The effect of rumen-protected lysine and methionine on milk yield, milk components, and body weight in grazing Holstein cows during spring calving season in the southern hemisphere

The objective of this study was to assess the effect of rumen-protected (RP) methionine and lysine on milk yield, milk components, and body weight (BW) of grazing Holstein cattle with different parities during the spring calving season in the southern hemisphere. The study was conducted in a typical southern Chilean dairy farm, without supplementation of RP amino acids. The study considered the supplementation of RP lysine (LYS) and methionine (MET) in a 2 x 2 factorial design. Twenty-six cows per treatment were randomly assigned during spring 2019 into four groups as follows: (i) control group; (ii) LYS group (product with 55% of lysine; 20 g/cow per day); (iii) MET group (product with 55% of methionine; 30 g/cow per day); and (iv) LYS+MET group. Experimental cows started the supplementation period at day 2 postpartum, and they were evaluated until 80 days postpartum. Daily milk yield and daily BW were averaged weekly for 11 weeks. Milk solids (kg/day and percentage) and urea concentrations were determined once a week and were averaged in four periods of 3 weeks each. Amino acids were supplemented individually in the milking parlour. All cows were handled in a common lot, grazed the same pasture daily, and consumed the same partial mixed ration before each milking. Outcome variables were analysed by ANOVA for repeated measures, fitting a mixed model. Within primiparous cows, there was a trend during weeks 8, 9, and 11 and a significant effect on week 10 for greater milk yield for LYS than the control group, and there was a significant effect on weeks 9, 10, and 11 for greater milk yield for LYS compared to MET. However, within multiparous cows, in weeks 4, 5, 6, 7, 8, and 9, MET produced more milk than LYS and LYS+MET produced more milk than LYS in weeks 3 and 4. In primiparous cows, LYS produced more fat (kg/day) than the control group in period 1 and 4; however, in multiparous cows, MET produced more fat (kg) than the control group (periods 2 and 3) and LYS (period 2). Within primiparous cows, LYS produced more protein (kg/day) than MET and the control group in period 4; however, in multiparous cows, MET produced more milk protein (kg/day) than LYS (period 1 and 2) and tended to produce more protein than the control group in period 1 and 4. In conclusion, RP lysine supplementation improved milk yield and the content of fat and protein in primiparous cows with spring parturition under grazing conditions, while RP methionine supplementation had a similar effect on multiparous cows. © 2023 The Authors.