In most cases, factors that tend to increase the milk yield of the cow tend to decrease milk fat percentage. Certain changes in the diet of the cow adversely affect the milk fat percentage. Most of these are related to a high concentrate, low roughage diet, which contains a low amount of fiber. The exact reason for the milk fat depression is not known except that the depressed fat percentage is accompanied by a change in rumen fermentation. There is a decrease in rumen acetate production, an increase in the rumen propionate production, and a decrease in the rumen pH. The feeding of sodium or potassium bicarbonate, magnesium bicarbonate, magnesium oxide, and calcium hydroxide, in part, prevent the depression in the milk composition caused by a restricted roughage ration.
Obvious differences exist in milk composition and yield among various breeds of dairy cattle.
B. Nutrition 1. In general, any ration that increases milk production reduces fat percentage in milk but not necessarily total fat yield. Feeding cod liver oil and other highly unsaturated oils markedly reduces milk fat percentage but not production. Also causes a reduction in b -hydroxybutyric acid used for fat synthesis. There is a gluconeogenic response to increased propionate production and a decline in blood lipids available for milk fat synthesis.