Evaluation of the variability in chemical composition and digestibility of different lupin (Lupinus angustifolius) kernel meals when fed to rainbow trout (Oncorhynchus mykiss)

The study examined the influence of variability in the chemical composition of lupin (Lupinus angustifolius) kernel meals on their digestibility when fed to rainbow trout. A range of lupin kernel meals were included into separate diets fed to rainbow trout and the feces collected by stripping. Comparison of the composition of feed and feces was undertaken to assess the extent of nutrient and energy digestibility. There was a strong correlation between protein content of a lupin kernel meal and the digestibility of that protein. Notably, the strongest correlation was that between kernel meal protein content and nitrogen digestibility (y=0.001(±0.00007)x−0.473, R2=0.981, P=0.0011). The identification of this relationship allowed the calculation of equations to predict the digestible protein content of lupin kernel meals based on their crude protein content (y=1.202(±0.032)x−149.118, R2=0.998, P=0.00004). The strong correlation between kernel meal protein content and its nitrogen digestibility had a direct effect on the relationship with energy digestibility. Notably, the kernel meal protein content had more influence on its energy digestibility (y=0.002(±0.000314)x−0.274, R2=0.927, P=0.0086) than that of the nitrogen digestibility. This is likely to be related to the relatively large contribution that protein makes to the dietary energy content of the lupin kernel meal relative to that of the carbohydrate content. The demonstrated relationship, between lupin kernel meal protein content and its nutritional value, provides a good support for the promotion of grain segregation by protein content and commodity pricing according to protein content. More specifically, this should focus on the value per unit of digestible protein when used as an ingredient for fish diets.