Effects of dietary incorporation of a co-extruded plant protein (rapeseed and peas) on growth, nutrient utilization and muscle fatty acid composition of rainbow trout ( Oncorhynchus mykiss)

Four experimental diets (diets CP 0, CP 5, CP 10 and CP 15) containing 42% crude protein were formulated to contain graded levels of colzapro, a co-extruded product of rapeseed and peas, by replacement of protein from fishmeal. Another diet (CP E) with a lower protein (34%) and higher fat content was formulated to contain a high level (45%) of colzapro. A 12-week growth trial was conducted with rainbow trout (initial body weight 40 g; 18°C) to evaluate the potential use of this plant protein in trout diets. Growth performance, digestibility of nutrients, and ammonia nitrogen excretion were measured. The effects of dietary colzapro on the fatty acid composition of trout muscle were studied. Compared to the control group, fish fed diets in which 5, 10 and 15% of protein was replaced by protein of colzapro showed equivalent or slightly betten growth performance. Increasing the dietary colzapro level to 45% (diet CP E) led to a significantly lower final weight, specific growth rate and an increase in feed to gain ratio when compared with the other groups. No significant differences were observed in the digestibility of protein and fat, regardless of dietary treatment, while those of dry matter and energy were affected by colzapro incorporation. Non-faecal nitrogen loss showed a decreasing trend with increasing levels of dietary colzapro. In the muscle, n−6 fatty acids increased with increasing levels of dietary colzapro, mainly due to a rise in linoleic acid levels, while the levels of 20: 4 n−6 did not vary much between treatments. Although the levels of 22: 6 n−3 fatty acids remained stable, a decreasing trend for the total percentage of n−3 fatty acids was observed, thus affecting the n−3 n−6 ratios. Histological observations did not reveal any significant difference in thyroid cells between dietary treatments. The results obtained showed that colzapro can be utilized in rainbow trout diets at levels of up to 20% without negative effects on growth, nitrogen or energy utilization and muscle fatty acid composition.