Abstract
Feeding corn dried distillers grains with solubles in low crude protein (CP) diets improves nitrogen utilization efficiency, however it can also reduce metabolizable AA supply, especially Lys, which compromises lactation performance. Therefore, our objective was to determine the effects of feeding by-pass protein/rumen-protected AA supplements in low or high CP diets containing 10% corn dried distillers grains with solubles. Six multiparous Holstein cows (619.3 ± 49.8 kg BW; 26.8 ± 6.2 DIM) were subjected to a split-plot 3 × 3 Latin square design with 21 d periods. The whole-plot factor was dietary CP content; low (14.6%; LP) or high (16.6%; HP), and the subplot was by-pass protein/AA supplement; control (no supplement), Supplement A (0.11 kg/cow/d) or Supplement B (0.45 kg/cow/d). Supplement A and B differed in both ingredient and chemical composition, resulting in differing AA concentrations. Dry matter intake, milk and milk lactose and protein yield did not differ in cows fed the LP than HP diet. However, reducing dietary CP content resulted in a decrease in N intake and apparent total tract CP digestibility, and a tendency for a decrease in milk protein yield. Furthermore, cows fed the LP diet tended to excrete a lower amount of total urinary N and urea-N and excreted a lower amount of total N than cows fed the HP diet. Similarly, BUN concentration was lower, and MUN concentration tended to be lower when N intake was restricted. However, there was no supplement effect on nutrient intake and digestibility, milk and milk component yields, and all measures of N utilization. Overall, feeding CDDGS in a LP compared to a HP diet had a marginal effect on production performance, which possibly negated the potential benefits of providing supplemental bypass protein/RP-AA under our experimental conditions. However, feeding CDDGS in a LP than HP diet improved N utilization efficiency, which may be beneficial from an environmental sustainability standpoint.