Effects of increased milk allowance through within-study comparisons on dairy calf growth and future performance

The objective of this study is to employ meta-analysis to comprehensively evaluate the relative effects of increased milk allowance through comparisons within studies on the growth, apparent digestibility, blood parameters, and production performance. A systematic review was conducted with literature searches in PubMed, Web of Science, and Google Scholar, using the keywords "milk level AND calf OR calves," resulting in 2,722 studies. After screening and evaluation, 43 full-text articles met the PICO criteria (participants, intervention, comparison, outcome) and were included in the meta-analysis. This study seeks to identify key factors affecting calf development by analyzing the relationship between milk feeding levels and performance indicators such as weight gain and milk production, considering the complexity and variability in calf nutritional research. Additional factors such as feeding frequency, milk type, feeding method, starter form, and environmental conditions were also considered. Meta-regression analysis evaluated the effects of milk feeding levels along with key moderators including feeding frequency, feeding method, and starter feed physical form on multiple outcomes: growth performance such as ADG, feed efficiency, and BW; digestibility metrics; 305-d first-lactation milk yield; and age at first calving. The total effect size was pooled using a random-effects model, which visually summarized the results, showing significant improvements in growth performance on calves fed a high milk level. Publication bias was identified through funnel plots and Egger's test, indicating bias in studies on total dry matter intake and ADG during the preweaning period. Key findings revealed that increasing milk feeding levels enhanced growth performance, particularly in ADG. However, high milk feeding levels were linked to decreased digestibility metrics, indicating a trade-off between growth and nutrient absorption. Based on regression analysis, we conclude that a mean daily milk allowance of 8.03 L/d, achieved through step-down weaning from birth to weaning, maximizes first-lactation milk yield. This meta-analysis provides valuable insights for optimizing milk feeding strategies to achieve the best long-term outcomes.