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USDA-ARS, Fort Keogh Livestock and Range Research Laboratory, Miles City, MT 59301 and University of Wyoming, Laramie 820716,6
Abstract
Fifteen Hereford and 47 crossbred heifers were allotted by breed and body weight to be artificially inseminated to one of two Angus sires selected for progeny birth weights (L = low; H = high). Forty-two of the heifers were randomly assigned to be slaughtered at 200, 215, 230, 245 or 260 d of gestation for measurement of fetal and placental characteristics. Twenty heifers were allowed to go to term and five calves from each sire group were randomly assigned to be euthanized and dissected within 24 h after birth. Sire differences in birth weight (BW) and dystocia score (32.9 vs 35.4 kg; 1.8 vs 3.1, L vs H sires, respectively) existed (P<.01), and there was a sire effect (P<.01) for fetal calf weights (FW) and eviscerated calf weights (EW). However, there was a sire x calf sex interaction for BW (P<.05), EW (P<.01), FW (P<.01), femur length (P<.05), heart weight (P<.05), kidney weight (P<.01) and pituitary weight (P<.01). Weight differences suggested these interactions were a result of the relationship of the organ weights to fetal body weights and the interaction effects on calf weights resulted from limitations in the maternal environment which restricted growth of H-sired male calves in utero. Sire x fetal age interaction effects were all nonsignificant (P>.10) except for cerebrum weight. This finding indicates that fetus and calf growth rates were parallel for the L and H sires. A sire effect was found for biceps (P<.01) and liver weights(P<.01), but not for cerebrum weights (P>.10). Increasing weight due to fetal age was attributed to hypertrophy for the cerebrum (P<.05) and liver (P
=.01), while the biceps increased (P<.05) by both hypertrophy and hyperplasia, as determined from deoxyribonucleic acid and protein analyses. All measured fetal organ weights except heart, when expressed as a ratio with EW, decreased (P<.05) with increasing fetal age. Brain (cerebrum + cerebellum):liver weight ratios were higher (P<.01) in L-sired calves (.32 vs. .28) than in H-sired calves. Total placentome weight (b' =91; P<.01) and placental fluid volume (b' =. 32; P<.01) were highly associated with FW, accounting for 84% of the variation in FW. We conclude that fetal growth rates were similar from d 200 of gestation to parturition for the two groups, even though birth weights differed. Thus, genotypic differences affecting birth weight likely were manifested before 200 d of gestation.
1 This study was a contribution to Western Regional Project W-112, Reproductive Performance in Domestic Ruminants. Published with the approval of the Directtor, Wyoming Agr. Exp. Sta. Journal Article No. 1299.
2 Research completed as partial requirement for the Ph.D. degree at the University of Wyoming, Laramie. Present address: Dept. of Biochem. J. Hillis Miller Health Center, Box J-245, Univ. of Florida, Gainesville 32610.
3 Fort Keogh Livestock and Range Res. Lab., USDA-Agr. Res. Serv.
4 Division of Anim. Sci., Univ. of Wyoming, Laramie 82071.
5 Direct reprint requests to this author.
6 The cooperation of the Montana Agr. Exp. Sta. is appreciated and acknowledged.
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