Abstract
BackgroundThe pre-weaning growth of lambs, an important component of meat production, depends on maternal and direct effects. These effects cannot be observed directly and models used to study pre-weaning growth assume that they are additive. However, it is reasonable to suggest that the influence of direct effects on growth may differ depending on the value of maternal effects i.e. an interaction may exist between the two components.MethodsTo test this hypothesis, an experiment was carried out in Romane sheep in order to obtain observations of maternal phenotypic effects (milk yield and milk quality) and pre-weaning growth of the lambs. The experiment consisted of mating ewes that had markedly different maternal genetic effects with rams that contributed very different genetic effects in four replicates of a 3 × 2 factorial plan. Milk yield was measured using the lamb suckling weight differential technique and milk composition (fat and protein contents) was determined by infrared spectroscopy at 15, 21 and 35 days after lambing. Lambs were weighed at birth and then at 15, 21 and 35 days. An interaction between genotype (of the lamb) and environment (milk yield and quality) for average daily gain was tested using a restricted likelihood ratio test, comparing a linear reaction norm model (interaction model) to a classical additive model (no interaction model).ResultsA total of 1284 weights of 442 lambs born from 166 different ewes were analysed. On average, the ewes produced 2.3 ± 0.8 L milk per day. The average protein and fat contents were 50 ± 4 g/L and 60 ± 18 g/L, respectively. The mean 0–35 day average daily gain was 207 ± 46 g/d. Results of the restricted likelihood ratio tests did not highlight any significant interactions between the genotype of the lambs and milk production of the ewe.ConclusionsOur results support the hypothesis of additivity of maternal and direct effects on growth that is currently applied in genetic evaluation models.
Highlights
The pre-weaning growth of lambs, an important component of meat production, depends on maternal and direct effects
Pre-weaning growth is a complex trait for which phenotypic observations recorded on the lamb result from effects contributed by two individuals: by the lamb via direct effects and by the mother via maternal effects
If the first mating of a ewe was performed with a male of the low direct genetic effects group, the second mating was performed with a male of the high direct genetic effects group, and vice versa
Summary
The pre-weaning growth of lambs, an important component of meat production, depends on maternal and direct effects. In the context of pre-weaning growth models, the standard assumption is that the observed phenotype is the sum of the environmental and genetic effects contributed by the ewe and its lamb [2] In some cases, these models produce surprising results, e.g., strong negative estimates of the correlation between maternal and direct genetic effects [3,4]. Wolf [8] gave a simple explanation of this in the case of mammals for growth performances: “...if offspring differ in how efficiently they process milk, and this difference has a genetic basis, the contribution of the maternal character (performance on milk production) would not be additive”, or in other words, growth = milk production*feed conversion efficiency This suggests the presence of an interaction between direct and maternal effects. Using a reaction norm model, we tested for the presence of an interaction between the direct genetic effect of the lamb and the maternal phenotypes
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