The Impacts of Palm Kernel Cake on Nitrogen Dynamics in Confined Ruminants: A Systematic Review and Meta-Analysis

Nitrogen (N) utilization by ruminants affects production efficiency, feeding costs, and environmental N losses in confined production systems. Palm kernel cake (PKC), an abundant agro-industrial by-product in tropical regions, has been increasingly used in ruminant diets, although its effects on nitrogen dynamics remain inconsistent. In this study, we systematically reviewed and meta-analyzed the effects of dietary PKC inclusion on N intake, excretion, absorption, and retention in confined cattle, goats, and sheep. Eleven studies published between 1995 and 2025, comprising 44 treatment means and 322 experimental units, were included in the meta-analysis. A random-effects model was applied, and the ruminant species was used as a moderator, defining a significant level at 0.05. Overall, the pooled effects indicated that species significantly influenced N intake (p < 0.01) and N absorption (p < 0.01). Species also showed a tendency to influence N in feces (p = 0.062) and manure N (p = 0.073), whereas N in urine (p = 0.194) and N retention (p = 0.170) were not affected. In subgroup analysis, PKC inclusion reduced N intake in goats (Standardized Mean Difference (SMD)) = −0.792; 95% CI (Confidence Interval) = −1.428 to −0.155; I2 (Heterogeneity) = 76.7%) and cattle (SMD = −1.576; 95% CI = −2.250 to −0.902; I2 = 65.7%), N in urine in cattle (SMD = −0.478; 95% CI = −0.806 to −0.150; I2 = 0%), N absorption (SMD = −0.873; 95% CI = −1.517 to −0.229; I2 = 77.1%), and N retention (SMD = −0.875; 95% CI = −1.338 to −0.412; I2 = 64.1%) in goats. Conversely, PKC had a positive effect on N absorption in sheep (SMD = 1.137; 95% CI = 0.016 to 2.258; I2 = 72.4%). Overall, this study highlights the species-dependent responses of N dynamics to PKC inclusion, emphasizing the importance of species-specific dietary strategies when using agro-industrial by-products to improve nitrogen utilization efficiency and potentially mitigate N losses in confined ruminant systems.