Impact of Flint corn grain processing and urea source on digestion sites, ruminal fermentation, and nitrogen metabolism of finishing nellore heifers

Abstract

This study aimed to evaluate the effects of corn processing method (CPM) and urea source (US) on intake, nutrient digestibility, nitrogen (N) metabolism, ruminal fermentation and kinetics, as well as the expression of genes involved in short-chain fatty acid (SCFA) transport and pH regulation in beef heifers. The trial was conducted using eight rumen-cannulated Nellore heifers (initial body weight ± SD; 295 ± 11.7 kg) in a replicated 4 × 4 Latin square design, with four 26-d experimental periods. Treatments were arranged in a 2 × 2 factorial scheme, combining two CPMs—dry-ground corn (DGC) or rehydrated and ensiled corn (REC)—with two US: Feed-grade urea (U) or coated urea (CU), the latter formulated to partially release N postruminally. Variables assessed included nutrient intake and digestibility, ruminal fermentation parameters, microbial protein synthesis (MCP), N excretion routes, and blood metabolite profiles. Overall, US effects were independent of CPM, except for intestinal digestibility of organic matter (OM), which was greater for the REC+CU treatment (P = 0.04). Diets containing REC yielded greater omasal flows of OM, crude protein (CP) (P < 0.01), and starch (P = 0.03), resulting in greater intestinal digestibility of these nutrients (P < 0.01), greater glucose absorption (P = 0.01), and lower fecal starch excretion (P = 0.02). Dietary inclusion of CU reduced both the peak value and range of ruminal ammonia-N concentration (P = 0.02), as well as plasma urea-N variation (P = 0.05). Moreover, CU diets led to lower maximum plasma glucose concentrations (P < 0.01), with tendencies for lower minimum values (P = 0.10), and narrower glucose concentration range over time (P = 0.09), suggesting a more consistent glucose supply to the intermediary metabolism. A tendency toward improved N utilization efficiency was observed with CU, both as a proportion of N intake (P = 0.10) and N digested (P = 0.09). Microbial protein synthesis efficiency was also greater with CU when expressed per unit of truly digested OM in the rumen (P = 0.07), total OM digestion (P = 0.10), and CP intake (P = 0.03). In summary, REC improved postruminal digestion and glucose availability, favoring microbial synthesis, whereas CU modulated N release, enhancing N status and nitrogen use efficiency.