Data of 5,390 Merino lambs born from 1986 to 2007 (~6.9 generations) were used to derive genetic parameters and trends for age-specific and overall lamb survival on the underlying scale, as well as for lamb birth weight, using Gibbs sampling. The majority of lambs were descended from lines that were divergently selected for the ability of ewes to rear multiples. The line selected in the upward direction was denoted as the high line (H line), whereas the line selected in the downward direction was the low line (L line). Analyses included the covariance between direct and maternal genetic effects, except where it was not estimable owing to small direct additive variance components, a high incidence of lambs surviving, or both. Direct heritability estimates were 0.02 for lamb survival at birth, 0.12 for lamb survival from birth to tail docking, 0.39 for lamb survival from docking to weaning, 0.28 for overall lamb survival, and 0.17 for birth weight. Corresponding estimates for the maternal genetic effect were 0.26, 0.14, 0.16, 0.14, and 0.29. Dam permanent environmental variance ratios were, respectively, 0.14, 0.09, 0.05, 0.07, and 0.07. Estimates of the direct-maternal genetic correlation were –0.60 for lamb survival from docking to weaning, –0.61 for overall lamb survival, and –0.15 to –0.23 for birth weight. Genetic, maternal genetic, and dam permanent environmental correlations between lamb birth weight and overall or age-specific lamb survival were not different from zero. Expressed relative to overall means, annual direct genetic change in the H line amounted to –0.01% per annum for lamb survival at birth, 0.52% per annum for lamb survival from birth to docking, and 1.3% per annum for lamb survival from docking to weaning. Corresponding trends in the L line were, respectively, –0.01, –0.42, and 0.042% per annum. Maternal genetic trends amounted to, respectively, 0.23, –0.11, and –0.24% per annum in the H line and –0.78, –0.50, and –0.16% per annum in the L line. It was concluded that sustained genetic progress in lamb survival is feasible if directed selection is applied to a correlated trait such as the ability of ewes to rear multiples.

Piglets of low birth weight exhibit a reduced total number of skeletal myofibers at birth and throughout life compared with piglets of middle and heavy birth weight, which is associated with impaired (lean) growth and quality of carcass and meat at market weight. We investigated the effect of l-carnitine supplementation to suckling piglets of different birth weights on early postnatal myofiber formation, muscle growth, and body composition. A total of 48 piglets of low (LW) and middle (MDW) birth weight from 9 German Landrace gilts received 400 mg of l-carnitine (carnitine, n = 25) or a placebo (control, n = 23) once daily from d 7 to 27 of age and were slaughtered on d 28 of age (weaning). Carnitine-supplemented piglets deposited less fat as indicated by a reduced proportion of perirenal (P = 0.1) and intramuscular fat (P = 0.05). Circulating glucose concentrations tended to be greater in supplemented LW piglets (P = 0.13). The concentration of carnitine in semitendinosus (STN) muscle was approximately doubled (P < 0.001) by supplementation, with emphasis on the proportion of esterified carnitine. The ratio of lactate dehydrogenase to isocitrate dehydrogenase tended (P = 0.12) to be smaller in STN muscle of supplemented piglets, indicating a more oxidative muscle metabolism. The total number of STN myofibers was increased by 13% (P = 0.02) in supplemented LW piglets, thereby reaching the unchanged level of MDW littermates. In addition, supplemented LW piglets displayed a 2.4-fold mRNA expression of the gene encoding the embryonic isoform of the myosin heavy chain in STN muscle than control piglets (P = 0.05), but there were no differences in the proportion of fibers positively staining for the embryonic myosin isoform. l-Carnitine-supplemented piglets exhibited a greater DNA:protein ratio (P = 0.02) in STN muscle, which resulted from a greater DNA concentration (P = 0.04). However, the STN muscle of l-carnitine-supplemented piglets was not less mature as indicated by unchanged myofiber size, creatine kinase activity, and protein concentration. The results indicate that energy balance has been improved through intensified fatty acid oxidation. As a consequence, myogenic proliferation appears to be stimulated, which in LW piglets may have contributed to a compensatory increase in myofiber number. Thus, piglets, particularly those of low birth weight, could profit from an early postnatal l-carnitine supplementation, which may attenuate the negative consequences of low birth weight on body composition and meat quality at market weight.