Effect of concentrate extrusion and castration on diet digestion and performance of intensively reared male calves

Fifty-six male Friesian calves, half of them castrated, were used to study the effects of concentrate extrusion and animal castration on diet digestibility and performance in two different growth phases (from 91 to 250 and from 250 to 400 kg live weight). Diets consisted of a compound meal, extruded or nonextruded, and barley straw, both of them fed ad libitum. Half of the animals were slaughtered at 250 kg and the remaining at 400 kg live weight. Extrusion increased the gelatinization grade, resulting in a higher (p<0.01) fermentation rate, and reduced nitrogen solubility and degradation after 12 h of rumen incubation (p<0.05). Calves receiving the extruded meal showed a lower rumen ammonia concentration just before the first meal (p<0.001), the differences disappearing afterwards. Neither the pH nor the total VFA concentrations were affected, although butyrate, isobutyrate and isovalerate proportions before the first meal decreased with the extruded diet. Concentrate extrusion did not affect the apparent OM digestibility of total diet but caused a reduction of NDF (p<0.01) and CF (p<0.05) digestibility. In the first growth phase, concentrate and total DM intake decreased with extrusion (p<0.001), without affecting the average daily gain (ADG). Consequently, the concentrate and total feed conversion ratios were significantly lower (p<0.001 and p<0.01, respectively) with the extruded meal. Bulls showed a higher ADG than steers (p<0.001), resulting in lower feed conversion ratios (p<0.01) given that intake was not affected by castration. In the second growth phase, extrusion did not affect any of the performance parameters, however bulls showed a higher DM intake and ADG (p<0.05), but no differences in the feed conversion ratio were found between entire and castrated animals.