Growth Rate Distribution and Potential Non-Linear Relationship between Body Weight and Walking Ability in Turkeys

SIMPLE SUMMARY: Intense selection has led to a significant increase in body weight of commercial turkeys. These successes have had some impact on the mobility of turkeys. There is a consensus about the negative genetic relationship between growth and mobility. Although it is reasonable to assume that such a relationship is likely to change with the rate of growth, little is known about the potential non-linearity of such association. To investigate the potential non-linear relationship between growth and mobility, the growth rates at three different age periods (0 to 12, 12 to 20, and 0 to 20 weeks) were calculated, and each bird was assigned to one of the quartiles of the growth rate distribution for each age period. Heritability estimates of walking ability ranged between 0.18 and 0.26 indicating the possibility for genetic improvement. The genetic correlations between adjacent growth rate quartiles were high and decayed as the interval between quartiles increased. There is no evidence to support that walking abilities across growth rate quartiles are different traits. However, the magnitude of the variation in the incidence of walking scores and genetic correlations across the different growth quartiles seem to point towards a potential non-linear relationship between growth and mobility. ABSTRACT: The aim of this study was to investigate the potential non-linear relationship between growth and walking ability (WA). The phenotypic data included body weights at 12 and 20 weeks and WA at 20 weeks of age measured on 276,059 male turkeys. The growth rate at three age periods (0 to 12, 12 to 20 and 0 to 20 weeks) was calculated. Each bird was assigned to one of the quartiles of the growth rate distribution for each age period. Between the first and fourth quartiles, the incidence of score 1 (bad WA) increased by 31, 18, and 33% for the first, second, and third age periods, respectively. For good WA (scores 4, 5, and 6), the incidence decreased by 55, 66, and 72% between the first and fourth quartiles for the first, second, and third age periods, respectively. Estimates of heritability of WA ranged between 0.18 and 0.26. The genetic correlations between adjacent growth rate quartiles were high and decayed as the interval between quartiles increased. The magnitude of the variation in the incidence of walking scores and genetic correlations across the growth rate quartiles point towards a non-linear relationship between growth and mobility suggesting other factors may affect walking ability.