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Fetal Growth of Beef Calves. I. Efffect of Prepartum Dietary Crude Protein on Birth Weight, Blood Metabolites and Steroid Hormone Concentrations¹

Fort Keogh Livestock and Range Research Laboratory, Miles City, MT 59301 and University of Wyoming, Laramie 82071^,6

Abstract

Fifty-nine crossbred heifers (427 kg) bred to one Hereford sire were randomly assigned at 75 d prepartum to two diets. Heifers were individually fed, and diets were isocaloric but contained either a low (LP = 81% NRC, .56 kg/d) or high (HP = 141% NRC, .98 kg/d) level of crude protein. Jugular vein cannulae were inserted into 16 LP and 16 HP heifers at 10 d prepartum. Daily preprandial blood samples that were collected until parturition were analyzed for serum estradiol-17β (E₂), progesterone (P₄), glucose (G) and urea nitrogen (UN). Heifers fed LP gained slower than HP-fed heifers before calving (.73 vs 1.02 kg/d; P<.01); immediate post-calving weights and condition scores were 418 vs 444 kg (P<.01) and 5.4 vs 6.1 (P<.01; LP vs HP, respectively). Calf birth weights (35.3 vs 36.1 kg), average calving difficulty score (1.6 vs 1.6) and percent assisted births (35.5 vs 35.7%) did not differ (P>. 10; LP vsHP, respectively). Prepartum concentrations of UN (6.2 vs 13.5 mg/dl) and G (52.9 vs 58.2 mg/dl) were lower (P<.05) and P₄ (5.94 vs 4.26 ng/ml) was higher (P=.07) in LP heifers. Prepartum concentration profiles were related to calving difficulty score (CD, 1 = no assistance to 3 = hard pull) for E₂ (CD1 vs CD2 + CD3, P<.01; CD2 vs CD3, P.01), P⁴ (CD1 vs CD2 + CD3, P<.05), G (CD1 vs CD2 + CD3, P<.05) and UN (CD2 vs CD3, P<.05). After calving, all dams were maintained together on pasture and supplemented with alfalfa hay and grain mix until adequate range forage was available to maintain weight gains. Dams that were fed LP prepartum gained faster than HP dams during this period (.49 vs .15 kg/d; P<.01). Prebreeding weights (443 vs 453 kg; LP vs HP) and condition scores (5.1 vs 5.1) did not differ, nor was the postpartum interval affected (44 vs 40 d; LP vs HP). There was no effect of dietary protein on dystocia or postpartum interval, although there were diet-induced differences in body weight and condition of the dams at calving. Results indicate that differences in prepartum profiles of serum steroid hormones and metabolites may be related to dystocia, in addition to relative fetal oversize.

¹ This study was a contribution to Western Regional Project W-112, Reproductive Performance in Domestic Ruminants. Published with the approval of the Director, Wyoming Agr. Exp. Sta., Journal Article No. 1297.

² Research completed as partial requirement for the Ph.D. degree at the Univ. 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.