Population Analysis of a Commercial Beef Cattle Herd

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Abstract
Twenty-four years of records (1958 through 1981) from a commercial Angus herd in Wyoming were utilized in a population analysis to characterize the herd as a biological unit. The herd averaged 632 ± 129 cows and 123 ± 6 replacement heifers, with all replacement heifers and many bulls selected from within the herd. The model concerned females only, seasonal breeders with overlapping generations, at a time immediately postpartum. Nine life table statistics were reported: age-specific survivorship (Lx); age-specific survival and mortality rates (Px and Qx); expected herd life (Ex); average number of daughters per cow of age × (Mx); reproductive value (Vx); net reproductive rate (Ro) ; generation interval (T) and instantaneous rate of population increase (r). Average life table statistics were calculated to characterize the herd. The Lx curve was a standard quadratic survivorship curve (R2=.91); Px and Qx reflected culling for replacements and increased mortality after age 11 yr; the highest Ex was 5.73 ± .31 at 1 yr of age; Mx ranged from .33 to .47 with no age-specific pattern; Ro, T and r equaled 1.26 ± .18, 5.62 ± .13 and .03 ± .03, respectively. For other variables to characterize the herd, mean values were 4.49 ± .09 for productive years in the herd, 3.78 ± .09 for total offspring per cow and 1.10 ± .03 for replacement daughters per cow. Three measures of genetic contributions of foundation females to future generations were calculated. Each of them followed a gamma distribution, suggesting that although cows, on the average, replaced themselves in the herd, many cows produced no replacement daughters, while others produced numerous daughters and more distant descendants.

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