GENETIC-ASPECTS OF DIET SELECTION IN THE CHIHUAHUAN DESERT

Fecal microhistology and chemistry were used to evaluate botanical com position and quality of diets selected by Brangus cattle grazing Chihu ahuan desert range in 3 seasons; fall: October, 1991; winter: January, 1992; and summer: July, 1992. Fecal samples were collected from 100 h ead in fall (58 2-8 year cows and 42 carves), 53 head (2-8 year-old co ws) in winter, and 44 head (2-8 year-old cows) in summer. Paternal hal f sib analyses were used to estimate genetic and phenotypic variances and heritability estimates. Heritability is the proportion of total (p henotypic) variation which is due to additive genetic effects. The eff ect of sire within age was observed for percentage of Aristida spp. (P = 0.01), Sporobolus spp. (P = 0.09), total grasses (P= 0.02), Croton pottsii (Klotzsch) Muell.-Arg. (P = 0.03), and total forbs (P = 0.02) in fall diets, The number of grass species in diets was also affected by sire (P = 0.03). Heritability estimates were 0.87, 0.51, 0.78, 0.76 , and 0.79 for percentages of Aristida spp., Sporobolus spp., total gr asses, Croton pottsii, and total forbs, respectively. Heritability est imates for number of grass and forb species in fall diets were 0.68 an d 0.26, respectively. Heritability estimates for winter samples were 0 .40, 0.00, 0.37, and 0.27 for percentages of Sporobolus spp., total gr asses, Yucca elata Engelm., and total shrubs, respectively. Heritabili ty estimates for the number of grass and total species observed in win ter diets were 1.11 and 0.47, respectively, Heritability estimates for percentages of Bouteloua spp., total grasses, Croton pottsii, Dalea s pp., and total forbs in summer samples were 0.20, 0.55, 0.58, and 0.46 , respectively, Heritability estimates for the number of grass and tot al species in summer diets were 0.49 and 0.79, respectively. These dat a suggest that genetic composition of beef cattle may affect diet sele ction under Chihuahuan desert conditions.