Nutrition and milk composition (AG0534)
John Roche and Dawn Dalley, Ellinbank
June, 1996
Introduction
Milk composition is economically important to milk producers and processors. It consists of approximately 13% solids and 87% water.
It is the solids content that determines the value of the milk as more countries are placing a negative weighting on carrier (volume) costs. Technology allows processors to disassemble milk into its various component parts and these can be combined in different ways to make new products or used as specific ingredients in other foods. Approximately 80% of total world milk delivered to milk factories in 1993, was processed into products. This shows the importance of milk composition to the processor. Milk composition is also of extreme importance to the consumer from a nutrition point of view. In this health conscious age, changing the ratio of protein to fat or indeed changing the fatty acid composition of the milk is now perceived as being desirable. It should be remembered that changes in the diet of cows may influence the yield of constituents as well as their concentrations in milk.
Milk fat concentration
Milk fat concentration can be altered by diet to a much greater extent than milk protein concentration. Until recently, it was the main component of milk included in milk payment schemes and therefore was the most researched. The type of fermentation in the rumen is very important in affecting milk fat concentration. In order to prevent a decline in milk fat concentration it is important that the rumen fluid acetate:propionate ratio is greater than 2. The principal dietary factors affecting milk fat concentration are:
Dietary fibre content
The level of dietary fibre has a large effect on the rumen volatile fatty acid profile. A high fibre content generally leads to a high acetate:propionate ratio and is thus conducive to a high milk fat concentration. It has been shown that the relationship between milk fat concentration and dietary acid detergent fibre (ADF) concentration is linear in the range 90-220g/kg ADF and that approximately 220 g/kg ADF in the dry matter (DM) is the minimum to prevent milk fat depression. The NRC (1988) recommend a minimum of 210 g/kg DM ADF and 280 g/kg DM of neutral detergent fibre (NDF) in early lactation to prevent milk fat depression. Previous work reported very low levels of fat (as low as 2%) in milk of cows offered >40-50% of their diet as a supplement of crushed wheat, or a high energy pelleted concentrate, to good quality pasture. This decline in milk fat was attributed to a low fibre intake.
Fibre quality
While the amount of fibre in the diet is very important, the quality of this fibre is equally so. The quality aspect refers to the source of and particle size of the fibre. Approximately 75 % of the NDF in the diet should come from forage. Forages of short particle length, although providing adequate fibre in chemical terms, do not promote salivation and mastication and are fermented relatively rapidly in the rumen. This can lead to a low acetate:propionate ratio and hence milk fat depression. A particle length of 2mm will maintain normal rumen function but particles greater than 6-8mm are required to maintain milk fat concentration.
Forage:concentrate ratio
Diets with a high proportion of concentrates and hence a low forage:concentrate ratio can result in a low milk fat concentration. The low fibre content of these diets, lowers the acetate:propionate ratio. Starchy concentrates (eg. barley, maize grain, etc) would lower the concentration more than digestible fibre based concentrates (eg. sugar beet pulp, citrus pulp). Thus it is not possible to give an optimum forage:concentrate ratio without defining the type of concentrate fed.
Concentrate feeding frequency
Infrequent feeding of concentrates (eg twice daily at milking) results in high peaks of propionate in the rumen and hence high peaks of insulin in the blood. This tends to direct lipogenic precursors away from mammary tissue and towards adipose tissue. It seems that more frequent feeding helps reduce the milk fat depression obtained when feeding high concentrate diets. This is probably a consideration when concentrate feeding levels rises to 10kg DM and above.
Starch degradability
The starch in cereal grains degrades at different rates in the rumen. Oat starch is the most quickly degraded, then wheat, barley and maize. The more rapidly degraded starch can lead to a rapid drop in rumen pH and a consequent depression in milk fat percentage. Whether these grains are whole, cracked or crushed will have an increasingly negative effect on milk fat.
Buffers
The need for buffers has increased with the increased use of grains and the increased use of precision chopped forages. A buffer is a chemical substance that resists changes in acidity of a solution to which it is added. When used in ruminant diets, buffers act to maintain pH of the digesta within a suitable range (6.2-6.8). This facilitates the digestion of fibre and the production of acetate.
It has been shown that buffer feeding increases milk fat percentage when high levels of concentrates are fed twice daily. Therefore the use of buffers should be considered when the forage portion of the ration makes up less than 45% of the diet or for high producing cows in early lactation, especially those receiving more than 1 kg grain/100 kg bodyweight (5-6 kg grain/cow).
Lipid supplementation
In general the feeding of rumen protected lipids increases milk fat concentration by up to 1.5% units in a low milk fat situation. Whereas feeding moderate quantities of unprotected fats tends to decrease it by up to 1% unit, especially if the fats are of the unsaturated type (eg vegetable oils).
Energy intake
Victorian and New Zealand studies have shown that the effect of underfeeding, in early lactation is variable but generally tends to increase milk fat per cent. For example a 50% reduction in intake leads to 0.4% increase in milk fat, but over the whole lactation this effect of underfeeding in early lactation reverses to become a reduction of 0.2% in milk fat.
During late lactation this trend is reversed. Improving milk composition by underfeeding is unprofitable because of the decrease in total milk solids yield brought about by reduced milk volumes with restricted feeding.
Condition score
There is an immediate increase in milk fat per cent in the first five weeks of lactation by having cows in good condition at calving (0.3%/condition score increase) but the overall lactational effect is negligible.
Milk protein concentration
In many countries, protein is now the most important constituent of milk, both to the processor and the producer. Due to changing consumer attitudes towards fat, protein is now more in demand and processors that manufacture protein based products want milk with an increased protein concentration. The main nutritional factors that can influence protein concentration of milk are:
Energy intake
As the energy of the diet increases so too does the protein yield and concentration of the milk. Ellinbank work would indicate that restricted intake (50% reduction) in early lactation, decreases protein concentration during early lactation by 0.3% units. This restricted feeding in early lactation results in an overall drop of 0.14% milk protein over the entire lactation. New Zealand work suggests a drop of up to 0.3% units. During late lactation this trend is reversed due to increasing the rapidity at which milk composition changes with decreasing yield (the dilution effect). Irish and Australian work would suggest that restricted feeding can also produce milk that is less suitable for processing at the factory.
Condition score
There appears to be no effect, either immediate or long term, on milk protein per cent by having cows calve down at a higher condition score.
Lipid supplementation
Including lipid (fat) in either rumen protected or unprotected form generally decreases milk protein concentration (0.05-0.15%). The inclusion of feed ingredients with a high oil content, such as whole cottonseed or distillers grains can also depress milk protein concentration. In some studies, niacin supplementation has been effective in alleviating lipid induced milk protein depression with whole cottonseeds and soybeans. Supplementation of protected protein in conjunction with lipid supplementation can alleviate the milk protein depression associated with fat.
Starch and sugar supplementation
In theory, increasing starch in the diet of dairy cows should decrease the ratio of acetate:propionate, thus increasing milk protein and decreasing milk fat. However, in practice, feeding starch-based supplements compared with digestible fibre-based supplements has not increased either milk protein concentration or milk protein yield. Treating the cereal with NaOH or feeding maize instead of barley starch are two strategies that give less starch digestion in the rumen and increase the quantity digested in the small intestine, but they seem to have little effect on milk protein.
Protein level and source
Provided there is not severe protein under-nutrition, increasing protein level in the diet has only a small and inconsistent effect on milk protein concentration. Generally there is a positive effect on protein yield. Protein source can have an effect through increasing either the quantity or the quality of protein reaching the small intestine of the cow (bypass protein). On grass silage based diets, fish meal supplementation generally increases milk protein concentration and yield. When corn gluten meal replaces soybean meal in the diet, milk protein concentration either drops or remains unchanged. The feeding of protected protein to cows at Ellinbank resulted in a slight increase in milk yield, but not concentration.
Amino acid supplementation
The underlying principles of increasing milk protein via dietary manipulations are generally to either increase the overall quantity of amino acids reaching the small intestine or to alter the profile of the amino acids so that more of the essential and milk protein-limiting amino acids are available. Recent data has shown that adding rumen protected lysine and methionine to increase the degradable protein in the small intestine by 7.3% and 2.5% respectively, increases both protein concentration and yield. It has also been shown that supplementing with both amino acids can give a lift in milk protein composition of 0.8 g/l but supplementing with methionine alone has a much lower effect (0.2 g/l).
Forage quality and type
Forage quality has an influence on the dry matter intake of cows and so would be expected to have an effect on milk protein. It has been shown in Scottish work that feeding a higher digestibility silage increases milk protein. The type of forage also has an effect. White clover can give a higher milk production response than perennial ryegrass and it generally tends to increase protein concentration by up to 0.1% unit. A similar response can also be obtained from grazing cows fed additional energy. Similarly, increasing maize silage as a proportion of a maize silage:grass silage diet can also increase milk protein yield and concentration.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
