A meta-analysis of the effects of supplemental dietary fat on protein and fibre digestibility in the horse

Any associative effects between supplemental dietary fat and nutrient digestibility would have an influence in the calculation of digestible energy and nutrient values of horse diets. However, these effects are inconsistently reported in the literature. The use of meta-analysis, which accounts for variation among studies, could quantify differences in nutrient digestibility caused by fat supplementation. Too few studies have presented data on intake to evaluate the relation between dietary fat intake and nutrient digestibility through a modelling approach. Consequently, unitless standardised effect sizes (Hedges' g) were calculated within studies for differences in digestibility between supplemented and non-supplemented diets. Twenty-two studies were identified that provided 43 comparisons for apparent total tract digestibility of crude protein (CPD), 44 for neutral detergent fibre digestibility (NDFD), and 30 for acid detergent fibre digestibility (ADFD). The calculation of mean effect sizes across studies by fixed effects models presented significant (P < 0.001) heterogeneity with all outcomes. Given that the causes of heterogeneity could not effectively be identified by meta-regression and subgroup analyses, random effects models, which include between-study and within-study variation, were used to summarise effect sizes. Compared to no supplementation, added fat had no effect (mean effect size = 0.204; 95% confidence intervals = − 0.101 to 0.509; P = 0.191) on CPD. Dietary fat supplementation did not affect NDFD (mean effect size = − 0.126; 95% confidence intervals = − 0.429 to 0.176; P = 0.413); however, it caused a significant (P = 0.008) decrease in ADFD (mean effect size = − 0.430; 95% confidence intervals = − 0.749 to − 0.111). With the limited value of acid detergent fibre in practical feed formulation, current results indicate that the calculations of digestible energy values of fat supplemented horse diets will not be influenced by associative fibre effects.