Effects of ratios of non-fibre carbohydrates to rumen degradable protein in diets of Holstein cows: 2. Rumen and plasma parameters and nitrogen metabolism

Abstract

Nine multiparous mid-lactation Holstein cows, averaging 171 ± 17 days in milk and producing 24.1 ± 3.3 kg of milk/d were assigned to a replicated 3 × 3 Latin square design to study the effects of altering the non-fibre carbohydrates (NFC) to rumen degradable protein (RDP) ratios on rumen and plasma parameters and nitrogen (N) metabolism. Diets were based on a barley concentrate and 45 forage to 55 concentrate ratio. Rumen degradable protein (g/kg) and NFC : RDP ratios averaged 98 and 4.10; 108 and 3.71 and 118 and 3.34 for the LRDP, MRDP and HRDP treatment diets, respectively (dry matter (DM) basis). Ratios were achieved through altering the RDP content of the diets while NFC was held constant at 401 g/kg DM. Urea was supplemented as a source of RDP to decrease the ratio. The effect of treatments on NH3-N concentration in rumen fluid and plasma urea nitrogen (PUN) was significant and increased linearly. Nitrogen intake, absorbed N and urinary N increased linearly between LRDP to HRDP. As a proportion of N intake, faecal N tended to decrease linearly from the LRDP diet to the HRDP diet, i.e. as dietary RDP increased. As a proportion of N apparently absorbed, milk N tended to decrease linearly from LRDP to HRDP as dietary the NFC: RDP ratio decreased. Results showed that indicators of the metabolic efficiency of N were more affected than milk yield, confirming that low producing cows are less likely to respond to altering NFC : RDP ratios. Hence, balancing rations to further improve the N utilization efficiency may be of a higher priority.

Keywords: Dairy cow, nitrogen metabolism, non-fibre carbohydrates, rumen degradable protein