxidise the increased supply of NEFA. It showed cows which developed ketosis had liver TAG levels 8 times higher than unaffected cows whilst it was showed ketosis cows had an increased risk of mastitis due to reduced production and activity of neutrophils. Furthermore, when cows with fatty livers were experimentally challenged with E coli, a significant increase in the time taken to clear this infection was noted, suggesting such cows had a compromised immune system.

To avoid such problems, either the load of mobilised fat presented to the liver must be reduced or the processing capacity of the liver increased. Nutritional strategies to reduce body fat mobilisation are emerging (promote feed intake by better diet formulation, reduce tissue mobilisation by reduced dietary protein levels) but mobilisation of some body fat will be inevitable in almost all cows during early lactation. The alternative strategy to increase the metabolic capacity of the liver with respect to fat relies heavily on increasing the export of TAG as VLDL. It has been indicated that the provision of additional methionine or methionine and lysine reduced total liver TAG levels as well as plasma ketones. Other studies have shown choline to be an important substrate in the synthesis of phospholipids by the liver, it did report an increased supply of ruminal protected choline caused a substantial reduction in liver TAG levels, when measured at calving and day 21 of lactation.

Recently, a commercial product based on ruminal protected choline along with biotin, folic acid, vitamin B12 and gluconeogenic supplements (propylene glycol and propionate salts) was shown to improve performance during the first 22 weeks of lactation, with associated improvements in fertility and reduced mastitis. It may appear that this strategy is conferring benefits related to the health of the cow’s liver, but given the number of nutrients contained in the product it is difficult to establish the exact cause. As indicated, the product also contains biotin which stimulates pyruvate carboxylase activity, which in turn catalyses the hepatic glucose production from lactate and amino acids. In this respect, it showed a marked increase in lactate production by the liver from the dry period through to lactation day 83, with peak production at day 11 being more than double the output noted at day 83, which coincides with other studies which have shown plasma biotin levels to be reduced at day 25, compared with day 100. Whatever the mechanisms, these studies suggest real opportunities to improve hepatic function and provide evidence of substantial improvements in milk yield when cows received 20mg/day biotin supplement.

Author: Denis Dreux

During the dry period, dietary levels of starch only need to be relatively modest, at best to provide some readily available energy for the rumen microbes whilst allowing some adaptation of the rumen microbes. Based on price, availability, as well as overall performance however, farmers recognise the value of including starch in lactation rations and are keen to introduce relatively high levels as soon as possible after calving. On this basis, including a small amount of the lactation ration prior to calving is advisable, as this will allow development of the rumen papillae as well as the rumen microflora prior to the cow being fed increased levels of starch in the lactation ration.

Starch occurs principally in cereals and some roots (eg potatoes) and most starch sources will be almost completely digested in the alimentary tract, provided some pre-processing has occurred where necessary. Furthermore, the rumen is the principal site of starch digestion, although digestion rate is affected by both chemical and physical form of starch. It follows that the amounts of starch escaping rumen digestion are usually modest, but can be affected by type of starch fed.

When high levels of rapidly degradable starch are fed, the ruminal production of volatile fatty acids (VFA) increases dramatically, with an associated increase in rumen VFA concentrations, due to VFA production rate exceeding the rate of VFA absorption from the rumen. As a consequence, rumen pH falls and below pH 6.0, fibre digestion rate will be reduced due to negative effects on rumen fibrolytic microbes. Furthermore, such dietary regimes are often characterised by increased levels of lactic acid which is a stronger acid then any of the major VFA, thus increasing the acidifying potential of the rumen.

Below pH 6, cows are considered to be suffering from subclinical acidosis, which may be manifested in general feed inappetence and mild diarrhoea. If not treated, conditions are likely to deteriorate further and clinical acidosis is generally assumed to occur at pH levels below 5.5. In this case, rumen digestion will have almost ceased, rumen contractions will be virtually non-existent and the cow will be totally off–feed and showing few signs of rumination. If such conditions are not dealt with rapidly, the cow may die, most probably due to the invasion of gut microbial toxins through the compromised intestinal wall of the cow.

Avoidance of all forms of rumen acidosis rely on improved nutritional management. Dairy cows can cope quite successfully with relatively high levels of starch in the total ration, but only if the form and rate of delivery of that starch to the cow are controlled. Feeding high levels of high starch containing concentrates in the parlour is not advisable as this will undoubtedly accentuate the problem. An obvious alternative is to include a significant proportion of the starchy feeds along with the forage components in a well-mixed ration. Even here however it is advisable to balance the forms of starch in the ration with respect to rapidly and slowly degradable sources but provided this balance is correct it is possible to feed relatively high levels of starch (24-26% DM basis), assuming that overall levels of sugar in the ration are not excessive. It is important to ensure that the ration is well mixed in order that cows cannot select the cereal components in preference to the forage, whilst ensuring adequate feed space for all cows, thus avoiding excessive bouts of feeding with large sized single meals.

Adopting this strategy and at the same time ensuring that the transition ration contains some starch prior to introduction of the full lactation ration should minimise the incidence of acidosis, whilst the prophylactic inclusion of sodium bicarbonate can be considered in some instances. It is noteworthy that the freshly calved cow is particularly prone to acidosis and may in part be attributable to a compromised mineral status at this time which prevents adequate buffering of the rumen. In studies at CEDAR to examine rumen acidosis, when rumen fistulated cows were fed the same amount of a starch rich ration, those cows with higher milk yields were less able to control rumen pH resulting in an overall difference of over 0.5 pH units.

Author: Denis Dreux

As already indicated in our previous blog, low blood calcium levels affect ruminal function, with associated effects on rumen and abomasum contractions leading to reduced and often erratic feed intakes during the immediate post calving period. In such situations, cows are likely to increase body tissue mobilisation in order to meet their lactation demands; which in turn may give rise to fatty livers and ketosis, due to failure of the cow to completely metabolise mobilised adipose tissue.

Of most immediate concern however, is the possibility of displaced abomasum (DA’s), which can occur when feed intake is erratic and muscle function is poor. The incidence of this condition is increasing on many farms can often result in the need for veterinary intervention and undoubtedly affects the cow’s subsequent lactation performance. Avoidance of DA’s is certainly a better route to providing a cure and is best achieved by providing a well-presented and palatable ration, at the same time ensuring the cow’s calcium status is not compromised.

It is important that this ration contains adequate levels of physically effective fibre, as DA’s can also occur in freshly calved cows turned out to fresh lush pasture or when rations containing extremely short chopped silage (either grass or maize) are fed. Such rations contain inadequate levels of physically effective fibre, which will reduce rumination times, with knock-on negative effects on rumen health and feed intake. In such situations rumen fill will be reduced, therefore allowing the abomasum to move within the abdominal cavity and hence becoming displaced.

Good nutritional management is the only solution to the avoidance of displace abomasum.

Author: Denis Dreux

The importance of steaming up the late gestation cow in preparation for the next lactation has been recognised for several decades. With increasing knowledge of the physiology of dairy cows and the important contribution that good pre-calving management can make to both parturition and the subsequent lactation, transition management has taken on a new dimension within the last 5-10 years. Parturition itself represents a major challenge/insult to the cow and is a period when serious problems can occur, which to differing extents, may affect cow health, subsequent lactation performance overall profitability.

This short review of transition management will attempt to examine the principals involved in the physiological changes that occur as the non-lactating in-calf cow moves to a non-pregnant and lactating state. In particular, it will examine the underlying causes of some of the major production disorders as well as focussing on the establishment of good feed intakes post-calving to achieve satisfactory levels of milk production. From this base of physiological knowledge the review will attempt to provide some practical aspects involved in managing cows to achieve uneventful calving, successful lactations intakes and good overall fertility.

The transition period, drying off

There is no specific definition of the transition period in terms of duration and whilst it is generally acceptable to consider commencement at the time of drying-off, it is debatable as to when it is concluded. Drying-off normally occurs at 60 days prior to expected calving; earlier drying off can result in over conditioned cows prior to calving whilst delayed drying off, often seen with higher yielding cows, can affect yields in the subsequent lactation. Cows yielding 10-12 litres milk per days are relatively easy to dry off, with either intermittent or abrupt cessation of milking. Higher yielding cows can be more difficult but a strategy of providing less feed of lower nutritive value will usually work; withholding water is not recommended. Dry cow therapy is recommended at this time together with monitoring of udder health to ensure that mastitis does not become a problem.

Prior to drying off, cows could be eating upwards of 3% body weight which will fall rapidly to only 2% after removal of the cow’s lactation demands. Involution of udder tissue occurs at this time, although the mass of secretory tissue will have been declining since achievement of peak lactation. At the same time the developing foetus will require additional nutrients as the cow moves to completion of the third trimester of pregnancy. Overall, this increased demand will be relatively modest but the associated increased proportion of the cow’s abdominal cavity being occupied by the gravid uterus will have a major effect on space available for the rumen. As this constriction on the rumen continues, overall feed intake will be affected and in such situations, it may be advisable to compensate for possible reductions in total nutrient intake by increasing overall ration nutrient density.

Calf weight

As indicated, total feed consumption during this period is unlikely to exceed 2% of body weight but in most circumstances will be sufficient to meet the animals total requirements provided the ration is of suitable energy and protein density. Where underfeeding does occur, for whatever reason, birth weight and survival rate of the calf may suffer.  A recent study found a twofold range in the birth weight of heifer calves born to multiparous cows, which was rather surprising given that all cows received the same pre-calving management. As indicated low birth weight can affect calf survival rate but may also affect subsequent lactation performance of the heifer calf, although to date these effects have not been quantified. In contrast, over feeding during the transition period can result in cows becoming over conditioned, often resulting in oversized calves. Both conditions can increase the incidence of dystocia, whilst overfat cows may encounter other problems such as poor intakes, fatty liver and ketosis once lactation has commenced.

Feed intake

After drying off, the cow’s appetite declines rapidly but for at least the first part of the transition period, levels approaching 2% body weight are achievable under most dietary regimes. Thus a 625kg non-lactating pregnant dairy cow should consume between 11.5 and 12.5kgDM/d, with few cows likely to consume more than 13kgDM/day. Closer to calving, total feed intakes start to decline, most noticeably from approximately 7 days prior to actual calving date. Maintaining satisfactory levels of feed intake throughout the peri-parturient period is important in respect of subsequent lactation performance.

Managing feed supply during the transition period needs to take account of the initial reduction in nutrient demand due to cessation of lactation, the reducing abdominal space to accommodate the rumen as uterine contents increase, the need to provide opportunity for rumen adaptation to the lactation ration whilst avoiding overfeeding (over fat cows, large calves) and the possible pre-partum onset of milk secretion. At all times it is essential that good rumen function is maintained in order to achieve good levels of feed intake post-calving. From drying off until approximately 3 weeks prior to calving (far-off period) reasonable levels of forages of relatively high fibre contents can be fed, targeting between 50-60% of the total ration (e.g. 1.0 to 1.2% bodyweight supplied as forage).

Straw feeding

Feeding good quality straw during the transition period is becoming quite common, as it not only provides physical fill but due to its lower potassium level can be valuable when attempting to control dietary potassium levels. Too much straw in the ration at this time may however affect subsequent lactation performance.

A good amount of straw in the dry cow diet would be around 5kg per head, per day. Quantity is important, but how it is presented to the animal is even more critical. By respecting a certain length of this fibre mixed in a TMR, you would assure the right volume of feed intake. Correctly chopped straw will participate in enhancing the rumen functions.

Author: Denis Dreux

Harvest is coming and a lot of farmers are thinking to use whole cereals to feed their ruminant with the aim of decreasing overall food costs.

Whilst sheep can successfully utilise whole grains, the same does not apply to bovine livestock and to optimise nutrient digestion when cereals are fed, some processing of the grain prior to ingestion is required. This permits the ingress of rumen microbial enzymes. Grains can be physically processed by grinding or dry rolling whilst more recently, crimping of moist (immature) grain and treatment with a suitable preservative has been proposed. Various chemical treatments have been used of which sodium hydroxide appears to have been the most successful.

Grinding of cereals generally results in a relatively rapid rate of degradation in the rumen following ingestion. This inevitably increases the rate of production of volatile fatty acids (VFA) resulting in reduced rumen pH levels. At moderate to high levels of grain inclusion, this reduction in rumen pH may be sufficient to reduce the ruminal digestion of fibre, with consequential effects on total tract digestion of feed DM and level of feed intake. In worst-case scenarios, especially when the treated grain is fed in discrete meals, the outcome may be subclinical (> rumen pH 6.0) or clinical (> rumen pH 5.5) rumen acidosis. When clinical rumen acidosis occurs, irreversible damage to the rumen epithelium can occur whilst the production of microbial toxins and their subsequent absorption through a compromised rumen wall can result in death of the animal.

Rolling dry grains is considered to be less harsh compared with grinding but the process needs to be carefully controlled to avoid the grains being over-processed. When suitably processed, rolled cereals are considered to have a slower rate of digestion than ground grain but feeding high levels can still have undesirable effects on rumen fermentation principally through changes in rumen pH and the rumen microflora. Whilst both grinding and rolling of grain can be undertaken on farm (provided suitable farm equipment is available), the process of flaking grains has to be undertaken in a dedicated plant. This involves steam treatment of the grain followed by rolling and is likely to produce feed of similar digestion characteristics to ground cereals. In most countries, steam flaking is generally confined to the treatment of maize grain.

Crimped grain appears to have good intake and nutritional characteristics, provided moulding of the treated grain during storage does not occur. Its major advantage appears to be the ability to harvest immature grains, circumventing the need to wait until the crop is mature or to dry the harvested grain before storage. Such issues can be very important in marginal cereal growing areas, which is often where considerable amounts of home produced grain are grown for livestock feeding. An additive must be applied at the time of crimping to ensure safe storage and both biological and chemical based products have been proposed. Some manufacturers claim conferred benefits on nutritive value with specific additives but these appear to be marginal.

Urea treatment of moist wheat at harvest is a cheap and simple system, which preserves the grain and increases its crude protein content. Treated wheat can be fed whole without further processing. It consists of a concentrated solution of urea combined with an enzyme activation, ‘urease’, which on contact with moist grain works quickly to release ammonia which penetrates the heap of grain giving complete, long lasting protection.

Sodagrain is chemically grain treatment which reduces the degradation of starch in the rumen and increases the amount of starch entering the small intestines. Sodium hydroxide is highly caustic and can lead to safety issues if not used appropriately. However, its use is still permitted and provided suitable precautions are taken, it should present no major hazards to the farm operator. Furthermore, as the pH of treated grain drops quickly over the first 24-48hrs after preparation, due to the conversion of sodium hydroxide to sodium bicarbonate, the product is safe to handle after 2-3 days and should present no problems to the animal when fed after 4 days.

It is claimed that sodium hydroxide treated grain has a slower rate of digestion in the rumen than physically processed grain, there have been relatively few studies to quantify the possible impact of such on ruminal and post-ruminal digestion of starch and other nutrients. Compared with other methods of processing cereals, the sodagrain process has several nutritional benefits:

• The alkaline nature of the product helps buffer acidity within the rumen.
• The slow breakdown of the starch reduces digestive problems often associated with feeding high levels of cereals.
• Sodagrain when fed in a MechFiber ration improves level of production especially milk protein, milk yield and live weight gain.

Although grass composition is undoubtedly important, the level of grass intake achieved by the cow is likely to be even more crucial in respect to overall levels of milk production achieved. It is known that a range of factors affect feed intake, but a useful and simple equation as described by Leaver (1981) describes intake as:

“ACTUAL intake = POTENTIAL intake – feed constraints – environmental constraints”

Potential intake is determined by the bodyweight of the cow as well as actual level of milk production whilst feed constraintsinclude herbage availability, quality and contamination, along with possible effects due to supplementary feeding. Environmental constraints include weather and day length, both of which affect the amount of time animals will or are able to graze freely. Grass dry matter intakes of between 16 and 18kg/cow/day, often equivalent to in excess of 100kg fresh grass/cow/day, can be achieved under well-managed rotational grazing and this is capable of supporting milk yields of 25 to 30litres/cow/day. However, it is accepted that in practice such levels of intakes and milk production will only be achieved in good grazing conditions and for a relatively short period during spring and early summer. In poorer weather, grass will typically be wetter, due to both increased levels of internal and surface water and levels of grass intake as well as milk production are likely to be considerably less than the accepted optimal. Grass intakes also typically decline as the season progresses, with increasing soil and dung contamination as well as reducing day length all contributing to reduce grazing times. There has been considerable detailed research carried out on the grazing behaviour of cows and the various factors affecting grass intakes. Grass intakes are known to depend on grazing time, grass biting rate and grass intake per bite, which all vary according to prevailing conditions.

This can be described as:

Grass DM intake = Grazing time x bite rate x bite size

With a grazing time of 600 minutes per day, an average bite rate of 60/min and an average bite size of 0.4gDM/bite, then a grass DM intake of 14.4kg DM/day is predicted according to Arnold, 1981 “60 typically cows graze for about 9 hours/day, but this can range from 7-12 hours/day”. Bite rate varies from about 45-65 bites/minute, whilst Philips & Leaver (1986) suggested that cows are constrained to a maximum of about 40,000 bites/day (11hrs grazing at 60 bites/minute). This effectively restricts the ability of the cow to compensate for any reductions in bite size which may be caused by limited herbage availability. Bite size varies considerably, typically from 0.3-0.7g DM/bite, with larger bites only possible when the cows are presented with longer, rotationally grazed grass.

Availability of forage is obviously a major factor affecting grass intake and overall levels of consumption are known to be compromised when grass stubble height is reduced. Generally, sward heights after grazing in a paddock system of 8-10cm are recommended for higher yielding cows. Continuous or set-stocked systems can often be more efficient as these tend to minimise grass wastage, but at the same time they can limit grass intakes because of the need to graze longer and/or faster to compensate for the smaller bite size typical of short, set-stocked grass.

For this reason, rational grazing is generally recommended for higher yielding cows (McGilloway & Mayne, 2002). From this, it is obvious that a number of factors can limit grass intake in practice. Very high levels of grass dry matter intake (>16kg/cow/day) can sometimes be achieved in spring/early summer, but only in ideal conditions. Typically, there are likely to be lower (12-14kg/cow/day) even though grass availability may not be a problem at this time. This reduction is considered to be due to lower grass quality which will need extra time to be processed by the cow. Much lower intakes (<6kgDM/cow/day) can occur when grass growth is reduced, typical of many areas in a hot dry summer, due primarily to declining forage availability. Finally, supplementary feeding will also affect grass intakes, the consequences of which are discussed below.

0mins x 60bites/min x 0.4gDM/bite =14,400gDM/day.

Typically, cows graze for about 9 hours/day, but this can range from 7-12 hours/day. Bite rate varies from about 45-65 bites/minute, but some suggest that cows are constrained to a maximum of about 40,000bites/day (11hrs grazing at 60 bites/minute). This effectively restricts the ability of the cow to compensate for any reductions in bite size which may be caused by limited herbage availability. Bite size varies considerably, typically from 0.3-0.7g DM/bite, with larger bites only possible when the cows are presented with longer, rotationally grazed grass.

Availability of forage is another major factor affecting grass intake and overall levels of consumption are known to be compromised when grass stubble height is reduced. Generally, sward heights after grazing in a paddock grazing system of 8-10cm are recommended for higher yielding cows. Continuous or set-stocked systems can often be more efficient as these tend to minimise grass wastage, but at the same time they can limit grass intakes because of the need to graze longer and/or faster to compensate for the smaller bite size yielding cows. From this it is obvious that a number of factors can limit grass intake in practice. Very high levels of grass dry matter intake (>16kg/cow/day) can sometimes be achieved in spring/early summer, but only in ideal conditions. Typically, they are likely to be lower (12-14kg/cow/day) even at this time of year. As the season advances, grass intakes are likely to further reduce (>10kg DM/cow/day) even though grass availability may not be problem at this time. This reduction is considered to be due to lower grass quality which will need extra time to be processed by the cow, as well as increased soil and dung contamination. Decreasing day length in the later summer and autumn will also have an effect. Much lower intakes (<6kgDM/cow/day) can occur when grass growth is reduced, typical of many areas in a hot dry summer, due primarily to declining forage availability.

Measurement of dry matter intake and feed conversion efficiency (FCE)

Assessing dry matter intakes is a key element for success. Measuring milk production, knowing how much cows are consuming in dry matter terms and predicting feed conversion efficiency will ensure that action can be taken to adjust the diet. The efficiency can be calculated by; expressing the energy corrected milk production as a function of dry matter intake. Ratios of 1.1-1.2 or less, are common in non-buffer feeding situations. FCE values of 1.2 –1.4 or more (more efficient) are achievable by feeding a balanced TMR besides grazing.

Grass growth

So far this year, according to AHDB data, grass growth has been more segmented compared to previous years; more than likely due to a sudden change of weather. Countries from the midlands have the highest growth over the UK.

​

​

​

​

Grass protein

Grass quality from this year’s AHDB figures seems normal and nicely follows the values of the previous year. As the crude protein rate is normally decreasing, farmers need to think to increase the protein level of the buffer diet.

​

​

AHDB sources show a big variation into the protein value of fresh grass. Mid June, it did vary 30% to as low as 15%.

​

​

InTouch customer data:

Across England, Scotland and Wales, data has been collected for April and May, looking at the grass DM intake VS. Feed Efficiency and cost.

April 2017

April was not a month where the amount of fresh grass in the diet was important. Farmers were saving the crop for their first cut silage, and the lack of rain made the growth very slow.

May 2017

Sun and rain were excellent during May, fresh grass intake shot up in most of the diets.

Grazing grass is a cheap way of producing milk, but can be an expensive one if not managed properly. With the data collected, we can see that there is a certain correlation between grass intake, feed efficiency and profit.

The use of well formulated grazing TMRs provides a much more considered route when optimisation of cow performance and feed utilisation are recognised as crucial to the financial viability of the business. It brings better opportunities to exploit the cheapness of grazed grass and optimise returns from higher yielding cows.

The role of grass in modern milk production

The ruminant has a unique ability to thrive on a multitude of diets, ranging from 100% grass and conserved forage to only arable by-products and concentrates. However, grass (usually grazed) is the major forage for dairy herds in much of Northern Europe and many argue that as it’s often the cheapest source of feed, so that its potential should be maximised. But it is also accepted that there are limitations to grass for higher yielding dairy cows, largely because of reduced control and consistency of feeding.

The aim of this blog is to discuss the role of grass for the modern high yielding dairy cow and to give some practical recommendations on grass feeding.

Grass composition

One of the difficulties when relying on grass is that its composition varies widely with season, species and management. This can be seen from the data summarised in table 1, based on 244 samples of fresh grass. Dry matter (DM) contents ranged from 11-42% of grass fre4sh weight with crude proteins (as % DM) between 5 and 36%, sugars 2 to 28% and NDF levels of 41 to 76%. It must be noted that this data includes primary spring growths, as well as summer and autumn re-growths harvested at different stages of maturity. There it may not be wholly representative of well-managed grazed grass, but this can still be of highly variable composition, often making it difficult to manage in respect of optimising feed utilisation.

Although grass composition can vary widely, there are important trends related to grass maturity. Young immature grass comprises mainly of leaf with little if any stem, so that cell contents typically make up <60% total dry matter, with cell walls (measured as neutral detergent fibre or NDF) often about 40%. Crude protein content is also typically high at this time at approximately 25% in dry matter, whilst sugar content is often low at about 10% in DM. However in mature (headed) grass, cell wall content (NDF%) increases to over 60% and cell content decrease to 40% or less. Crude protein content typically drops to <10% in DM whilst sugars can increase to 20% in DM, although this will depend upon weather conditions, especially the incidence of sunshine.

From this it follows, that well-managed, leafy grazed grass would be expected to be of high digestibility with a high metabolisable energy (ME) content, along with low fibre and high crude protein contents. However, there are known to be seasonal difference. In vivo studies have shown that the highest quality grass (12MJ/kgDM) only occurs in spring, when protein levels of about 25% are also common.

However, metabolisable energy contents of 11-11.5MJ/kgDM would probably be more typical of good grazing during the summer, and often with more moderate protein levels (18-20%). In the autumn, leafy regrowth grass is likely to have an ME content of between 10.5 and 11.0MJ/kgDM, often with relatively high protein levels (20 %+), although an increasing proportion of this may be present as non-protein nitrogen. There is substantial evidence that the efficiency of utilisation of ME is reduced in autumn- compared with spring-grass. This is likely to be a significant cause of the reduced levels of milk production typically observed on late summer/autumn grass compared with spring grass, though as suggested later other factors may be involved.

Author: Denis Dreux

Increasingly, the levels of protein and fat in milk are being recognised as major determinants of milk price and many dairy farmers are becoming concerned about falling levels when herd performance is judged on a year on year basis. Both stage of lactation and seasonality, especially with spring calving herds, can influence milk fat and protein levels, whilst research evidence and experience of milk processors have shown that at certain times of the year, the production of specific milk products is compromised an, in more severe situations, may be precluded. The level of protein in milk can have a major effect on cheese yield and quality, whilst the level of milk fat and the structural integrity of the milk fat globule, can affect the yield and quality of high-fat milk products.

Part of the problem in respect of poor milk fat and protein levels can be attributed to the higher milk yields being achieved with the present day Holstein. Research studies have shown cows with overall lactation yields in excess of 10,000 litres produced 30% more milk than average yielding cows, but their combined yield of milk fat and protein was only 18% higher, with average fat and protein levels of 3.90 and 3.10% compared with 4.37 and 3.45% respectively, when fed similar rations. Selection for higher milk component contents has brought only modest gains and some farmers have chosen cross-breeding as a possible route to improve fat and protein contents. The first cross, perhaps Holstein and Jersey, will bring improvements in line with the mean of two parents plus a small bonus (average 5%) attributable to hybrid vigour, although lower milk yields will mean more cows need to be milked to meet milk quota. Ideally the first cross animal should be bred out to another breed before the resultant offspring is bred back to one of the original breeds. This brings complexity to the farm’s breeding decisions and may reduce commercial value of the cattle, whilst there are unlikely to be any further benefits from hybrid vigour. Overall, cross-breeding may not provide the long-term solution that many farmers seek.

Principally, the yield and the content of milk fat are influenced by the digestion of feed in the rumen. Provided the ration contains adequate levels of digestible fibre and the cow efficiently digests that fibre, the acids produced in the rumen will support efficient milk fat synthesis, bringing responses in both fat yield and content. But in many cows the rumen may be less efficient than we believe. Short chop length silages will reduce cud chewing which in turn will reduce the nutrients the cow gets from that forage. Feeding starchy feeds in large single meals, such as occurs at milking on many farms, will cause a marked increase in rumen acidity immediately after the meal, with the resultant drop in rumen pH reducing the extent of fibre digestion and the quantity of nutrients obtained from that intake of fibre. Turning cows out to spring grass may be seen by many as a final release from winter feeding but high levels of sugar and low levels of fibre in spring grass can have the same effect as high starchy concentrates, with low rumen pH values, compromised fibre digestion and resultant drops in milk fat. Recent research showed cows grazing fresh lush pasture had rumen pH levels below 6 for 16hrs each day, indicative of sub clinical acidosis. Getting sufficient fibre into the cow and ensuring that fibre is digested in the rumen, not forgetting the importance of cud chewing, provides a serious opportunity for improving milk fat content and yield.

Improving milk protein requires a different nutritional approach. Feeding more protein in the ration usually increases milk protein yield but generally does nothing for milk protein content, unless initial levels are seriously depressed. High levels of dietary protein can increase body tissue mobilisation in early lactation and with the negative effects this can have on cow fertility, it seems prudent to avoid such strategies. There is however clear evidence that increasing the level of starch in the ration will increase milk protein content and yield. Simply replacing 1/3^rd of the grass silage component of winter rations with maize silage which contained good levels of starch (30% of total silage dry matter) gave consistent improvements in milk protein content (3.37 v 3.27%) together with marked improvements in milk protein yield (1.40 v 1.16kg/day). Feeding higher levels of maize silage should bring bigger responses whilst other options on the farm include whole crop cereal silage and cereal grains, with sodagrain (sodium hydroxide treated) providing some impressive gains, with milk proteins of 3.32% in a study which compared a grass: concentrates control ration with 3.01% protein, whilst milk fat levels were maintained at 4.19%. Others have tried fodder beet, which when grown in the correct environment can produce phenomenal crop yields, and in a recent research study, milk protein and fat content responses (3.21 and 4.29% v 2.99 and 41.7% respectively) were noted.

Deciding what to feed can impact on milk composition but how it is fed is equally important. Avoiding large slugs of feed at milking is a start, recognising that spring flush of pasture may not be an ideal feed for the production of milk of good composition is another whilst optimising rumen health to achieve optimal fibre digestion and not compromising starch digestion is crucial. After that controlling dietary protein levels to ensure that cows don’t milk excessively off their backs is worthy of serious consideration.

Taking a serious look at nutrition can bring large rewards in milk composition, improving milk price and overall profitability, especially where the use of home-grown feeds is increased.

Author: Denis Dreux

Unless extended lactations are being considered, dairy cows should rebreed every 12 months. Given an average lactation length of 305 days, this allows a dry period of 60 days during which the cow can recover from the previous lactation, complete the growth of the unborn calf and prepare for the next lactation. Failure to meet a calving interval of 12 months often results in an extended dry period where cows are not only non-productive but can become over-conditioned prior to calving.

Achieving 12 month calving intervals is most important for those farms that block calve in the spring; in order to exploit the potential of grazed grass. Those calves that calve later due to previous breeding problems not only miss the opportunities which the spring flush of grass brings to such systems, but will need to be managed differently from the rest of the herd during the subsequent Autumn and winter periods to drying off.

In contrast, where all year round calving is adopted and higher milk yields are being targeted, there may be merit in a modest increase in calving interval to 13-14 months, as such cows can often be quite difficult to dry off when still giving in excess of 25kg milk/day at lactation day 305. This should not however be seen as an excuse to accept longer calving intervals with longer dry periods. There is no biological evidence to suggest that dry periods of more than 60 days are of value to most cows.

With the aim of most dairy farmers being to achieve an average herd calving interval of 365 days, it follows that the cow must be in-calf before 90 days post-calving. Most cows will start to ovulate again at 20-25 days post-calving and on most dairy farms, rebreeding will commence after 55-60 days in milk. As the average length between oestrus bouts is 21 days, this only gives two opportunities to establish a successful pregnancy.

In higher yielding cows, the re-commencement of oestrus activity may be delayed (circa 8-14 days), further shortening the period available for rebreeding if a 365 day calving interval is still being pursued.

Getting cows back in calf has huge financial implications in all systems of milk production. For spring calving herds it is important that tight calving patterns are achieved to optimise pasture utilisation, whilst in all year calving herds, extended dry periods can be expensive and should be avoided wherever possible. Reproductive function is quite complex and as this report has shown, is affected by the nutritional and the metabolic state of the cow. As feeding for high peak yields loses some of its importance, so there is considerable opportunity to manage the nutrition of the cow to achieve good milk yields of good milk composition, with satisfactory levels of fertility placing similar emphasis on all 3 of these important components.

The current trends to lower first service conception rates and increased number of serves required per pregnancy need to be reversed. Improved management with better detection of oestrus activity is important but equally the benefits from improved nutritional management can be considerable. Whilst many herds have first service conception rates at or below 40%, there are well-managed herds where values in excess of 60-65% are being achieved – this applies equally to large herds as well as those with fewer cows.

Author: Denis Dreux