Productivity of Barzona, Brangus, and Beefmaster cattle was evaluated on conservatively (n=2) (40 ha AU-1) and moderately (28.5 ha AU-1) stocked pastures (n=2) in the Chihuahuan Desert of south-central New Mexico. Equivalent numbers of suckled, first-calf heifers of each breed (n = 31) weighing 333±11 kg were randomly assigned to the study pastures in the spring of 1992. Pastures were grazed continuously and herd productivity data were collected from 1992-1994. In late August 1994, all pastures were destocked due to onset of severe drought. No effect of breed (P>0.10) was detected in the analyses, so data were pooled across breeds and compared between the stocking rates. Calf crop percentages (1993, 1994) were higher in conservative than moderate stocked pastures (82 vs 62%, respectively, P<0.01). Financial analyses standardized to a hypothetical medium size (8,094 ha) New Mexico Chihuahuan Desert cattle range showed net returns per ha did not differ (P>0.10) between stocking rates. However, the main effect of year and stocking rate x year interaction were significant (P<0.05). These analyses suggest that the drought in 1994 lowered returns per ha compared to 1993 when precipitation was near average and that conservative stocking may present less financial risk than moderate stocking when drought occurs. These data are consistent with other studies from arid and semi-arid rangelands demonstrating that conservative stocking can give financial returns from cattle production equal to or greater than those from moderate stocking.

Fecal microhistology was used to estimate botanical composition of samples taken from Hereford (N = ll), Angus (N = 11) and Brangus (N = 37) 3 to 5 year-old cows in 3 seasons (October, 1991 and January and July, 1992) and from Hereford (N = lo), Angus (N = 9) and Brangus (N = 34) calves in October. Breed differences in botanical composition of diets and relationships between dam and offspring botanical composition of diets were examined. Breed diierences were observed for cows in all 3 seasons and for calves in October. Brangus cows showed greater preference (P < 0.05) for Sporobolus than Hereford cows in October, January, and July. Brangus cows also showed greater preference for Sporobolus than Angus cows in January and July. Brangus and Angus calves showed greater preference for Sporobolus than Hereford calves in October (P < 0.05). Brangus cows had a stronger preference for Yucca and total shrubs in January than either Hereford or Angus cows. Hereford cows and calves had stronger preference for Aristkia than either Angus or Brangus in October (P -z 0.05). Regression of October calf botaui-Cal components on dam botanical components indicated significant relationships for only 2 genera, Aristida (2' ~0.01) and Sporobolus (P

Balancing growth and reproductive performance in beef cattle managed in desert environments is challenging. Our objectives were to 1) evaluate trends in growth and reproductive traits, and 2) assess associative relationships between growth characteristics and reproductive performance in a Brangus herd managed in a Chihuahuan Desert production system from 1972 to 2006. Data were from bull (n = 597) and heifer calves (n = 585; 1988 to 2006) and cows (n = 525; repeated records of cows, n = 2,611; 1972 to 2006). Variables describing the growth curve of each cow were estimated using a nonlinear logistic function (each cow needed 6 yr of data). Mixed-effect models and logistic regression were used to analyze trends across years in growth and reproductive traits (both continuous and categorical). For continuous traits of calves, a slight cubic response (P < 0.01) described the dynamics of birth weight, 205-d BW, and 365-d BW across years. For categorical traits of females, positive linear trends (P < 0.05) across years were observed in percent pregnant as yearlings, calved at 2 yr of age, and first-calf heifer rebreeding (slopes ranged from 0.007 to 0.014%/yr). Autumn cow BW increased gradually until 1997 (509 kg +/- 8.8) and then decreased gradually by 0.6 kg/yr, whereas pregnancy percentage decreased gradually until 1995 (78.4% +/- 1.0) and then increased slightly by 0.2%/yr. A quadratic effect best described the dynamics of these 2 variables across years (P < 0.01) as well as estimates describing the growth curve of each cow. Specifically, asymptotic BW and age increased (P < 0.05) from 1972 to 1983 and 1990, respectively. Asymptotic age then decreased by 27% from 1983 to 1996 (P < 0.05). The maturing rate index was negatively correlated with age at first calving and calving interval (r = -0.42 and -0.18, P < 0.01), which suggested that early-maturing cows had enhanced fertility in this environment and production system. In summary, minimal changes were observed in measures of growth in bulls and heifers in a Brangus herd managed in the Chihuahuan Desert. Opposing relationships were observed among measures of cow size and fertility; as growth curves shifted toward earlier maturity, measures of reproductive performance suggested that fertility improved.

Maternal performance of 134 Hereford (H), Brangus (B), and reciprocal crossbred (H x B and B x H) cows from 2 to 7 yr of age was evaluated under semidesert conditions in this study. Calves produced by 2- and 3-yr-old cows were sired by Brangus and Hereford bulls. Calves produced by 4- to 7-yr-old cows were sired by Charolais bulls. Breed of sire and breed of dam of cow affected kilograms of weaning weight, 205-d weight, weaning weight as a percentage of cow weight, and 205-d weight as a percentage of cow weight produced annually. Brangus (either as sire or dam of cow) was superior to Hereford in all cases. Observed maternal heterosis on 2- to 3-yr-old cows was 23.0, 20.1, 30.0, 29.1, 23.9, and 23.0% for calf birth date, weaning percentage, weaning weight per year, 205-d weight per year, weaning weight as a percentage of cow weight per year, and 205-d weight as a percentage of cow weight per year, respectively (P less than .01). Observed maternal heterosis from mature cows was 19.8, 12.8, 21.0, 18.7, 17.4, and 15.4% for calf birth date, weaning percentage, weaning weight per year, 205-d weight per year, weaning weight as a percentage of cow weight per year, and 205-d weight as a percentage of cow weight per year, respectively (P less than .01). Results indicate large heterotic effects on annual cow productivity and an adaptive advantage for cows with Brangus sires and(or) dams under semidesert conditions.

Fecal microhistology and chemistry were used to evaluate botanical composition and quality of diets selected by Brangus cattle grazing Chihuahuan desert range in 3 seasons; fall: