Estimating genetic parameters for growth traits is crucial to plan breeding strategies for improving meat production in indigenous sheep breeds. The study first tested the effects of environmental and maternal effects on five growth traits, including birth weight (BWT), weight at 120 days (WT120), weight at 180 days (WT180), weight at 270 days (WT270) and weight at 365 days of age (WT365) and then estimated genetic parameters for these traits using data obtained in 1215 Lohi sheep. The effects of factors, including year (YOB), season (SOB) and type of birth (TOB), age of dam (AOD) and sex on growth traits of Lohi sheep, were examined using analysis of variance (ANOVA) in R software. Sex, TOB and YOB significantly affected all studied traits. The estimates of direct and maternal heritability for BWT and WT120 were 0.15 ± 0.08 and 0.20 ± 0.06, and 0.45 ± 0.16, 0.21 ± 0.08, respectively. The direct heritability estimates for WT180, WT270 and WT365 were 0.20 ± 0.07, 0.21 ± 0.07 and 0.19 ± 0.08, respectively. Due to the high heritability estimate obtained for WT120 compared to other studied traits, and its strong genetic correlation (>0.9) with post-weaning growth traits, it is recommended that selection must be practiced on WT120 to improve the growth performance of Lohi sheep. The results could be used for the development of genetic/genomic selection programs aiming to improve the production performance of the Lohi sheep.

Modelling the growth profiles of sheep not only provides information about how body weight changes with age but also distills these serial measures into biologically important parameters which can be used in genetic evaluation programs. The objective of the present study was to evaluate a series of different functions applied to serial body weight measures of lowland sheep and, in doing so, also explore the relationship between the parameters within and across functions. The evaluated functions were Brody, Gompertz, Logistic, negative exponential, Richards and von Bertalanffy. The data set used consisted of 158,463 body weight records (range of 6 to 38 records per animal) from 13,090 female sheep. The functions were fitted to each animal separately. The coefficient of determination (R2) and root mean square error (RMSE) per animal, along with the ease of model convergence, were used to evaluate model fit. The percentage of animals that achieved convergence per function ranged from 82.39% (Richards) to 100.00% (negative exponential and Logistic). The mean R2 value for the Logistic function was the lowest (0.94), while that for the Richards function was the highest (0.98). A weak negative correlation (r = −0.23 to −0.13) was estimated between the A (asymptotic weight) and B (integrated constant related to initial weight) parameters for all the functions except for Richards. The A and K (maturity rate) parameters of all the functions were negatively correlated and ranged from −0.55 (Brody) to −0.41 (Logistic). The values of the A parameter across all the functions were strongly positively correlated. The estimates for the B parameter of Logistic and Richards functions exhibited a very weak correlation (r = −0.04). A weak to strong correlation between the K parameter values across all functions was observed. Results suggested that all the evaluated functions, except the Richards function, can be applied to model the growth of lowland sheep. The Gompertz and von Bertalanffy functions were considered as the best fitting functions to describe the body weight profiles of Irish female lowland sheep based on their fit to the data, the ease of convergence, and the biological sensibility of the estimated function parameters.