EFFECT OF DIETARY PROTEIN QUALITY IN SUPPLEMENTS TO DAIRY COWS.

The authors postulated that the performance of dairy cows grazing ryegrass pastures may be enhanced if the amino acid balance in the small intestine is enhanced. Being normally in short supply on ryegrass, supplementation with rumen-protected methionine (RPM) and lysine (RPL) could be beneficial. This was then investigated in the study.

Sixty high-producing multi-lactation Jersey cows in mid-lactation were enrolled in a randomised complete block design according to milk production, days in milk, lactation number, and randomly allocated to three groups within block. Each group was then randomly allocated to one of three experimental treatments: (1) control concentrate (C), concentrate supplemented with about 53 g per cow per day of RPL (estimated to provide approximately 22g of intestinally absorbable lysine), and (3) concentrate supplemented with about 42g per cow per day RPM and 53g per cow per day RPL respectively (estimated to provide approximately 9.3 and 22g intestinally absorbable methionine and lysine (RPML) respectively). The concentrate was supplemented to the cows at 8kg per cow per day, fed in-parlour in two equal portions. Otherwise they strip-grazed ryegrass pasture for the remainder of the time. The parameters measured were milk production, milk composition, body weight, body condition change, milk nitrogen fractions, plasma amino acid concentrations and microbial protein synthesis. The data was statistically analysed as a randomised block design with the GLM model for average effects over time, but distinguishing between higher producing (>24l per cow per day) and lower producing (<20l per cow per day) cows.

The results showed a tendency that RPL reduced milk protein production (P = 0.09) but significantly increased the plasma lysine-methionine ratio (P < 0.05). The RPML treatment increased plasma concentration of methionine and glycine, as well as body condition (P < 0.05) and tended to increase plasma lysine and cysteine. It also decreased the plasma lysine-methionine ratio (P < 0.05). It was interesting that the lower-producing cows responded negatively to RPL supplementation as measured by lower milk protein production (P < 0.05).

The lack of response in milk production and composition with RPL and RPML supplementation was probably due to the fact that metabolisable energy was the first limiting factor, the cows were in later lactation and therefore the response was in body condition, and that the protein quality of the base concentrate met requirements. The results therefore suggested that lysine and methionine were supplied in excess of requirements and were probably metabolised and repartitioned to other metabolic pathways than milk production and composition.