Utilization of low-quality roughages; effects of supplementing with casein treated or untreated with formaldehyde on digesta flows, intake and growth rate of cattle eating wheat straw

Abstract

1. Expt 1. Forty 200 kg heifers were given wheat strawad lib. plus one of five pelleted supplements, each of which supplied 40 g/nitrogen as urea/d, Treatment A did not supply other sources of N, and other treatments supplied daily 40 g digestible N as casein and formaldehyde-treated casein (HCHO-casein) in the following proportions (w/w): 100:0 (B), 70:30 (C), 30:70 (D), 0:100 (E). After 5 weeks (period 1) all supplements were withdrawn from half (four) of the animals in each treatment group (NS) over a period of 8 weeks (period 2). At the end of period 2. N balances were measured in four animals on each of treatments E and NS.2. In period 1 the mean intake of straw by animals on treatments D and E was higher than the mean intake by animals on treatments A, B and C (3·32v. 2·83 k/d respectively,P< 0·01) and live-weight changes also differed significantly (+ 72 and – 126g/d respectively:P< 0·01). Intakes of straw in period 2 were 3·74 and 3·20 kg/d for animals with and without supplements respectively (P< 0·01) and live-weight changes were +110 and – 157 g/d on the respective treatments (P< 0·01). For animals receiving supplements in period 2, intakes of straw did not differ significantly between supplements; live-weight changes were −14 g/d on treatment A and +141 g/d on treatments B, C, D and E (P< 0·01).3. N balances on treatments E and NS were +11·4 and – 3·3 g/d respectively (P< 0·01), although digestibility of organic matter (OM) was similar on the two treatments.4. Expt 2. Four 185 kg steers with rumen and abomasal cannulas were given wheat strawad lib. plus one each of treatments A, B and E in a randomized block sequence. Dry matter (DM) intakes were 3·44, 3·89 and 4·05 kg/d on treatments A, B and E respectively (P< 0·05). N intakes were 29 and 37 g/d higher on treatments Band E respectively than on treatment A, but abomasal flows of N were only 4 and 14 g/d higher on the respective treatments. The latter value indicates that approximately 0·62 of HCHO-casein was degraded in the rumen. Efficiencies of bacterial protein synthesis were 31, 24 and 26 g bacterial N/kgOM apparently digested in the rumen (P> 0·05) on treatments A, B and E respectively.5. It was concluded that efficiency of bacterial protein synthesis in the rumen was not limited by the supply of peptides and amino acids, and that protein supplements do not consistently stimulate intake of low-quality roughages when requirements for rumen degradable N have been met. Higher flows of N to the intestines when HCHO-casein, and to a lesser extent casein, were given were associated with a shift from negative to positive live-weight changes. These live-weight changes were not significantly correlated with DM intakes from which it appears that effects of casein supplements on live weight may have been attributable to effects of absorbed amino acids on efficiency of tissue protein synthesis either directly or through gluconeo genesis.