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

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 protein supplement with the level of milk yield was studied 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.d(-1) and 38 kg.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% maize silage and 40% concentrate) with different protein sources and levels: low-protein supplement (LP) (5% soybean meal +1.2% of urea in the diet), high-protein supplement containing soybean meal (HP-SB) (19% soybean meal) and high-protein supplement containing formaldehyde-treated soybean meal (HP-FTSB) (12% formaldehyde-treated soybean meal +1.2% of urea in the diet). HP treatments significantly increased (P < 0.05) DMI (+1.2 kg.d(-1)), milk yield (+1.8 kg.d(-1)), 4% FCM (+2.5 kg.d(-1)) and fat content (+1.9 g.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.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.