Studies on adult body size and its effects on pre-weaning kid weight of Mubende goats in Uganda: population screening

Abstract

Paper II is an investigation of the effect of parent's adult body weight on pre-weaning kid weight of Mubende goats. Information from 2 studies is used to evaluate the efficiency of selecting for large body size among a population of goats using heart girth measurements screening method under field conditions. In both studies, initial surveys were conducted to screen for adult goat body size under on-farm field conditions using heart girth measurements. The adult goats were categorised into two body-size groups, large (L) and ordinary (O) using initially only heart girth for screening under field conditions and then both heart girth and body weight measurements under on-station conditions. Four mating groups were created, i.e. large bucks x large does (LB x LD), large bucks x ordinary does (LB x OD), ordinary bucks x large does (OB x LD) and ordinary bucks x ordinary does (OB x OD). Matings were carried out by randomly allocating does within a specific group to specific bucks. Part of the goats used in study I was also used in study 2 with animals maintained in their respective size category groups. The combined data did increase sample size and balance. Kid body weights measured from birth to weaning (16 weeks of age) was analysed as the dependent variable. The statistical model used included effects of year (Y), month (M), size of buck (B), sex of kid (S), type of birth (L) and partial regression of initial regression of initial dam body weight (b1W). The combined data analysis showed that dam weight influenced pre-weaning kid body weight only at birth and attributed to maternal genetic, maternal environment and direct additive genetic effects. Large dams have better body nutrient reserves for the foetus resulting in heavier kids at birth. Large size was found to directly influence body weight of offspring. Buck size influenced pre-weaning body weights indicating influence of additive genetic effects. Direct additive genetic effects and environmental effects could not; however, be separated as part of the population was not randomly sampled. However, use of more animal numbers and more balanced sample sizes could contribute to reliability of the analysis and improved results. Kid sex and litter size influenced weight at birth and pre-weaning period with male kids consistently heavier than their female counterparts and single birth kids being heavier than twins and triplets. The fixed effect of year and month of birth significantly influenced kid weight at pre-weaning and weaning period but not at birth. This is generally associated with seasonal influences during pre-weaning period related to availability of quality herbage.