MILK FAT AND DIET
Milk fat level is quite variable and reasonably responsive to diet. A large portion of the variation observed in milk fat can be attributed to changes in fatty acids produced in rumen fermentation. More precisely, the higher the ratio of acetate to propionate in the rumen, the higher the fat content of the milk will be. Many of the dietary factors discussed below exert at least part of their influence by altering the ratio of fatty acids.
Forage Level in the Diet
The most common dietary cause of a low fat test is a diet containing a low level of forage and a high level of concentrate. The low fat test observed in this situation is often confounded by the fact that low forage diets are often associated with high milk production and high milk production itself tends to lower milk fat test.
Many of the rules of thumb applied to forage feeding are aimed at maintaining the type of rumen fermentation that will give a normal milk fat test. Commonly used guidelines are a minimum of35% forage in the diet dry matter or a daily forage dry matter intake equivalent to 1.5% of body weight.
A diet which maintains "normal" fat test without sacrificing production is usually a healthy one and often the most economic. For these reasons, fat test is often used as a barometer of the nutritional status of a herd. Since fat test is so highly variable between cows one must be cautious in using individual cow data for ration evaluation - group or herd averages will always give a more accurate picture.
Fibre in the Diet
Although we often view forage as the dietary component required to maintain fat test, it is in fact the fibre in the forage that is crucial. Fibre, usually measured as ADF (acid detergent fibre) or NDF (neutral detergent fibre), is the dietary factor that most accurately indicates the likelihood of a diet maintaining fat test.
Since most feeds in a cow's diet contribute some fibre, the overall dietary fibre level needs to be considered, not just the contribution from forage. A guideline sometimes applied to dairy diets is a recommended minimum daily NDF intake equal to 1.1-1.2% of body weight.
Unfortunately, fibre level alone will not always predict a diet's ability to maintain milk fat test. An adequate length of cut of the forage (i.e. the length of the fibre) is also an important factor. Whether a forage is stored as finely chopped silage or as long hay will have no effect on the level of fibre as determined in a feed analysis lab. However, the forage stored as hay will be more effective in maintaining milk fat test. A large part of this difference comes about because the long forage (i.e. long fibre) stimulates cud-chewing and saliva flow. The high buffering capacity of cow saliva helps to maintain rumen pH and acetate levels and fat test.
A similar comparison can be made between a coarse grain mix and a similar pelleted one. Although a chemical analysis will say they are the same, the coarse feed will be a more effective source of fibre and will more likely maintain fat test.
Type of Grain
The type of grain fed also influences rumen fermentation and milk fat test. The starch in corn for example is quite slowly degraded in the rumen and tends to maintain higher and more uniform rumen pH, higher rumen acetate levels and higher milk fat test when compared to a more rapidly degraded starch such as barley.
However, rumen fermentation is such a dynamic and complex process that factors such as dietary fibre level and feeding schedule often override the effects of grain source.
Dietary Protein Level
In most practical feeding situations, dietary protein level does not influence milk fat test.
In some cases, however, researchers and farmers observe an improvement in fat test when more protein is fed. This is likely due to an improvement in rumen fermentation and fibre digestion with an accompanying increase in the proportion of acetate in the rumen resulting in a higher fat test.
On the other hand, in cases where extra protein causes an increase in milk volume, fat test percentage could drop simply due to the effect of dilution.
Despite the general lack of effect of dietary protein on fat test, feeding the amino acid methionine (a protein constituent) often does improve fat test. Some commercial protein topdresses include supplemental methionine for this reason.
Traditionally, dairy cows received relatively small amounts of lipid (fats and oils) in their diets. In more recent years however, many high producing cows have been fed fat either in the form of oilseeds, such as soybeans, or as added fat.
Data on the effect of dietary fat level on butterfat test can be confusing, with positive, negative and neutral effects all commonly recorded. The primary factors involved in these apparent discrepancies are the type of fat and level at which it is fed.
Unsaturated fats, ( they usually exist as oils at room temperatures), are detrimental to rumen fermentation and fibre digestion and often depress feed intake. Feeding this type of fat, especially when it is not incorporated in a seed such as whole soybeans, often causes a depression in fat test.
More saturated fats such as tallow however, do not have the same negative effects as unsaturated oils and, when fed in moderate amounts will often cause an increase in milk fat test.
Rumen escape or rumen inert fats such as the commercial product Megalac also have the potential for increasing fat test especially in 1st-calf heifers.
Another aspect in the feeding of rumen inert fats is how they change the composition of the milk fat, rather than the milk fat level. A significant portion of 'rumen by-pass' fat may be transferred unaltered into the milk, resulting in a change in the milk fat composition, especially the balance of saturated to unsaturated fat. This could have important implications as a way to alter milk fat composition to meet specific market requirements.
Buffers such as sodium bicarbonate and magnesium oxide have been used as feed additives in dairy diets for years. They can be effective in maintaining proper rumen pH and alleviating fat depression. They will not improve fat test unless the test has been depressed due to excessive acidity in the rumen. As with other strategies, buffers will not improve a "normal" or a genetically depressed test.
Feeding Schedule and Sequence
Some low-milk fat problems can be alleviated with changes in feeding schedules, feeding sequences or feeding system.
For example, TMR (total mixed ration) feeding has been promoted and used as a method of minimizing the pH-depressing effects of high levels of grain feeding.
Buffering the rumen by feeding forage prior to grain is another widely promoted practice that is aimed at stabilizing rumen pH and maintaining fat test.
Although research on multiple feedings has been limited, review articles on work-to-date indicate that an increase in milk fat is the most likely benefit to be obtained from multiple feedings.
Niacin is a B vitamin that is important in fat metabolism. Fat cows often show a milk production response to niacin supplementation in early lactation. Although research results are not conclusive or dramatic, this increased production is often accompanied by an increase in fat test.
For many years, dairy cows have been fed and managed to maximize milk fat production. Although these efforts were driven primarily by milk pricing formulas and were aimed at increasing the monthly milk cheque, they had many side-effects that were also beneficial.
Milk fat percentage is well established as a quick check on dairy herd nutrition. A milk-fat sustaining diet is usually accompanied by a better rumen fermentation resulting in improved forage digestion, less off-feed problems, less feet and leg problems, and generally speaking less stress on the cow.
The fact that diet can have a major effect on milk fat percentage has made this milk component of particular interest to dairy farmers. If our milk pricing schemes in the future put less emphasis on high butterfat, we will lose an important incentive for feeding healthy diets. If this happens we will need to further emphasize the many other reasons for feeding adequate fibre levels and maintaining proper and consistent feeding schedules.