Responses to supplementation with protected protein or protected amino acids in dairy cows grazing ryegrass-based pasture during early lactation

Abstract

The response to protected protein or protected amino acids in cows grazing ryegrass-based pasture during early lactation was investigated. Effects on nutrient intake, milk yield and composition, change in live weight and body condition score and blood metabolites were assessed.

Two hundred and five multiparous Friesian cows, aged between 4 and 10 years, were randomly allocated to one of nine treatment groups (n=22-23), three for early lactation (weeks 2-12), three for mid lactation (weeks 13-22) and three for late lactation (weeks 23-33). The results for the early lactation groups will be reported only. Cows were individually fed supplements of barley (1 kg/d; BR) or protected protein (1 kg/d; PP) or barley with protected methionine-lysine (1 kg/d and 17g/d methionine-lysine; ML). Pasture intake was estimated with subgroups of 8 cows using the alkane technique, with two measurements each lasting 13 days. Two non-lactating cannulated heifers were used to determine the degradability of feeds with the nylon bag method.

Intake of supplements was incomplete and the mean intake over the 11-week study period was 0.73, 0.41 and 0.67 kgDM/d for cows given BR, PP and ML respectively (P<0.01). Pasture dry matter intake (DMI) was similar for the three treatments (17.4 kg/d) (P>0.05). Supplementation with PP or ML did not affect the total DMI, crude protein intake (CPI), and estimated metabolisable energy intake (MEl) compared to cows given BR.

ML intake may have been too low to obtain an effect. Neither PP nor ML supplements significantly altered milk yield or milk composition, compared to cows given BR. Live weight nor body condition did not differ between treatments. Cysteine concentration in plasma was unchanged (P>0.05). Supplementation with ML increased concentration of casein in milk (P=0.05). Supplementation with PP increased plasma urea-N and β-hydroxybutyrate (BHB) concentration, and decreased plasma glucose concentration relative to BR cows, while ML had the opposite effect (P<0.01).

It was concluded that neither total MP supply nor methioninellysine supply appeared to be limiting production in cows grazing rye grass-based pasture during early lactation. There is a possibility of deleterious effects due to the excess supply of RDP, MP and/or methioninellysine, which deserves further study.