Phosphorus Availability in Low-Phytate Mutant Varieties of Barley: Digestibility Studies with Channel Catfish and Red Drum

Abstract

About 70% of the phosphorus in plant feedstuffs is typically in the form of organic phytates, but molecular phytogenetic techniques allow the development of variant seed forms that are capable of storing most of it in an inorganic form. In response to concerns over excessive phosphorus discharges from uneaten or unassimilated aquaculture feeds, two independent digestibility trials were conducted to evaluate the suitability of single-gene, low-phytic-acid mutations of barley Hordeum vulgare as feedstuffs for channel catfish Ictalurus punctatus and red drum Sciaenops ocellatus. Five isonitrogenous and isocaloric diets containing five different barley variants plus one reference diet (RD) were independently evaluated with each species. The RD for catfish contained 30% menhaden Brevoortia tyranus fish meal and 30% soybean meal, whereas the red drum RD included 48% lyophilized red drum muscle meal as the sole protein source. Each experimental diet was formulated so that a barley mutant replaced 30% of the RD. The diets were fed for 3 weeks to triplicate groups of juvenile channel catfish weighing 200 ± 10 g/fish (mean ± SD) and red drum weighing 400 ± 17 g/fish. Fecal samples were collected 8–12 h postfeeding. Apparent phosphorus, crude protein, and organic matter digestibility coefficients were determined by using chromic oxide as an inert marker. For catfish, the apparent digestibility coefficients for organic matter, phosphorus, and protein ranged from 28% to 62%, 47% to 98%, and 66% to 75%, respectively, for the different barley varieties. The same coefficients for red drum ranged from 69% to 93%, 54% to 86%, and 71% to 84%, respectively. Barley variant 955 resulted in the lowest fecal phosphorus content for both species. These results indicate that a considerable reduction of phosphorus excretion by fish could be achieved solely by substituting ordinary barley (or other plant ingredients) with low-phytate mutant grains. This information may lead to reductions in the amount of unavailable phosphorus added to commercial fish diets, with concomitant reductions in discharges of waste phosphorus.