Effect of ammoniation and urea addition on chemical composition and digestibility of mature timothy hay, and rumen fluid characteristics of growing steers

Abstract

Feed quality of timothy (Phleum pratense L.) harvested at the beginning of heading (H), with a crude protein (CP) content of 13.3%, was compared to timothy harvested at a late stage of growth either untreated (M) or treated with anhydrous ammonia (30 g kg −1 dry weight) (MA); or with urea added at feeding (MU) making it isonitrogenous with MA. These diets were fed to eight ruminally fistulated steers (220 ± 12 kg) in a 4 × 4 double Latin square. Treatments MA and MU increased the crude protein (CP) content of timothy from 8.3 (M) to 19.8 and 21.8%, respectively. Ammoniation improved the apparent digestibility of dry matter (DM) (68 vs. 62%), CP (66 vs. 47%), hemicellulose (HC) (85 vs. 67%), and cellulose (C) (79 vs. 65%), compared to M. In support, in sacco DM disappearance was about 18% units greater (P < 0.01) for ammoniated hay (MA) as compared with M. Ammoniated timothy hay had apparent digestibility of DM, CP and C (68, 66 and 79%, respectively) similar to that of timothy hay from early harvest (67, 65 and 75%, respectively) and greater HC digestibility (85%) than that of all other hays (77, 67 and 66% for H, M and MU, respectively). Except for CP, digestibility of the other constituents was greater following ammonia treatment than following urea application. Soluble protein was significantly higher in rumen fluid of steers fed MA compared to other treatments. Ammoniation increased energy released from structural carbohydrates, made available following the delignification process, which resulted in improved overall DM digestibility. Ammoniation compensated for deterioration caused by late harvest, resulting in forage constituent digestibility comparable to that of early harvested hay, while additional nitrogen (N) brought about by MU did not improve the overall digestibility of the forage. Ammoniation appears to be an efficient means of increasing N and structural carbohydrate digestibility of low-quality timothy forage. Key words: Ammoniation, urea addition, Phleum pratense L., digestion, rumen fluid