Effect of dietary protein quality and amino acid supplementation on performance of high producing Jersey cows grazing ryegrass pasture

Abstract

The performance of grazing dairy cows, when supplemented with high levels of a maize-based concentrate, could potentially be improved in terms of higher milk and milk protein production by improving the essential amino acid balance of the metabolisable protein reaching the cow’s small intestine. The objective of this study was to investigate the effect of supplementing rumen protected lysine or a combination of rumen protected lysine and rumen protected methionine on the production performance of high producing Jersey cows grazing ryegrass pasture in spring while receiving a maize-based concentrate. The performance parameters measured were milk production, milk composition, body weight and body condition change. The effect of supplementation on milk nitrogen fractions and plasma amino acid concentrations were also measured. A secondary objective was to evaluate a urine spot sampling technique and allantoin excretion as a non-invasive method to estimate rumen microbial protein synthesis. The study was conducted at the Outeniqua Research Farm situated near George in the Western Cape during spring. Sixty high producing multiparous Jersey cows in mid-lactation were used in a randomised complete block design experiment and blocked according pre-experimental milk production, days in milk, lactation number and randomly allocated to three groups within each block. Subsequently, each group was randomly allocated to one of three experimental treatments. These treatments were: (1) control treatment (C) supplemented with no rumen protected amino acids; (2) rumen protected lysine treatment (RPL) supplemented with 53.12 g/cow/d of RPL providing approximately 22 g of intestinally absorbable lysine/cow/d and (3) rumen protected methionine and lysine treatment (RPML) supplemented with 41.68 and 53.12 g/cow/d of RPML, providing approximately 9.3 and 22 g intestinally absorbable methionine and lysine, respectively. In addition to strip grazed ryegrass pasture the cows received 8 kg (as is) of a maize-based concentrate, fed in two equal portions in the milking parlour. The data were statistically analysed for the high (block 1 to 10) and low producing (block 11 to 20) cow groups. The supplementation of RPL did not affect milk production and composition, body weight, body condition, faecal starch content, plasma amino acid concentrations and microbial protein production (P > 0.05). However, compared to the C treatment RPL supplementation tended to reduce milk protein production (3.69 vs. 3.89 %; P = 0.09). Supplementation of RPL did, however, increase the plasma lysine:methionine ratio beyond the ratio represented by the C treatment (3.67 vs. 3.32; P < 0.05). The high producing group of cows (> 24 l/cow/d) did not respond to the supplementation of RPL, while the lower producing group of cows (< 20 l/cow/d) responded negatively in terms of milk protein production compared to the C treatment (3.85 vs. 4.16 %; P < 0.05). Supplementing the combination of lysine and methionine did not affect milk production and composition, faecal starch content, body weight and microbial protein production. However, in the presence of an increase in plasma methionine concentration (41.4 vs. 28.8 umol/l; P < 0.05) cow body condition increased (+ 0.43 vs. + 0.31 points (Scale 1 to 5; P < 0.05) compared to the C treatment. In addition to an increase in plasma methionine, glycine increased (448 vs. 405 umol/L; P < 0.05), and lysine tended to increase (109 vs. 95.5 umol/l; P = 0.09) along with cysteine (9.45 vs. 8.40 umol/l; P = 0.07) compared to the C treatment. Supplementation of RPML decreased the plasma lysine:methionine ratio below the ratio represented by the C treatment (2.63 vs. 3.32; P < 0.05). The higher producing group of cows (> 24 l/cow/d) and lower producing group of cows (< 20 l/cow/d) did not respond to the supplementation of RPML. Comparing the study results with predictions done with the CNCPS model gave relatively realistic and comparative predictions in terms of metabolisable energy allowable milk, metabolisable protein allowable milk, pasture dry matter intake and microbial protein synthesis, including the metabolisable protein and amino acid balance of the cows and support the findings of our study. Results indicate that the allantoin in spot urine samples are a valid method to be used to determine microbial protein synthesis for Jersey cows grazing pasture. In view of the experimental results the data indicate that the lack of positive responses in terms of milk production and milk composition when RPL or RPML were supplemented were as a result of metabolisable energy being the first limiting nutrient in addition to the cows being later in lactation and that the C diet met the cows metabolisable protein requirements. Results are interpreted to suggest that both lysine and methionine were supplied in excess of cow requirements and subsequently metabolised, repartitioning nutrients towards other metabolic pathways away from milk production and composition. The results further show that the amino acid requirements, limitations and responses in cow performance for cows grazing high-quality pasture while being supplemented with a maize-based concentrate is complex and requires further research.