361 Late-Breaking: Nitrogen utilization in cattle fed low-protein hay: Effects of protein supplementation, source, and processing.

Abstract Feeding high-fiber forages with low crude protein (CP) content may limit nitrogen (N) utilization in cattle. Protein supplementation is a common strategy to improve N retention and reduce environmental N losses. This study evaluated the effects of supplementing dried distillers grains with solubles (DDGS) or soybean-based supplement (SB; 52.5% soybean meal and 47.5% soyhulls), processed as meal or pelleted, on N intake, excretion, and retention in Jersey steers fed a grass hay-based diet. Five rumen-cannulated Jersey steers (BW: 424±45 kg) were used in a 5 × 5 Latin square design. Each experimental period consisted of 7 days of dietary adaptation followed by 5 days of total fecal and urine collection. Diets consisted of grass hay (8.66% CP) offered ad libitum, with no supplementation (control), or with protein supplements provided either as meal (DDGS: 32.46% CP; SB: 27.68% CP) or pelleted (DDGSP and SBP) at 0.5% of body weight (DM basis). Nitrogen intake, fecal and urinary N excretion, and N retention were determined daily. Data were analyzed using a mixed model in SAS (PROC MIXED), with treatment as a fixed effect and steer and period as random effects. Pre-planned contrasts assessed the effects of supplementation, source, processing, and their interaction. Effects were considered significant at P≤0.05 and trends at 0.05< P≤0.10. Supplementation increased N intake (P < 0.001), with greater values for DDGS than SB (P = 0.001). Fecal N excretion increased with supplementation (P=0.005) and was greater with DDGS (P< 0.001), with a tendency for lower values in pelleted treatments (P=0.071). Urinary N excretion was greater with supplementation (P< 0.001), higher for SB sources (P< 0.001), and also increased with pelleting (P=0.043). Supplementation increased N disappearance (P< 0.001), which was greater with DDGS (P=0.009) and tended to be higher with pelleting (P=0.051). Retained N increased with supplementation (P< 0.001) and with DDGS (P=0.001). Nitrogen digestibility (absorbed N/N intake) improved with supplementation (P< 0.001) and pelleting (P=0.001). Retention relative to disappearance (retained N/absorbed N) was greater with DDGS than SB (P=0.001). Retention relative to intake (retained N/N intake) also increased with supplementation (P=0.016) and DDGS (P< 0.001). No interaction effects were detected. In conclusion, protein supplementation increased nitrogen intake, disappearance, and retention, improving overall N use efficiency in steers fed low-protein hay. Retention relative to both intake and N disappearance was greater with DDGS, indicating improved metabolic utilization compared to SB. Pelleting enhanced N disappearance but did not affect retention. These results suggest that both the type and physical form of the protein supplement can modulate nitrogen dynamics and influence the efficiency of N utilization in cattle fed forage-based diets.