Nitrogen Metabolism in Bos indicus and Bos taurus Cattle Consuming Low-quality Forages

Five Angus (Bt) and 5 Brahman steers (Bi) fitted with ruminal and duodenal cannulas were used in concurrent 5 ? 5 Latin squares to determine the effects of supplemental protein degradability and level of supplemental N on utilization of rice straw. Treatments consisted of a control (CON; no supplement) and two levels (50 and 100 mg N/kg BW) of an isonitrogenous supplement (27% CP), either high (H; 72%) or low (L; 28%) in DIP.

Forage OM intake (FOMI) was greater for Bt than Bi (P = 0.05). Supplementation increased FOMI in both Bt and Bi (P < 0.05). Organic matter digestibility (OMD) was greater in Bi than Bt (P < 0.01). Supplementation increased OMD for Bi (P = 0.02) but not Bt. Total digestible OM intake (TDOMI) was similar between subspecies (P = 0.12). Bos indicus had greater ruminal NH3-N than Bt (P < 0.01). Plasma urea nitrogen (PUN) was greater for Bi than Bt (P < 0.01) for all treatments and at both 0 and 4 h after feeding. Supplementation tended (P = 0.06) to increase PUN versus CON in Bt, but not Bi (P = 0.82). Bos taurus had numerically (P = 0.19) greater total volatile fatty acids (VFA) across treatments than Bi. Total N intake increased versus CON (P < 0.01), and greater amounts of supplemental N increased total N intake within both subspecies (P < 0.01). Fecal N excretion was greater in Bt than Bi (P = 0.01). Supplementation increased (P < 0.01) fecal N versus CON for both subspecies. Urinary N tended to be higher for Bi than Bt (P = 0.10). Supplementation increased (P < 0.05) urinary N for both subspecies. Retained N was greater for Bt over Bi (P = 0.07).

While Bt had greater FOMI, increased OMD for Bi with supplementation resulted in similar TDOMI between subspecies. Overall, Bi had higher NH3-N and PUN than Bt; which increased as level and degradability of supplements increased in both subspecies. Increased supplementation raised N excreted and N balance for both subspecies; fecal N was greater in Bt, while urinary N was greater in Bi.