Effects of the level and source of dietary protein on intake and milk yield in dairy cows

摘要

The objectives of this trial were to determine the effect of dietary protein content and the effect of the source of protein supplements on dry matter intake (DMI) and milk yield in dairy cows fed ad libitum. In addition, the interaction of proteinsupplement with the level of milk yield was stud-ied to test the hypothesis that the DMI response to protein supplement depends on the nutrient requirements of the cows. Two groups of six lactating cows differing in milk yield (45 kg centre dot d~(-1) and 38 kg centre dot d~(-1) pre-treatment) were assigned to a double 3 X 3 Latin square design over periods of 3 weeks. The 3 treatments consisted of 3 complete diets (60 percent maize silage and 40 percent concentrate) with different protein sources and levels: low-protein supplement (LP) (5 percent soybean meal + 1.2 percent of urea in the diet), high-protein supplement containing soybean meal (HP-SB) (19 percent soybean meal) and high-protein supplement containing formaldehyde-treated soybean meal (HP-FTSB) (12 percent formalde-hyde-treated soybean meal + 1.2 percent of urea in the diet). HP treatments significantly increased (P < 0.05) DMI (+1.2 kg centre dot d~(-1)), milk yield (+1.8 kg centre dot d~(-1)), 4 percent FCM (+2.5 kg centre dot d~(-1)) and fat content (+1.9 g centre dot kg~(-1)) compared to the LP treatment. These improvements were not related to the source of protein used or the level of milk yield. Compared to LP treatment, the HP-SB treatment increased milk protein content (+1.4 g centre dot kg~(-1)) (P < 0.05), whereas the HP-FTSB treatment had no significant effect. Intake and milk yield were increased by higher protein content in the diets independently of the source of protein used or the level of milk yield.