Nutritional utilization in Malagueña dairy goats differing in genotypes for the content of alphaS1-casein in milk

A study was carried out with 20 goats of the Malagueña breed, half with a high (HG) and half with a low (LG) genetic capability for alpha(S1)-casein (AS1-CN) synthesis, to determine whether the 2 different genotypes (that cause differences in goat milk composition) are related to differences in nutritional feed utilization. Among the 10 HG goats, 7 had BB and 3 had AB genotypes for AS1-CN, whereas there were 7 EF and 3 FF genotypes in the 10 LG goats. The goats were fed diets differing in crude protein content (13.6 vs. 17.7% dry matter for diets 1 and 2, respectively). For each genotype group, a balance trial was conducted with each of the 2 diets in a 2-period balanced changeover designed with half the animals consuming diet 1 and the other half diet 2, determining individual feed intake and the utilization of N and energy in the diets. Greater voluntary feed intake on a metabolic body weight basis among the HG goats was identified as the first possible cause of their milk production. The HG goats also had a greater level of feed utilization, on a metabolic body weight basis, for N and energy intake. Greater ratios of N balance/ digestible N, milk protein N/digestible N, milk energy/ digestible energy, and milk energy/ME were found for HG goats compared with LG. These effects appear to be dependent on the level of protein in the diet, indicating interactive effects. The greater N and energy utilization of HG versus LG goats may explain the differences in milk composition between the 2 genotype groups.