Nitrogen loss from protein meals held in terylene bags in the rumen of cattle and the nutritive value of the residues

Abstract

The 'nylon' bag technique was used to measure in the rumen of cattle the loss of nitrogenous compounds ('proteins') from some protein meals available in Australia. There were apparent differences between animals in the proportion of nitrogen (N) remaining from a pelleted meal held in Terylene bags for 15 h in the rumen. When four steers were paired on the basis of minimized standard errors the estimates (¦ s.e.) of N remaining (46 ¦ 1.1 and 56 ¦ 1.3%) were different (P < 0.05). Pelleting reduced (P < 0.01) the rate of N loss from one of two insoluble N fractions, but had no significant effect on total N loss from a mix of meals. Feeding more protein to steers on a low quality roughage increased their intake of roughage from 14 to 30 g dry matter (kg liveweight)-0 92 day-1 (P < 0.05), and increased the rate of N loss from bags from 2.3 to 4 3 % h-1; this was associated with an increase of rumen ammonia from 5.6 to 140.3 mg N l-1 (P < 0.05). Fish-meal proteins were the most resistant of those tested to fermentative breakdown in the rumen. The estimated proportion of undegraded N (UDN) was 62-81% of total N. The vegetable protein meals were least resistant to losses of N in the rumen (UDN 48-5570 of total N), and the meat meals were the most variable group (UDN 38-61% of total N). Amino acid proportions in the residue of cottonseed meals were relatively unchanged from the original sample, but the remaining N tended to be resistant to an acid pepsin digestion, 46-56% of the residue N being insoluble. A pellet (Agrafpel 79) was compounded from several meals and contained a large fraction of UDN, which had an increased ratio of essential to non-essential amino acids compared with the ratios in the UDN of the separate meals. In addition, the UDN of the pelleted meal was readily digested in acid pepsin. A high proportion of total N as UDN, together with a ready digestion in acid pepsin, are considered important attributes of protein meals for improving the protein nutrition of young cattle.