Meta-Analysis to Assess Effect of Prepubertal Average Daily Gain of Holstein Heifers on First-Lactation Production

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Abstract
Decreased mammary development has been observed as prepubertal average daily gain (ADG) is increased; however, responses in first-lactation milk production to alterations in prepubertal ADG have been inconsistent across several experiments. Due to the continuous nature of ADG, body weight at calving (BWC), milk production, and milk composition, designing an experiment that encompasses a large range of ADG while maintaining an adequate number of animals at each ADG becomes prohibitive. Therefore, the objective of the current analysis was to quantitatively and statistically assess effects of alterations in prepubertal ADG and BWC on first-lactation production of milk, fat-corrected milk, milk fat, and milk protein. Eight studies that included Holstein heifers and were published within the past 15 yr were included in this analysis. The mixed model procedure of SAS was used to assess effects of prepubertal ADG and BWC on milk production independent of genetic or management differences that were evident between trials. Linear and quadratic effects of prepubertal ADG and BWC and their combinations were modeled including a random classification variable for each trial. Milk yield responses were associated quadratically with increasing prepubertal ADG; first-lactation production increased as prepubertal gains increased up to 799 g/d, the point of maximal milk production, whereas further increases in prepubertal ADG were associated with lower milk production. Increasing BWC within the range of 477 to 550 kg tended to linearly increase first-lactation milk production, but BWC significantly affected milk production in a model that included the linear and quadratic effects of ADG as well as linear BWC. In that model, which accounted for differences in calving weight, milk production was maximal at prepubertal ADG of 836 g/d. Milk protein yield was quadratically affected by alterations in prepubertal ADG, and milk protein yield was maximized when prepubertal growth occurred at 836 g/d. This analysis also revealed that yield of milk fat remained relatively constant independent of alterations in prepubertal ADG, which occurred despite the significant quadratic effect on milk and milk protein yield.

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