RESPONSE OF GROWING STEERS TO DIETS CONTAINING BIG BALE SILAGE AND SUPPLEMENTS OF MOLASSED SUGAR-BEET PULP WITH AND WITHOUT WHITE FISH-MEAL

Twenty four steers, either Limousin male or Simmental male X (British Hereford male X Friesian female) female genotype were used in a 77 day study. Initial mean live weights were 288 kg and the steers were allo cated by randomised block design to one of four treatments. The treatm ents were big bale silage fed alone ad libitum (C) or supplemented (kg dry matter (DM) day(-1)) with molassed sugar beet pulp (MSBP) 0.66 (S ), 0.66 MSBP + 0.12 white fishmeal (FM) (SF1) or 0.66 MSBP + 0.23 FM ( SF2), The silage (292 g DM kg(-1)) was of moderate fermentation qualit y and contained an estimated 9.7 MJ Metabolisable Energy and 93 g crud e protein kg(-1) DM, Supplementation with MSBP alone or together with FM significantly reduced silage DM intake although there was evidence of a reduction in the magnitude of this effect due to FM supplementati on. MSBP supplementation alone did not increase live-weight gain but s upplementation with FM significantly increased live-weight gain with n o significant differences observed between diets SF1 and SF2. Food con version ratio was non-significantly improved by MSBP alone, with furth er significant improvements due to FM. None of the supplementary treat ments significantly improved apparent digestibility of organic matter or nitrogen (N). N intake was significantly elevated by FM supplementa tion which was associated with trends towards a greater N retention an d significant increases in rumen ammonia and volatile fatty acid (VFA) and blood plasma urea concentrations. Molar proportions of rumen VFA were not altered by treatment. There was a trend towards an improvemen t in rumen microbial protein synthesis estimated by purine derivative excretion only on Diet SF2. This result coupled with the trends toward s increases in silage intake may suggest some improvement in digestive activity. In addition a major anabolic effect of FM supplementation o f the diets may have occurred post-absorptively and may relate to the supply of, for example, essential amino acids. The results indicated t hat moderate growth performance (0.6 kg day(-1) LWG) can be achieved w ith diets based on big bale silage with low crude protein concentratio ns of 95-100 g kg(-1) DM which were improved by supplementation of MSB P with FM. The interaction between big bale silage and MSBP and N sour ces and levels needs further investigation.