EFFECTS OF AMMONIUM NITRATE AND POTASSIUM SULFATE FERTILIZATION ON RATES OF RUMINAL IN SITU DISAPPEARANCE OF TALL FESCUE AND ORCHARDGRASS NITROGEN AND SULFUR

Tall fescue (TF) and orchardgrass (OG) were fertilized with 0, 294 or 882 kg NH 4 NO 3 /ha and 0 or 833 kg K 2 SO 4 /ha to determine the effect on rates of apparent ruminal disappearance of herbage dry matter (DM), nitrogen (N) and sulfur (S) using the in situ nylon bag technique. Losses of water-soluble and filtrable DM, N and S differed significantly due to herbage species, NH 4 NO 3 and K 2 SO 4 fertilization levels. After 24-h ruminal incubation, there was no difference between herbages in residual DM, N or S expressed as a proportion of original nutrient. Increasing level of NH 4 NO 3 fertilization reduced (P < 0.001) 24-h residual DM and N and increased (P < 0.001) residual S. Potassium sulfate fertilization reduced (P < 0.001) residual herbage S after 24-h incubation. Rates of insoluble herbage DM and N disappearance (k d ) did not differ between herbages but increasing level of NH 4 NO 3 fertilization increased (P < 0.001) insoluble DM and N disappearance rates. Rate of insoluble S disappearance was lower (P < 0.05) for TF than OG but unaffected by fertilization. Effective ruminal degradability (ERD) of DM (P < 0.01) and release (ERR) of N (P < 0.001) and S (P < 0.05) were lower for OG than TF. Increasing NH 4 NO 3 fertilization increased (P < 0.001) ERD and ERR of DM and N, respectively. Furthermore, NH 4 NO 3 fertilization reduced (P < 0.001) while K 2 SO 4 application increased (P < 0.001) ERR of herbage S. Simple correlation coefficients between k d of DM, N and S and original herbage total N concentration were r = 0.59 (P < 0.001), r = 0.51 (P < 0.001) and r = 0.36 (P < 0.05), respectively. The relationships between ERR of herbage S and total N or nonprotein S concentration in herbage were r = −0.74 (P < 0.001) and r = 0.70 (P < 0.001), respectively. Key words: Tall fescue, orchardgrass, sulfur, nitrogen, ruminal in situ disappearance