Letters to the Editors

Ileal digestibility and availability of lysine in protein concentrates ,for pigs The availability of lysine in protein concentrates for pigs and the relationship between ileal digestibility and availability have been investigated in two separate research projects : Dr Jane Leibholz of the University of Sydney, with weaner pigs, and myself at the Wollongbar Agricultural Institute, using grower pigs. The findings have been published in two separate series of papers in the British Journal of Nutrition. The results and conclusions have been diametrically opposed; Dr Leibholz found that ileal digestibility and availability were similar, we found that ileal digestibility overestimated availability in heat-damaged meals. In her latest paper Dr Leibholz (1992) offers explanations for these differences and why her results differ from mine. I would like to offer readers alternative explanations. The major differences in our findings are summarized in Table 1, using soya-bean meal and cottonseed meal as examples of highand low-quality meals. They indicate: (1) both centres found similar values for the ileal digestibility of lysine in cottonseed meal (0.56-0.74) and soya-bean meal (0.88-0.92); (2) Dr Leibholz found the availability of lysine was similar to the ileal digestibility, we found the availability of lysine was similar to the ileal digestibility for soya-bean meal, but only about half the ileal digestibility for cottonseed meal; ( 3 ) Dr Leibholz reported high estimated retentions of absorbed lysine from cottonseed meal (0.94) which suggest that all the absorbed lysine was available, we found much lower retention (0.36), which indicates it wasn't! ; (4) Dr Leibholz also reported high retentions of absorbed methionine from soya-bean meal (0.98), again we found much lower retention (0.45); (5) both centres conducted experiments which they claimed validated their values. The results described were the basis for Dr Leibholz concluding that the ileal digestibility of amino acids reflected availability. We concluded that the ileal digestibility assay overestimated availability in heat-processed meals. Clearly both conclusions can't be right !