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)

ABSTWCT:Calving records from 1969 to 1989 from the Roman L. Hruska U.S. Meat Animal Research Center were used to investigate how climatic conditions, in addition to dystocia, age of dam, size of calf, and sex affect calf survival from birth to 1 wk of age. Data were analyzed separately for cows calving with ( n = 11,094) or without ( n = 72,187) dystocia. Neonatal mortality was described by a logit model and parameters were estimated by maximum-likelihood procedures. Calves born to cows with dystocia were five times as likely to die neonatally than calves born without assistance. Of all calves that died, 43.6% were born with difficulty. Of these calves, survival was lowest for those that were small relative to their genetic group, sex, and age of dam. Large calves had markedly increased mortality only when born to 2-yr-old dams. Average ambient temperature and precipitation on day of calving affected survival nonlinearly and the magnitude of the effect depended on age of dam, sex and size of calf, and dystocia incidence. Calves born to 2-yr-old cows were more susceptible to severe weather conditions than calves born to older cows. The negative effect of precipitation on survival increased with decreasing temperature.

The objectives of this study were to determine the effects of dose of exogenous progesterone (P4) prior to artificial insemination on concentrations of 17 beta-estradiol (E2) and on conception rates in bovine females. Heifers (n = 100) and cows (n = 100) received P4-releasing intravaginal devices (PRIDs) to produce two different circulating concentrations of P4. All animals received a single PRID 10 days before (Day-10) the start of the breeding season (Day 0). In animals that received the low dose of P4 (1 PRID, target concentration of 2-3 ng/ml of plasma), the original PRID remained in place for 10 days. In animals that received the larger dose of P4 (2 PRIDs, target concentration of 5-8 ng/ml of plasma), an additional PRID was inserted on Day -9. To maintain concentrations of P4 in the 2-PRID group, the PRIDs inserted on Days -10 and -9 were replaced with new PRIDs on Days -5 and -4, respectively. Prostaglandin F2 alpha (25 mg) was administered to all animals on Days -9 and -3 to remove the endogenous source of P4. Following PRID removal, animals were artificially inseminated 12 h after signs of behavioral estrus were observed. A treatment-by-day interaction (p < 0.0001) was observed for concentrations of P4 in circulation of both heifers and cows. Animals that received 2 PRIDs had greater (p < 0.001) concentrations of P4 by Day-8 of treatment than animals that received 1 PRID. In cows that received 1 PRID, concentrations of E2 increased 2.4-fold from Day-10 (6.8 +/- 1.0 pg/ml) to Day-2 (16.7 +/- 1.4 pg/ml).(ABSTRACT TRUNCATED AT 250 WORDS)

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.