Effects of forage feeding to calves on performance, rumen fermentation, and nutrient digestibility: A meta-analysis

The literature on whether or not to feed forage is marked by inconsistencies, largely due to various factors associated with forage inclusion in calf diets. To elucidate these factors, we conducted a 3-level meta-analysis to comprehensively investigate the overall effects of forage provision in young calves. We searched for studies published between 2000 and 2023 in Google Scholar, ScienceDirect, and X-Mol. Moderator analyses were performed to evaluate the effects of different forage sources, forage provision methods, starter forms, milk levels, and forage levels on the heterogeneity of growth performance, rumen fermentation, and nutrient digestibility outcomes in studies feeding forage to calves. A mixed-effect model was used to predict the relationship between forage level and performance. Funnel plots and Egger's test were used to determine publication bias. A total of 86 treatment comparisons from 36 articles were included in the final dataset. The weighted mean difference was used to evaluate the effect size, and the statistical heterogeneity of the effect size was estimated using Cochrane's Q test. The results showed that forage supplementation improved growth performance, structural growth, and development of rumen fermentation in calves. However, moderator analyses revealed that several factors, including forage source, feeding method, physical form of starter, milk feeding level, and forage feeding level might contribute considerably to variability, resulting in significant heterogeneity in the effects of forage provision in calves. Larger effect sizes were found for oat hay (OH) supplementation rather than alfalfa hay (AH) or straw, and for feeding forage as a free choice (FC) compared with a TMR (defined as a mixture of calf starter and forage), especially when forage was fed as a FC resulting in higher total DMI and body barrel. Forage supplementation was more effective in boosting calf growth in calves provided with ground starter than in calves fed pelleted and textured starter. Greater effects were shown for AH supplementation than OH or straw in improving rumen fermentation parameters. Furthermore, compared with the preweaning period, the effect of providing forage on calves was more noticeable in the postweaning period. The results of the mixed-effects model analysis indicated that calves can be efficiently fed 12% of DM as forage during the preweaning period to support their growth and development. Meanwhile, 9% of DM might be the optimal level for feeding OH and straw to preweaning calves. More studies are essential to explore how different levels of AH dietary supplementation affects calves during the preweaning period and improve the consistency and accuracy of the dose-response curve predictions. Overall, growth performance and rumen fermentation of dairy calves were affected by forage inclusion. Moreover, forage source, feeding method, physical form of starter, milk feeding level, and forage level are essential factors that result in different degrees of effect on the calf's performance and rumen fermentation.