Replacement of Soya Bean Meal and Corn by Field Peas in Young Bulls Fattening Diets: Performance, Rumen Fermentation, Nitrogen Use and Metabolism

Abstract

This study explored the interest in field peas partially replacing soya bean meal and corn in beef fattening diets by assessing impacts on animal performance, ruminal fermentation, nitrogen (N) use and economic output. Thirty-two Parda de Montaña young bulls (210±24.3 kg BW) were randomly assigned to one of four treatments (0%, 15%, 30%, 45% pea in isonitrogenous and isoenergetic concentrates. After 23 d adaptation, fattening was divided into Growing (first 134 d) and Finishing (from d 135 to 500 kg target slaughter BW). Gains were higher (P<0.001) and the DM intake and feed conversion ratio were lower (P<0.001) during Growing vs. Finishing. The proportion of field peas influenced DM intake (P<0.05) but did not affect days on feed, daily gains, the feed conversion ratio or carcass traits. Ruminal NH3-N concentrations were lower and total volatile fatty acids (VFA) were higher during Growing vs. Finishing (P<0.001). Ruminal fluid pH was higher and NH3-N concentration, total VFA and propionic acid were lower in treatment 0% pea (P<0.01), likely because of lower dietary protein and starch degradability. The intake of N did not differ among diets. Faecal N excretion was the lowest and urinary N excretion the highest in treatment 30% pea (P<0.05) during both periods, which was associated with higher ruminal NH3-N and plasmatic urea concentrations. The economic performance of treatment 30% pea was the best in four scenarios considering different relative feed ingredient prices. These results support the economic interest in including up to 30% field peas in beef fattening diets but, given the different N partition patterns towards faeces and urine, these alternatives’ environmental interest should be assessed at a territorial scale.