Effect of dietary citrus pulp on performance and dry matter digestibility in dairy cows: A meta-analysis and meta-regression

Citrus by-products have been used as low-cost nutritional ingredient in ruminant diets to support growth and lactation. The current study aimed to evaluate the effect of dietary inclusion of citrus pulp on dairy cow performance through a meta-analysis approach. A systematic search was based on the PICO question (Participant-Intervention-Comparator-Outcomes): Compared to the control group, what is the effect of citrus pulp feeding on dairy cow performance? The bibliographic search was conducted by four experts using the scientific databases: PubMed, Google Scholar, Scopus and PRIMO-ExLibris. The outcome variables considered were dry matter intake (DMI), DM apparent total tract digestibility (ATTD), milk yield (MY), fat, protein, lactose content and yield, and ruminal propionic, butyric and acetic acids proportion. The effect size was measured through the raw and standardized mean difference (RMD and SMD respectively). The meta-analysis was conducted using the "Meta" package in R. The covariables included in the meta-regression were: genetic group, inclusion level of citrus pulp in the diet, dietary content and difference of CP, NDF and starch between citrus pulp and control diets, and milk production level. Dietary inclusion of citrus pulp in dairy cow rations had negative effect on DMI (-0.62 kg/d; P = 0.001), MY (-0.71 kg; P < 0.01), and protein (-0.08 g/100 g; P < 0.01) and lactose (-0.09 g/100 g; P < 0.01) concentrations with greater impact in high-producing cows, whereas DM ATTD was not affected by citrus pulp dietary inclusion. The volatile fatty acid proportion (mol/100 mol tVFA) increased for acetic (+0.64; P < 0.01) and butyric acids (+1.09; P < 0.01) but decreased for propionic acid (-1.18; P = 0.0001) in response to citrus pulp inclusion. Regarding meta-regression, inclusion level of citrus pulp was negatively associated with MY and DMI. The inclusion of citrus pulp in the diet of dairy cows has adverse effects on MY and some milk constituents such as lactose and protein, which are probably mediated by a decrease in DMI, dietary starch content and changes in the ruminal proportions of propionic and butyric VFAs. Finally, the decrease in DMI and milk yield is more marked in high yielding cows.