Genetic parameters and genetic trends for growth traits of the Red Maasai sheep and its crosses to Dorper sheep under extensive production system in Kenya

The Red Maasai are indigenous sheep breeds known for their adaptability in arid and semi-arid environments while the Dorper is known for its good mutton production. A field experiment cross breeding program for the two breeds was set up in 2003 to exploit these traits. In this paper, we present the genetic parameters for birth weight (BWT), weaning weight (WWT) and weight at nine months (MWT) for pure Red Maasai (RRRR), pure Dorper (DDDD), 75% Dorper (DDDR) and 50% Dorper (DDRR) breeds. The genetic parameters were estimated by fitting four different multivariate animal models while ignoring or including maternal effects. The model with the lowest Akaike's Information Criterion (AIC) was chosen as the best model for the estimation of genetic parameters. The average annual genetic gain of the studied traits for each breed group was estimated by regressing the estimated breeding values on year of birth. Records from 5442 lambs recorded over a period of 17 years (2003-2019) were used in this analysis. The breed group, sex, type of birth, season of birth and age of the dam significantly (P < 0.001) affected the weights of the animals. The direct heritability estimates for BWT, WWT and MWT were highest in the RRRR (0.40 +/- 0.08, 0.39 +/- 0.08, 0.45 +/- 0.08 respectively). Direct heritability was lowest for BWT in the DDDR (0.18 +/- 0.06), and for WWT and MWT in the DDDD (0.10 +/- 0.07, 0.08 +/- 0.06 respectively). The maternal heritability estimates were highest in BWT for DDRR (0.12 +/- 0.05) and at WWT for DDDD (0.05 +/- 0.03), and lowest in BWT for DDDR (0.03 +/- 0.03) and at WWT for DDRR (0.00 +/- 0.01). The maternal heritability for MWT for all the breed groups was close to zero. Genetic and phenotypic correlations between the traits were all high and positive across all breed groups. The phenotypic correlations ranged from 0.41 to 0.73 while the genetic correlations ranged from 0.40 to 0.99. The pure Red Maasai had an average annual genetic gain of 0.012 +/- 0.005 kg, 0.144 +/- 0.037 kg and 0.170 +/- 0.047 kg for BWT, WWT and MWT respectively while the pure Dorper had an average annual genetic gain of 0.006 +/- 0.006 for BWT, 0.047 +/- 0.033 for WWT and 0.069 +/- 0.044 for MWT. Between the crosses, the 50% Dorper had higher annual genetic gains for all the traits. The observed genetic gains demonstrate that selective breeding based on estimated breeding values can lead to significant genetic improvement in the Red Maasai sheep and its crosses.