Apparent digestibility coefficients and nutritional value of cottonseed meal for rainbow trout ( Oncorhynchus mykiss)

Abstract

Cottonseed meals (CSM) from four different locations in the southern US were evaluated to measure apparent digestibility coefficients (ADC) of nutrients using yttrium as an inert marker. One type of CSM was used at 0% (diet 0), 5% (diet 5), 10% (diet 10), 15% (diet 15), and 20% (diet 20) inclusion rates in experimental diets (44% crude protein, 22% crude fat) to evaluate its nutritional value. A total of 450 rainbow trout (initial mean body weight 11.2±0.4 g) were randomly stocked into fifteen 150-l fiberglass tanks with 30 fish per tank and 3 tanks per diet. Fish were fed to apparent satiation 3 times a day and 7 days per week. The ADC of the four CSM groups were: dry matter, 50.8–75.5%; fat, 60.2–78.6%; crude protein, 81.6–87.9%; potassium, 98.9–99.6%; magnesium, 45.1–60.5%; sodium, 89.1–92.7%; phosphorus, 53.3–56.2%; copper, 53.3–65.6%; iron, 34.0–60.4%; manganese, 15.1–31.6%; zinc, 6.1–33.4%; essential amino acids, 80.2–96.1%; and non-essential amino acids, 62.5–94.3%. After a 6-week growth period, average weight gain of fish fed diets 0 to 20 was: 42.4, 39.1, 39.4, 34.2, and 31.8 g, respectively. Average feed conversion ratio of fish fed diets 0 to 20 was: 0.94, 0.99, 1.01, 1.12, and 1.13, respectively. Survival was >98% for fish fed all diets. Results showed that the ADC of most nutrients in CSM obtained from different locations were different. Fish fed CSM diets at the 5% or 10% inclusion rates were not significantly different compared to fish fed the fish meal control diet in terms of weight gain, feed conversion ratio, and survival ( P>0.05), indicating that CSM could be used at the 10% inclusion rate in rainbow trout feed formulations.