1. The availability of lysine in nine vegetable-protein concentrates was assessed using the slope-ratio assay for growing pigs and rats. Diets were equalized for crude fibre using solka floc to minimize any possible effects of variation in fibre content on availability estimates.
2. The availability of lysine in the nine proteins for pigs, using food conversion efficiency (FCE) on a carcass basis as the criterion of response were (proportion of total): cottonseed meal 0.39, lupin (Lupinus angustifolius) seed meal no. 10.37, no. 20.65, no. 30.54, no. 40.54, field peas (Pisum sativum) 0.93, peanut (groundnut) meal 0.57, soya-bean meal no. 10.98, no. 2 0.89.
3. Estimates of available lysine for rats as assessed by the slope-ratio assay using FCE on a carcass basis were in close agreement with the pig estimates for cottonseed meal (0.35) and soya-bean meal no. 1(0.91) and no. 2(0.89), higher for lupin-seed meals (range 0.70–0.94 with a mean of 0.81) and peanut meal (0.76) and lower for field peas (0.76).
4. The differences in available lysine were not detected by the chemical Silcock available-lysine test (Roach et al. 1967) or by the direct 1-fluoro-2,4-dinitrobenzene procedure (Carpenter, 1960).
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