Response of growing steers to diets containing big bale silage and supplements of molassed sugar beet pulp with and without white fish meal

Abstract

Twenty four steers, either Limousin ♂ or Simmental ♂ × (British Hereford ♂ × Friesian ♀) ♀ genotype were used in a 77 day study. Initial mean live weights were 288 kg and the steers were allocated by randomised block design to one of four treatments. The treatments 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 quality and contained an estimated 9.7 MJ Metabolisable Energy and 93 g crude 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 supplementation. MSBP supplementation alone did not increase live-weight gain but supplementation with FM significantly increased live-weight gain with no significant differences observed between diets SF1 and SF2. Food conversion ratio was non-significantly improved by MSBP alone, with further significant improvements due to FM. None of the supplementary treatments significantly improved apparent digestibility of organic matter or nitrogen (N). N intake was significantly elevated by FM supplementation which was associated with trends towards a greater N retention and 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 improvement in rumen microbial protein synthesis estimated by purine derivative excretion only on Diet SF2. This result coupled with the trends towards increases in silage intake may suggest some improvement in digestive activity. In addition a major anabolic effect of FM supplementation of the diets may have occurred post-absorptively and may relate to the supply of, for example, essential amino acids. The results indicated that moderate growth performance (0.6 kg day −1 LWG) can be achieved with diets based on big bale silage with low crude protein concentrations of 95–100 g kg −1 DM which were improved by supplementation of MSBP with FM. The interaction between big bale silage and MSBP and N sources and levels needs further investigation.