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In egg-type pullets, an appropriate BW and body composition at the end of rearing are required for optimal production results. The industry direction toward extended commercial laying cycles demands accurate evaluation (allometric) of growth in laying hens. Mathematical models have been used to describe and evaluate growth, where biologically relevant parameters could be related to performance, such as the rate of gain or mature weight. Most growth models published in the literature have been based on meat-type poultry. The aim of the present study was to compare Gompertz and logistic models describing the BW and gain in individually fed free-run laying hens. The models were evaluated based on their ability to identify multiple growth phases. The addition of random terms was evaluated by introducing random terms within the preferred multiphasic model.
Lohmann Brown Lite laying hen chicks (n = 15) were fed with a precision feeding (PF) system, which allocated feed and measured feed intake on an individual basis. Photoschedule was set at 12L:12D during the entire experiment. For the first 3 wk, chicks received a standard wheat-based starter diet (2,726 AME, 21% CP, 1.0% Ca); from week 4 to week 23 pullets received a wheat-based grower diet (2,703 AME, 16.0% CP, and 1.1% Ca); from week 23 to week 43 hens received a wheat-based layer diet (2,689 AME, 15.0% CP, and 3.3% Ca). For the first 3 wk, pullets were weighed manually on a daily basis. After individual feeding started, the PF system recorded individual BW and feed intake on a per-visit basis, multiple times per day. Gain was calculated per week by subtracting the BW of each bird at the first day of each week from the BW at the last day of each week. Daily BW data were used to fit the weight-age relationship models. The 2-weekly moving average of weekly gain was used to fit the gain-age relationship models. Two nonlinear models were evaluated to describe the BW as a function of the age. The models evaluated to describe gain as a function of age were the derivatives of the previously described Gompertz and logistic functions and also included one, 2, or 3 phases.
The average BW was 1,471.4 ± 17.54 g at week 16 and 2,058.7 ± 22.27 g at week 43. The CV for the BW was 5.4 ± 1.45% at week 16 and 4.9 ± 1.45% at week 43. The cumulative feed intake was 5,630 ± 67.2 g from day 24 to week 16 and 17,950 ± 287.1 g from week 17 to week 43. Convergence was achieved for all models describing the weight-age relationship. Based on the R-squared and the RMSE, all models showed a similar goodness of fit to the BW data. In monophasic models the Gompertz curve showed a better fit than the logistic function. This is likely the result of the difference in the inflection points between the Gompertz and the logistic functions, that is, the moment of the highest rate of gain. The inflection point of the Gompertz function is at 37% of its asymptote, whereas the inflection point of the logistic function is 50% of its asymptote. In practical terms, the Gompertz model being a right-skewed distribution predicts slower growth in the later stages of each growth phase. In the present study, the mature BW was reduced by only 0.04 kg for the monophasic model. Convergence was achieved for all models describing the gain-age relationship. Based on the R-squared and the RMSE, the diphasic and triphasic models showed a better fit than the monophasic models. The F-test established that triphasic models fitted the data better than the diphasic models for both the Gompertz and logistic functions. The results indicate that modeling the gain-age relationship is more robust, as the biological growth phases identified by the Gompertz and logistic models are similar when modeling the gain-age relationship.
The present study identified that the multiphasic mixed Gompertz growth model fitted data of ad libitum–fed Lohmann Brown Lite hens best. The multiphasic mixed Gompertz model identified similar growth phases by using a weight-age or gain-age function and the phases aligned with developmental biology of growth. In addition, including the random component to the inflection point of the diphasic mixed Gompertz model allowed for future study of the relationship between the individual shape of the BW function and egg production parameters.