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Abstract

An experiment was conducted to study the effect of crude protein, X₁ (0.06, 0.12, 0.18, 0.24 and 0.30 Kg/100 Kg of liveweight/day), and the proportions of X₁ contributed by urea, X₂ (0.0, 17.5, 35.0, 52.0 and 70.0%), on the performance of steers receiving rice straw as a basal diet. Sixty-five Zebu steers were used in an incomplete 5 x 5 factorial design (13 treatments). Fishmeal was the true protein source. A constant metabolizable energy allowance (5.20 Mcal/100 Kg liveweight/day) and a fixed amount of rice straw (1.10 Kg D.M./100 Kg liveweight/day) were provided. Average daily gains were increased by the use of supplementary crude protein in an exponential manner; however, as the proportion of urea in the diet increased, the response decreased almost linearly. These effects are expressed by the following equation: Y₁= –21.658 + 22.764e ^{– (0.0015/X₁ + 0.0002X₂)}, R²= 0.86, P<.01), where Y₁ represents the average daily gain (Kg/head). The total substitution of the true protein by urea resulted in a 45% lower daily gain. The effect of total crude protein, X₁ Kg/100 Kg liveweight/day) and the percentage of protein substitution by urea, X₂%, on the average daily gain, Y₂ (Kg/head), is expressed by the equation: Y₂= – 67.848 + 69.309e ^{– (0.002/X₃ + 0.0001X₄)}, (R²= 0.83, P<.01). The efficiency of total protein utilization was affected in a curvilinear manner by the level of total crude protein, and in a negative manner by the urea content of the ration. Consumption of metabolizable energy was considered relatively low, and probably, this fact limited the daily gains. From an economical point of view, the best combination X₁ and X₂ was 0.170 and 65%, respectively. This combination resulted in a daily gain of 0.629 Kg/animal and gave a profit of 5.2%.