Effects on Dietary Monensin and Sex of Calf on Profiles of Serum Progesterone and Estrogen in Late Pregnancy of First-Cross Brahman-Hereford Cows

Two groups of eight first-cross Brahman-Hereford cows of equal parity within groups were fed monensin and no monensin, respectively, from day 246 ± .4 of gestation. The cows were group fed Coastal bermudagrass hay ad libitumand either 0 (group NMON) or 200 mg (group MON) of monensin incorporated in 1.8 kg of standard range cube (daily per cow). The experimental purpose was to study concentration profiles of sex steroids prepartum. Blood was sampled via tail vein or artery on alternate days through day 270 of gestation and at least daily thereafter until 3 days postpartum as well as within 1 hr after calving (day 0). Progesterone (P4), estrone (E1) and estradiol-17β (Eβ) and -17α (Eα) were measured in blood serum by radioimmunoassay. The data were analyzed by least-squares and split-plot analysis of variance and by stepwise-addition multiple regression. Diet did not affect body weight and length, wither height, heart girth or condition score of cows, calving traits or birthweight and vigor of calves. Periparturiently, concentration profiles of the serum steroids were characteristic of other published data. Concentrations of the serum steroids were not different (P>.25) between groups initially, but serum P4increased (P<.01) about 3 ng/ml in cows of group NMON during the first 22 days of the feeding trial and serum P4was higher (P<.01) on day 22 than in cows of group MON. In contrast, concentrations of serum estrogens did not differ significantly between diet groups until days 2 to 0 prepartum when concentrations of serum E1and Eβ were higher (P<.05) in cows of group NMON than in cows of group MON. Concentrations of serum Eα started to decrease on either day −4 (group MON) or day −1 (group NMON). Cows carrying a female calf had higher concentrations of serum P4(P<.05) and lower concentrations of serum estrogens [E1, P>.10; Eβ and Eα, P<10] prepartum than cows carrying a male calf. Differences in concentrations of the serum estrogens associated with calf sex were most evident during the last 18 days before calving.