Net protein requirements and metabolizable protein use for growing ram lambs fed diets differing in concentrate level and roughage source

Two trials were conducted to determine net protein requirements and the use of metabolizable protein (MP) for growing ram lambs when fed diets with different concentrate levels and roughage quality. In a comparative slaughter trial, eighty-four ½ Dorper × ½ Santa Inês ram lambs (18.0 ± 3.3 kg of BW) were individually penned and divided into 2 diet groups differing in roughage source: low-quality (sugarcane bagasse) roughage (LQR) or medium-quality (coastcross hay) roughage (MQR). In each group, 7 lambs were randomly selected and slaughtered after a 10 days adaptation period (baseline). Other 21 lambs from each diet group were fed ad libitum and slaughtered at 25, 35, or 45 kg of BW, and the remaining 28 lambs (14 from each diet group) were submitted to 1 of 2 levels of feed restriction (70 or 50% of the ad libitum intake). Body N content and retention were determined. In the second trial, 6 ram lambs (44.3 ± 5.6 kg of BW) were kept in metabolic cages and used in a 6 × 6 Latin square experiment designed to determine N digestibility and microbial N synthesis, and to compute MP supply by the 2 diets and 3 levels of intake. Nitrogen intake and microbial N synthesis were greater for the animals fed the MQR diet than for those fed the LQR diet (P < 0.05). Neither the endogenous and metabolic N losses, nor the net protein requirements for growth differed between diets (P > 0.05). However, the proportion of MP used for both maintenance and growth was lower for the animals fed the MQR diet (P < 0.05), likely because of an excess of MP reaching duodenum of these animals. We concluded that a reduction of the concentrate level in diets for growing ram lambs, achieved by improving the quality of the diet roughage, allows a greater supply of MP and may contribute to reduce diet protein concentration.