Alteration of milk fat by variation in the source and amount of starch in a total mixed diet fed to dairy cows.

The effect on milk yield and composition of the supplementation of the diets of dairy cows with wheat or potato peelings was studied at three different starch intakes (< 5, 6, and > 7.5 kg/d) for dry matter intakes around 20 kg/d. Starch supply was varied using different dietary concentrations of corn silage. Trials were conducted using Latin square designs, and all cows were fed a total mixed diet composed of corn silage, grass silage, or both; soybean meal; a mixed meal with formaldehyde treatment; minerals; and one of the following energy concentrates: potato peelings or wheat. Dry matter and energy intakes varied significantly only at the low starch concentration; higher intakes were observed when the potato peelings were fed. Body weight, milk yield, true protein content, lactose content, and protein yield were not affected by treatment. Milk fat content was higher (+3.3 g/kg) when potato peelings were fed at the high starch concentration but was unaffected at the low and medium starch concentration. Slow degradation of starch from the potato peelings in the rumen could enhance a higher delivery of precursors of milk fat synthesis in the udder. The effect on fat yield differed among starch concentrations. Milk fat content appeared to decrease for cows fed diets containing quickly degradable starch at a starch intake > 7 kg/d.

Interactions of ruminally protected methionine and lysine with protein source or energy level in the diets of cows.

Interactions between supplementation with ruminally protected Met and Lys and the nature of protein or energy concentration of the diet were studied using 16 and 12 multiparous lactating dairy cows in two trials of 8 and 12 wk, respectively, commencing on approximately d 40 of lactation. In trial 1, cows received a semicomplete diet plus concentrates. The diet consisted of 62 to 63% corn silage, 2.2% corn gluten meal, .4% urea, 11% soybean meal (untreated or treated with formaldehyde), and 23 to 24% barley. In trial 2, cows received a complete diet with corn silage, untreated and formaldehyde-treated soybean meal, and barley in the ratio 78:12:9:0 or 49:13:4:33. All treatments were replicated in a 4 x 4 Latin square design. In both trials, Met plus Lys (10 g/d of intestinally available Met and 30 g/d of Lys) has no significant effect on DMI, milk yield, fat content, casein as a percentage of true protein, or urea content of the milk. Mean increase of milk protein yield was 46 g/d with Met plus Lys, and mean increase of true protein content was 1.1 g/kg of milk. The increase in content of milk true protein was greater for cows receiving the low energy diet. Protein source had no effect on milk yield or composition. Glucose, urea, NEFA, BHBA, and total free AA in plasma were unaffected by supplementation of ruminally protected Met plus Lys. However, concentrations of Met and Lys in blood were slightly, but not significantly, higher in supplemented cows.