Effects of nitrogen and sulfur on digestion and nutritive quality of warm-season grass hays for cattle and sheep.

The influence of N and S on the usage of warm-season grasses was examined in two metabolism trials with cattle and sheep. Effects of N fertilization (75 kg N from urea/ha) on digestibility, intake, and ruminal mineral solubilization of switchgrass (Panicum virgatum L.; SWG) and big bluestem (Andropogon gerardii Vitm.; BB) hays were determined in a 4 x 4 Latin square experiment with mature steers. Effects of N and S applied as urea and sodium sulfate in spray form to SWG hay were estimated in a 2 x 3 factorial experiment using sheep. Dry matter and NDF digestibility was greater (P less than .03) for BB than for SWG, and intake of SWG was 10% greater (P less than .06) than that of BB. Fertilizer N increased DMI (P less than .02) of SWG and BB by cattle by 11.4 and 16.1%, respectively. Fertilization decreased (P less than .04) ruminal turnover times by 9.3 and 18.5% for SWG and BB, respectively. In situ DM degradation rates were faster (P less than .02) for fertilized than for unfertilized forages and were faster (P less than .06) for BB than for SWG. In the sheep trials, levels of CP in SWG diets were 7.2 and 9.5%: levels of S were .12, .20, and .29%, respectively. Urea supplementation increased (P less than .01) hay intake by 9.4% and also increased (P less than .01) digestibility of DM and NDF. Supplemental S had no effect (P greater than .05) on any measurement. There was no effect (P greater than .05) of supplemental N on ruminal retention times, rates of passage, or apparent retention of N and S. The provision of extra N by fertilization or dietary supplementation improved the nutritional quality of the low-protein, warm-season grass hays studied in this experiment, whereas no response to dietary S was detected.