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Animal growth models are essential for optimization of feed formulation and breeding programs in agricultural production. They are needed to understand when and how quickly animals gain weight, to be able to optimize this weight gain for desired production purposes. However, a limited number of studies are available investigating growth models in turkeys. Multiphasic models allow for multiple sets of coefficients and multiple inflection points, therefore are able to describe several overlapping growth spurts. Thus far, only two studies have been found investigating multiphasic growth in turkeys. Hence, the aim of the current study was to 1) compare sex and line differences in turkeys in the parameter estimates of mono- and di-phasic Gompertz growth models, and 2) evaluate mono and diphasic mixed Gompertz growth models to determine the variation in parameter estimates in a group of female line turkey toms. The models were assessed based on parameter estimates and their ability to identify multiple growth phases. These models can evaluate differences in growth curve shape between genetic lines and sexes and demonstrate variation in growth curve parameter estimates within genetic lines and sex in turkeys.
Two datasets were provided by Hendrix Genetics. The first dataset included male and female BW data from a male line and a female line of Large White turkeys. Data included 913 weekly observations from hatch to 20 wk of age. The data also consisted of 68 observations on hens between 28 and 32 wk of age and 75 observations on toms around 24 wk of age. The second dataset included individual BW data from 68 turkey toms of a different genetic female line from 5 to 24 wk of age. Birds were wing banded for individual identification with a radio frequency identification (RFID) wing band. All birds were ad libitum fed a standard North American commercial turkey diet and water was provided ad libitum during the entire growth period. A modified Gompertz growth model was used to describe BW as a function of age. Genetic line and sex differences in model fit were evaluated based on the monophasic version of the model to allow for comparison with previous literature. Monophasic and diphasic versions were evaluated within genetic line and sex for goodness of fit. Based on the analysis of the fit of monophasic and diphasic models within the dataset with individually collected BW data, three nonlinear mixed models were defined.
Growth curve shapes were different between male line toms, male line hens, female line toms, and female line hens (P < 0.001). However, inflection points were similar between male and female line toms and between male and female line hens (14.06 vs. 13.72 wk and 11.22 and 10.71 wk, respectively), while mature BW differed between lines by 6.49 and 3.81 kg for toms and hens, respectively. The normalized growth rate constant (growth rate constant corrected for mature weight) was around the same magnitude between male and female line toms (0.0031 vs. 0.0038, respectively), but slightly lower in male line hens compared to female line hens (0.0072 vs. 0.0091, respectively). Diphasic Gompertz models described growth better in all line × sex combinations compared to the monophasic models (P < 0.001) and mixed diphasic Gompertz models showed improved fit over mixed monophasic Gompertz models. The correlation structure of the random components identified that individuals with a higher mature weight had a later inflection point and lower growth rate coefficients.
The current study identifies differences in growth curves in modern male and female line turkey toms and hens. ¬Diphasic Gompertz models described male line toms, female line toms, and female line hens better compared to monophasic functions. Mixed multiphasic Gompertz models showed variation between individuals in estimates of mature BW and inflection point of each phase. The models described here could form a basis for new trait development based on individual BW curves for improved breeding practices and provide tools for studies evaluating the effect of dietary or environmental factors on growth trajectories in turkeys.