There is evidence that in growing lambs, net nitrogen requirements are affected by breed, sex, weight and level of feeding but little is known about the effect of dietary concentration. The purpose of the present experiment was to examine the effect of feeding straw diluted diets on nitrogen retention in Rasa Aragonesa lambs from weaning to slaughter.

Eight male lambs, 40-45 days old and 12.3 ± 0.47 kg live weight, were allocated to one of two pelleted diets containing 6 (C) or 23 (D) percent of barley straw and barley, soya bean meal and fish meal to give a crude protein content of 193 (C) and 186 (D) g/ kg dry matter. Both diets were fed ad libitum and nitrogen balances were carried out at 14, 21 and 29 kg live weight. For diet C at the last weight only the balance of two lambs was recorded.

The results are shown in table 1. Although the digestible organic matter (DOM) content was higher (P ≤ 0.005) for diet C (690 g/kg DM) than for diet D (606 g/kg DM), DOM intake was not significantly affected by dietary dilution. Nitrogen retention was higher at the lowest live weight for both diets and decreased at each live weight with dietary dilution (P ≤ 0.005). No significant period x diet interaction was found.