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 replacing soya bean meal and corn at different rates in beef fattening diets by assessing impacts on animal performance, ruminal fermentation, nitrogen 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 % peas in isonitrogenous and isoenergetic diets). After 23 d adaptation, fattening was divided into Growing (first 134 d) and Finishing (from d 135–500 kg - target slaughter BW). Daily weight gains were higher (P < 0.001) and the DM intake and feed conversion ratio were lower (P < 0.001) during Growing vs. Finishing. Ruminal NH₃-N concentrations were lower and total VFA were higher during Growing vs. Finishing (P < 0.001). Increasing proportion of field peas did not affect daily gains or carcass traits, tended to have a cubic effect on DM intake (P = 0.06) and a quadratic effect on days on feed (P = 0.09), but did not affect the feed conversion ratio. Ruminal fluid pH decreased and total VFA increased linearly with increased pea inclusion, whereas NH₃-N concentration (P = 0.06) and the proportions of propionic (P = 0.06) and butyric acids (P = 0.06) tended to display quadratic patterns, but acetic acid was not affected (P = 0.18). N intake did not differ among diets, but faecal N excretion decreased linearly (P = 0.002) and urine N excretion increased linearly (P = 0.02) with increasing proportion of pea. The gross margin obtained with 30 % peas was the best in four scenarios considering different relative feed ingredient prices. These results support the interest in including up to 30 % field peas in beef fattening diets but given the shift in the route of N excretion from faeces to urine, irrespectively of the period, these alternatives’ environmental interest should be assessed at a territorial scale.