Impact of energy supply of ewes on genetic parameters for fertility and carcass traits in Merino Long Wool sheep

The aim of the present analysis was to estimate genetic co(variance) components between fertility of 12,778 Merino Long Wool ewes defined as litter size (LS) and fattening and carcass traits measured on 2998 male lambs of the same breed on performance station. Fattening and carcass traits included daily weight gain (DG), muscle depth (MD), fat depth (FD), and kidney fat (KF) assessed by a new system for ultrasonic measurements. Litter size was defined as a binary trait (single or multiple births), and threshold–linear models in a Bayesian framework were applied. Results were compared to linear–linear models using REML methodology. The impact of the amount of available energy on fertility (LS) and on the estimates for genetic parameters was investigated through a stratification of the fertility data of ewes in three different subsets, i.e. all matings, matings in August (=energy deficiency), and matings excluding those in August. Genetic correlations between LS and traits recorded on station varied with the stratification of the data, but were similar for threshold and linear models. The genetic correlation between LS and KF from the threshold model was 0.62 in the whole dataset, increased to 0.85 when restricting the data to matings in August, and dropped to a substantially lower value of 0.35 when excluding the data from August. The correlations between LS and FD for the entire data, August data, and the subset excluding August matings were 0.10, −0.18, and 0.48, respectively. Results indicated a change of the physiological mechanisms when energy supply in ewes is restricted, and the importance of body fat reserves for such a situation. Heritabilities from univariate models and applying threshold methodology for LS were 0.05, 0.37, 0.24, 0.16, and 0.15 for LS, DG, MD, FD, and KF, respectively. Nearly identical results were obtained from the REML procedure. Because of relatively similar frequencies of both categories for LS (53.5% multiple births and 46.5% single births), the transformation of variance ratios from the observed scale (REML estimates) to the underlying liability scale (Bayesian estimates) led to nearly identical results. The impact of the permanent environmental component of the ewe and the lamb sire component at the total variance for LS were below 3%. The current breeding program for Merino sheep in the analyzed region of Thuringia is focused on August matings. Hence, it is recommended to use genetic parameters from the August subset when developing a combined breeding goal.