title: Evaluation of alternative nonlinear mixed effects models of duck growth.
abstract: Two nonlinear growth functions were evaluated on 6 groups of 32 ducks. Ducks were randomly assigned to 1 of 6 dietary treatments and weighed weekly from 1 to 43 d of age. The Weibull function had the form: BW(it) = A - (A - B) exp - [(C - 1)/C] (t/IP)**C, where BW(it) is the BW of the ith duck at age t in days and A, B, C, and IP are fixed parameters. The variable A represents mature BW and the variable IP is the age (inflection point) at which maximum average daily gain is achieved. The addition of a single random effect to the Weibull growth function (ip), in which the age to reach the BW at the overall population inflection point of each duck varies, provided a substantially better fit than any other alternative fixed or mixed models. Overall, the Weibull function under-predicted the d-1 BW (46.7 vs. 55.1 g, P < 0.05) and over-predicted the d-8 BW (233.6 vs. 211.7 g, P < 0.05). The predicted BW from d 15 to 43 was very close to the actual mean BW at each age. This model predicts that the between-duck variance in BW increases with age and that the CV increases from 1 to 8 d of age, reaches a plateau from 15 to 22 d of age, and then slowly declines. This mixed effects model predicts the mean age and approximate variation in age that ducks require to reach a specific BW, and is easily adaptable to stochastic modeling.
authors.affiliation: Department of Animal Sciences, Purdue University, West Lafayette, Indiana 47907, USA. aschinck@purdue.edu
authors.forename: A P
authors.initials: AP
authors.lastname: Schinckel
authors.forename: O
authors.initials: O
authors.lastname: Adeola
authors.forename: M E
authors.initials: ME
authors.lastname: Einstein
mesh_terms: D000375:Aging; D000821:Animal Feed; D000818:Animals; D004032:Diet / Q000662:veterinary*; D004372:Ducks / Q000254:growth & development*; D008954:Models, Biological; D015233:Models, Statistical; D015430:Weight Gain / Q000502:physiology*