Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/24/2012
Publication Date: 2/1/2013
Publication URL: http://handle.nal.usda.gov/10113/56442
Citation: Jacobs, B.M., Patience, J.F., Lindemann, M.D., Stalder, K.J., Kerr, B.J. 2013. The use of a covariate reduces experimental error in nutrient digestion studies in growing pigs. Journal of Animal Science. 91(2):804-810. Interpretive Summary: Variation is inevitable in animal research with experiments evaluating body composition, feed intake, growth, amino acids, and meat quality often utilizing covariates in data analysis. In general, nutrition studies do not utilize covariates, but instead utilize blocking relative to initial body weight or previous treatment. Use of covariance to control experimental error may also minimize the number of experimental units required to obtain statistically valid results as questioned by Institutional Animal Care and Use Committees. Based on data obtained in this experiment, if initial diets are known, subsequent treatments should be balance for the initial diet because of potential of initial diet by final diet interactions. If the initial diets are not known, then the initial digestibility coefficient would be effective in reducing the variation associated with subsequently obtained data, and should be considered as a covariate in future grow-finish swine digestibility research. Research results described in this report provides nutritionists at universities, feed companies, allied industries, and swine production facilities data on how to minimize experimental variation to improve the ability to detect treatment differences and to minimize the number of animals needed to obtain significant treatment differences.
Technical Abstract: Covariance analysis limits error, the degree of nuisance variation, and overparameterizing factors to accurately measure treatment effects. Data dealing with growth, carcass composition, and genetics often utilize covariates in data analysis. In contrast, nutritional studies typically do not. The objectives of this study were to: 1) determine the effect of feeding diets containing dehulled degermed corn, corn-soybean meal, or distillers dried grains with solubles on nutrient digestibility coefficients, 2) evaluate potential interactive effects between initial and final treatment diets on the final treatment diet effects, and 3) determine if initial criterion (digestibility or physiological values) would effectively correct for variation among pigs that could thereby affect final treatment diet digestibility coefficients. Seventy two crossbred barrows [(Yorkshire × Landrace × Duroc) × Chester White] were randomly assigned to one of three dietary treatments for Phase-1 (P1; 14 d). On d 14, pigs were randomly reassigned to one of the three diets within initial dietary treatment for Phase-2 (P2; 14 d). Fecal and blood samples were collected on d 14 of P1 and d 14 of P2. Fecal samples were dried and analyzed for C, ether extract, GE, N, NDF, P, and S. Plasma samples were analyzed for plasma urea nitrogen and triacylglycerides. Pigs fed diets differed widely in CP, NDF, and P, resulting in an overall decrease in C, GE, NDF, N, P, and S digestibility, and plasma urea nitrogen triacylglycerides as dietary fiber increased in P1 and P2 (P < 0.10). There were no differences in P2 criteria due to blocking for the P1 diet. There tended (P = 0.10 to 0.20) to be a significant P1×P2 interactions for NDF and S, indicating that the response of pigs to the P2 diet may depend upon the P1 diet. In contrast, when the P1 parameter was used as a covariate for P2 data, it was significant for GE, NDF, N, S, and plasma urea nitrogen (P < 0.10), while C and ether extract showed tendencies, but not for P digestibility or plasma triacylglycerides. In conclusion, if initial diets are known, subsequent treatments should be balance for the initial diet because of potential of initial diet by final diet interactions. If the initial diets are not known, then the initial digestibility coefficient would be effective in reducing the variation associated with subsequently obtained data, and should be considered as a covariate in future grow-finish swine digestibility research.