Graduate Thesis Or Dissertation
 

Estimates of combining abilities, heterotic effects and phenotypic correlations among in-bred lines of beef cattle and their line-crosses

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  • The investigations were based on a complete diallel cross among three inbred lines of Hereford cattle. Records of Angus calves born during the same period were used to obtain more accurate estimates of environmental effects and to determine breed differences among the various traits included in the study. Estimates of general combining ability, specific combining ability and reciprocal effects were obtained. In addition, heterotic estimates of the linecross calves were determined as well as phenotypic correlations among all the performance traits, blood constituents, carcass traits, organoleptic measurements and endocrine gland weights. General and specific combining ability effects were either small or nonexistent for all of the preweaning traits of the males except conformation score at 227 kg. Among the females, significant general combining ability effects were detected for preweaning ADG, age at 227 kg. and condition score at 227 kg. Specific combining ability effects were nonsignificant among both sexes except for birth weights of the 2 X 3 and 3 X 2 cross females. Angus calves were smaller at birth, had higher preweaning gains, were younger at 227 kg. and had higher conformation and condition scores at weaning than the Hereford calves. A significant heterotic effect was detected for percent stillbirths with the linecross calves showing more vigor and fewer dead at birth. The Angus exceeded the Herefords in percent calves born and weaned (P < .01). All other preweaning and weaning traits did not exhibit overall linecross superiority likely as a result of the low milk production of the inbred dams. Angus calves were lower in postweaning rate and efficiency of gain, higher in conformation and condition scores and younger at the end of the feed test than the Herefords. General combining ability effects were significant or highly significant for conformation and condition scores at the end of the feed test for both sexes. A significant general combining ability effect was detected for feed efficiency of the males. Specific combining ability effects were significant for postweaning ADG of males and females. Heterotic effects were significant for postweaning rate of gain and feed per unit gain among the females and for age at 363 kg. among the males. Heterosis for conformation and condition scores were not found for either sex. Angus calves had higher blood levels of amino acids and urea than Herefords. Amino acids and urea were higher in females than in males and increased with increasing age in both sexes. Creatinine levels in the blood were quite consistent between sexes and breeds. Differences in general combining ability, specific combining ability and heterosis were nonsignificant for all blood constituents at all weights. The Angus excelled the Herefords in components of carcass grade, thyroid gland weight and testicle weight, but Herefords excelled in loin-eye area and percent lean in the 12th rib. General combining ability effects were significant or highly significant for components of carcass grade, percent drip, tenderness, juiciness and thyroid gland weight. No specific combining ability differences were detected. Heterotic effects were important only for testicle weight of the 1 X 2 and 2 X1 cross males. For the entire study, line 1 excelled the other lines in general combining ability for most of the traits. Correlation coefficients indicated that compensatory effects for poor suckling gains occurred during the early part of the post-weaning period. Birth weight was highly associated with preweaning growth rates in Hereford females but was not associated with pre-weaning growth rates in the Angus females. The relationship between suckling gain and conformation and condition scores at all weights were important, but no relationship between postweaning rate of gain and conformation and condition scores were found. Less efficient animals had higher blood levels of amino acids and larger amounts of fat in the carcasses. Conformation scores of the live animals were positively associated with carcass conformation, marbling, carcass grade and loin-eye area. This would indicate that muscling and carcass grade can be determined in the live animal.
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