Our working hypothesis was that the low concentrations of progesterone (P4) and synthetic progestins administered in hormonal regimens to control estrous cycles of cows would have similar effects on secretion of LH and 17 beta-estradiol (E2). In addition, we hypothesized that concentrations of exogenous P4 typical of the midluteal phase of the estrous cycle and the corpus luteum (CL) would have similar effects on LH and E2, and the effects would be different from those of synthetic progestins and low concentrations of P4. Cows (n = 29) were randomly assigned to one of five treatment groups: 1) one Progesterone Releasing Intravaginal Device (1PRID; n = 6); 2) two PRIDs (2PRID; n = 6); 3) norgestomet, as in Syncro-Mate-B regimen (SMB; n = 6); 4) melengestrol acetate (MGA; 0.5 mg/day; n = 5); and 5) control (CONT; n = 6). Treatments were administered for 9 days (Day 0 = initiation of treatment). All cows from 1PRID, 2PRID, SMB, and MGA groups were injected with prostaglandin F2 alpha (PGF2 alpha) on Days 2 and 5 of the treatment period to regress CL. Cows in the 1PRID and SMB groups were also administered exogenous estrogen according to the respective estrous synchronization protocol for these products. Daily blood samples were collected from Day 0 to 35 to determine concentrations of P4. On Day 8, blood samples were collected at 15-min intervals for 24 h to determine pattern of LH secretion. On Day 9, all treatments ceased and cows in the CONT group received injections of PGF2 alpha.(ABSTRACT TRUNCATED AT 250 WORDS)

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Two experiments were conducted to test the following hypotheses: 1) exposure of beef heifers to sterile bulls increases the proportion of heifers attaining puberty by 14 mo of age and 2) rate of growth interacts with bull exposure to influence age at puberty in beef heifers. In Exp. I, heifers were assigned to one of two treatments: 1) heifers were exposed to bulls (BE approximately 7 0 d period of exposure) or 2) heifers were isolated from bulls (NE) and served as controls. In Exp. 11, heifers were assigned to either BE or NE treatments (175-d period of exposure to bulls) and were fed to gain at a moderate (MG, .6 kdd) or high (HG .8 kdd) growth rate. Blood samples were collected twice weekly to determine concentrations of progesterone indicative of onset of corpus luteum function and puberty. In Exp. I a greater (P e .05) propodon of heifers receiving the BE treatment than of heifers receiving the NE treatment initiated corpus luteum function by 14 mo of age. In Exp. 11, there was a bull exposure x growth rate interaction (P e .05). The effect of bull exposure was greater within the HG groups than within the MG groups. However, heifers fed to attain a moderate or high growth rate and exposed to bulls attained puberty at younger ages than heifers not exposed to bulls and fed to attain a moderate or high growth rate. Mean ages at puberty were 375, 422,428, and 449 (pooled SEM = 8.6) d for heifers in the BE-HG, BE-MG, NE-HG, and NE-MG groups, respectively. Therefore, we accept our working hypothesis that exposure of heifers to bulls increased the proportion of heifers attaining puberty by the initiation of breeding at 14 mo of age. Furthermore, growth rate interacts with the stimulatory influence(s) of bulls to influence age at puberty in beef heifers.

Our working hypotheses for this study were that 1) the profile of intrapituitary LH and FSH isoforms would be shifted toward acidic forms as sexual maturation progresses in the bovine female; and 2) concentration of 17 beta-estradiol (E2) in circulation during sexual maturation would be a major factor modulating the percentage of the more acidic isoforms. In addition, the biological-immunoreactive (B:I) ratios of each isoform of LH were evaluated at selected stages of sexual maturation. Heifers (7 mo of age) were assigned to one of three treatment groups: 1) ovariectomized (OVX; n = 16); 2) OVX and administered E2 (OVXE; n = 16); or 3) ovary-intact (INTACT; n = 14). Pituitaries were collected from heifers in each group at an estimated 120 days (prepubertal) of 25 days before puberty (peripubertal). A fourth group of 6 heifers remained intact (postpubertal INTACT) to determine time of puberty during the experimental period. Pituitaries of heifers assigned to the postpubertal INTACT group were collected during the follicular phase of the first or second estrous cycle postpuberty. Pituitaries were used for determination of relative amounts of gonadotropin isohormones. Tissue extracts of the pituitaries were chromatofocused on pH 10.5-4.0 gradients. The LH of all pituitaries resolved into thirteen isoforms that were designated isoforms A-L, and S, with isoform A the most basic form. Isoforms F and G (basic pH range) were the predominant isoforms of each chromatofocusing profile and comprised 50-60% of the immunoreactive LH. Isoforms J and K were the major isoforms eluting in the acidic pH range.(ABSTRACT TRUNCATED AT 250 WORDS)

Lymphocyte blastogenic response, B-lymphocyte population and antibody-producing activity of lymphocytes were determined to evaluate the lymphocyte function in neonatal calves during the first 4 weeks of life. The mean percentage of B-lymphocytes ranged from 10.2 to 12.5 % during the first 14 days of life and from 15.3 to 17.5% in calves from the day 21 to day 28 after birth. The absolute number of B-lymphocytes increased significantly (P< 0.05) from 370/p1 at birth to 736/p1 on day 28 after birth. The mean stimulation index of blastogenic response, measured by fluorometric assay, ranged from 5.75 to 6.61 with Con A, from 5.29 to 5.98 with PHA and from 1.89 to 2.50 with PWM. The mean (? S . D.) number of plaque forming cells ranged from 22.0 (+ 12.0) to 24.7 (+ 9.2) in cultured lymphocytes from 5 calves at birth to 14 days after birth and their levels increased markedly from 137.8 (+ 88.3) to 162.0 (? 57.8) in lymphocytes from 20 days to 28 days after birth. The present study showed that antibody-producing activity of lymphocytes is lower in calves within 3 weeks after birth compared to that of calves 3 weeks after birth, indicating that neonatal calf lymphocytes have a low antibody-producing activity at least up to 1 month after birth. U.S.