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The objective was to determine the relationships between early-life parameters [including average daily gain (ADG), body weight (BW), milk replacer intake, starter intake, and birth season] and the first-lactation performance of Holstein cows. We collected data from birth years 2004 to 2012 for 2,880 Holstein animals. Calves were received from 3 commercial dairy farms and enrolled in 37 different calf research trials at the University of Minnesota Southern Research and Outreach Center from 3 to 195 d. Upon trial completion, calves were returned to their respective farms. Milk replacer options included varying protein levels and amounts fed, but in the majority of studies, calves were fed a milk replacer containing 20% crude protein and 20% fat at 0.57 kg/calf daily. Most calves (93%) were weaned at 6 wk. Milk replacer dry matter intake, starter intake, ADG, and BW at 6 wk were 21.5 ± 2.2 kg, 17.3 ± 7.3 kg, 0.53 ± 0.13 kg/d, and 62.4 ± 6.8 kg, respectively. Average age at first calving and first-lactation 305-d milk yield were 715 ± 46.5 d and 10,959 ± 1,527 kg, respectively. We conducted separate mixed-model analyses using the REML model-fitting protocol of JMP (SAS Institute Inc., Cary, NC) to determine the effect of early-life BW or ADG, milk replacer and starter intake, and birth season on first-lactation 305-d milk, fat, and true protein yield. Greater BW and ADG at 6 wk resulted in increased first-lactation milk and milk component yields. Intake of calf starter at 8 wk had a significant positive relationship with first-lactation 305-d yield of milk and milk components. Milk replacer intake, which varied very little in this data set, had no effect on first-lactation 305-d yield of milk and milk components. Calves born in the fall and winter had greater starter intake, BW, and ADG at 8 wk. However, calves born in the summer had a higher 305-d milk yield during their first lactation than those born in the fall and winter. Improvements were modest, and variation was high, suggesting that additional factors not accounted for in these analyses affected first-lactation performance.

Meta-analytic methods were used to determine statistical relationships between metabolizable AA supplies and milk protein yield, milk protein percentage, and milk yield in lactating dairy cows. Sixty-three research publications (258 treatment means) were identified through a search of published literature using 3 search engines and met the criteria for inclusion in this meta-analysis. The Cornell Net Carbohydrate and Protein System (CNCPS) version 6.5 was used to determine dietary nutrient parameters including metabolizable AA. Two approaches were used to analyze the data. First, mixed models were fitted to determine whether explanatory variables predicted responses. Each mixed model contained a global intercept, a random intercept for each experiment, and data were weighted by the inverse of the SEM squared. The second analysis approach used classical effect size meta-analytical evaluation of responses to treatment weighted by the inverse of the treatment variance and with a random effect of treatment nested within experiment. Regardless of the analytical approach, CNCPS-predicted metabolizable Met (g/d) was associated with milk protein percentage and yield. Milk yield was positively associated with CNCPS-predicted metabolizable His, Leu, Trp, Thr, and nonessential AA (g/d). Milk true protein yield was also associated with CNCPS-predicted metabolizable Leu (g/d). Predicted metabolizable Lysine (g/d) did not increase responses in production outcomes. However, mean metabolizable Lys supply was less than typically recommended and the change with treatment was minimal (157 vs. 162 g; 6.36 vs. 6.38% metabolizable protein). Experiments based solely on Lys or Met interventions were excluded from the study database. It is possible that the inclusion of these experiments may have provided additional insight into the effect of these AA on responses. This meta-analysis supports other research indicating a positive effect of Met and His as co-limiting AA in dairy cows and suggests Leu, Trp, and Thr be given greater consideration in future research.

A well-managed, high-producing commercial dairy herd was used to test the impact of supplementing β-carotene (425 mg/d per cow) to lactating multiparous Holstein cows with normally low serum β-carotene (<3 µg/mL). Milk production was not affected by treatment (43.83 and 43.65 kg/d for the β-carotene and control diets). There was no difference because of treatment in overall 3.5% FCM (43.23 vs. 42.24 kg/d for the β-carotene and control diets) but early-lactation cows (0 to 100 DIM) and mature cows (3+ lactation) tended to produce more 3.5% FCM when supplemented with β-carotene (P < 0.10). Milk fat percentage was higher (P < 0.05) with supplemental β-carotene (3.28 and 3.18% for β-carotene and control cows), especially in early-lactation cows and mature cows. Overall milk fat yield (kg/d) was unaffected by treatment but early-lactation and mature cows supplemented with β-carotene tended to produce more milk fat than control cows. Milk true protein percentage and yield were not affected by treatment. Milk urea N levels were higher for cows supplemented with β-carotene (16.06 vs. 15.72 mg/dL for β-carotene-supplemented and control cows; P < 0.05). Somatic cell count was not affected by treatment. Overall pregnancy rate was unaffected by treatment, but after 105 d of β-carotene supplementation, pregnancy rate was 22% for β-carotene-supplemented cows compared with 11% for control cows. β-Carotene supplementation increased percentage of milk fat and tended to increase 3.5% FCM and milk fat yield in early-lactation and mature cows.

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