The goal of this study was to determine the effects of short-term feed withdrawal on reproductive and metabolic hormones during the luteal phase of the estrous cycle in mature ewes. Mature ewes observed in estrus were assigned randomly to control and fasted groups (n = 10 per group Trials 1 and 2). For Trials 1 and 2, control ewes had ad libitum access to feed, whereas fasted ewes were not fed from d 7 through 11 of their estrous cycle; on d 12, all ewes were treated with 10 mg of PGF2alpha, and fasted ewes were gvien ad libitum access to feed. For Trial 1, blood samples were collected daily through fasting and at 2-h intervals following PGF2alpha for 72 h. Serum concentrations of insulin (P < or = 0.002) and IGF-I (P < or = 0.01), but not GH (P > or = 0.60), were decreased during fasting compared with fed ewes. Serum concentrations of 29 (P = 0.02) and 34 kDa (P = 0.04) IGFBP were greater in fasted ewes at 96 h after initiation of fasting than in control ewes. Two control and four fasted ewes in Trial 1 did not exhibit a preovulatory surge release of LH by 72 h. Therefore, Trial 2 was conducted so that the timing of the LH surge could be predicted following the collection of blood samples at 2-h intervals for 112 h and then at 6-h intervals until 178 h following PGF2alpha administration and realimentation. The magnitude of the preovulatory LH surge in Trial 2 was decreased (P = 0.009) and delayed (P = 0.04), and serum concentrations of estradiol were diminished (P < or = 0.03) 12 h before the LH surge in fasted ewes. Ovulation rates were not influenced (P > or = 0.32) by fasting in Trials 1 and 2. Serum concentrations of progesterone in both Trials 1 and 2 were, however, greater (P < 0.001) in fasted than in control ewes. A third trial with ovariectomized ewes was conducted to determine whether the increased serum concentrations of progesterone observed in fasted ewes during Trials 1 and 2 were ovarian-derived. Ovariectomized ewes were implanted with progesterone-containing intravaginal implants and allotted to control (n = 5) or fasted (n = 5) treatment groups and fed as described for Trials 1 and 2. Similar to intact ewes, serum concentrations of progesterone were approximately twofold greater (P < 0.001) in fasted than in control implanted ovariectomized ewes. In summary, feed withdrawal for 5 d during the luteal phase of the estrous cycle increased serum concentrations of progesterone and evoked endocrine changes that could perturb the subsequent estrous cycle.

Brahman cows were used to evaluate dietary fat (3.74% [control] and 5.20% [treated]) effects on blood hormone and lipid concentrations, follicular populations, and in vitro steroidogenesis. Cows were fed and ovaries were monitored by ultrasound from d 1 of the first estrous cycle (EC) until the first follicle of the second EC reached 8 mm, at which time ovaries were harvested. Follicular fluid (FF) was collected from large- and medium-sized follicles and assayed for progesterone (P4), estradiol-17 beta (E2), testosterone, cholesterol, and triglycerides. The corpus luteum was removed, minced, treated with LH, and incubated for 4 h. Granulosa cells harvested from the largest follicle were treated with forskolin, LH, or FSH and incubated for 48 h. Blood was collected during the treatment period and plasma assayed for 13,14-dihydro-15-keto-prostaglandin F(2 alpha) (PGFM), growth hormone (GH), insulin, P4 and E2. Cholesterol and triglycerides were assayed only from blood samples collected every 7 d. Treated cows had greater (P < .01) plasma E2 during the first EC and greater (P < .01) P4 during the second EC than controls. Treated cows had elevated (P < .01) insulin following d 16 of treatment, but GH and triglycerides were similar (P > .10) in both treatment groups. Treated cows tended (P = .10) to have greater PGFM peak concentrations than controls. Plasma cholesterol was elevated (P < .01) in treated cows on d 7 of the first EC. Treated cows tended to have greater (P < .09) follicular populations during the ovulatory and first wave of the first and second EC. Treatment did not affect (P > .10) FF concentrations of P4, E2, testosterone, cholesterol, or triglyceride from either large- or medium-sized follicles. There were no differences (P > .10) in steroid hormones produced in vitro.

During prolonged periods of reduced dietary energy, there is a reduction in secretion of LH in females. To test the hypothesis that decreased secretion of LH is due to reduced secretion of GnRH, 18 ovariectomized ewes were fed either a low-energy diet (LOW, 60% of the National Research Council recommendations, n = 12) or a normal diet, (control, n = 6), for 127 days. Each ewe received basal levels (approximately 5 pg/ml) of estradiol via sc Silastic implants. After 127 days, serum concentrations of FSH and LH were reduced (P less than 0.05) by 63% and 77%, respectively in LOW ewes compared to control ewes. Pituitary concentrations of FSH and LH in LOW ewes also were reduced by 56% and 80%, respectively. Compared to levels in control ewes, concentrations of messenger RNAs for alpha-, FSH beta-, and LH beta-subunits were reduced by 75%, 76%, and 91%, respectively. Pulsatile administration of GnRH (250 ng/2 h) for three weeks to LOW ewes restored each of the parameters to levels not different from those in controls. By the end of the study, serum concentrations of GH in all LOW animals had risen dramatically, but not in control ewes. Therefore, it appears that exogenous GnRH is capable of restoring synthesis and secretion of gonadotropins in ewes receiving low-energy diets. These results provide support for the hypothesis that reduced dietary energy results in decreased secretion of GnRH.

Replacing toxic, wild-type Neotyphodium coenophialum-infected tall fescue (E+) with nontoxic, N. coenophialum-infected tall fescue (NE+) has improved cow performance, but producer acceptance of NE+ has been slow. The objective was to compare performance by spring- and fall-calving cows grazing either E+ or NE+ at different percentages of the total pasture area. Gelbvieh×Angus crossbred cows (n=178) were stratified by BW and age within calving season and allocated randomly to 1 of 14 groups representing 5 treatments for a 3-yr study: i) Fall-calving on 100% E+ (F100); ii) Spring-calving on 100% E+ (S100); iii) Fall-calving on 75% E+ and 25% NE+ (F75); iv) Spring-calving on 75% E+ and 25% NE+ (S75); and v) Spring-calving on 100% NE+ (SNE100). Groups allocated to F75 and S75 grazed E+ until approximately 28 d before breeding and weaning, then were then moved to their respective NE+ pasture area for 4 to 6 wk; those allocated to F100, S100, and SNE100 grazed their pastures throughout the entire year. Samples of tall fescue were gathered from specific cells within each pasture at the time cows were moved into that particular cell (∼1 sample/mo). Blood samples were collected from the cows at the start and end of the breeding season. Stocking rate for each treatment was 1 cow/ha. Forage IVDMD, CP, and total ergot alkaloid concentrations were affected (P<0.05) by the treatment×sampling date interaction. Hay offered, cow BW, and BCS at breeding, end of breeding, and at weaning were greater (P<0.05) from fall-calving vs. spring-calving. Cow BW at weaning was greater (P<0.05) from F75 and S75 vs. F100 and S100. The calving season×NE+ % interaction affected (P<0.05) calving rates. Preweaning calf BW gain, actual and adjusted weaning BW, ADG, sale price, and calf value at weaning were greater (P<0.05) from fall-calving vs. spring-calving and from SNE100 vs. S75 except for sale price which was greater (P<0.05) from S75 vs. SNE100. Cow concentrations of serum prolactin at breeding and serum NEFA at the end of breeding were affected (P<0.05) by the calving season×NE+ % interaction. Serum Zn and Cu concentrations from cows were affected (P<0.05) by calving season. A fall-calving season may be more desirable for cows grazing E+, resulting in greater calving rates, cow performance, and calf BW at weaning, whereas limited access to NE+ may increase calving rates, serum prolactin, and NEFA concentrations during certain times in the production cycle, particularly in spring-calving cows.

A study to examine the relationships between milk intake, forage intake, and performance of Hereford-Angus suckling range calves was conducted during July, August, and September of 1984 and 1985. Twenty calves were used each year. The study was conducted at the Red Bluff Research Ranch located 56 km west of Bozeman, Montana. Average daily gain, milk intake (MI), forage digestibility, and fecal output (FO) were measured at 28-d intervals, beginning when the average calf age was 66 +/- 4 d. Milk intake was estimated using weigh-suckle-weigh techniques. Total fecal collections were used to measure FO. Forage digestibility and rates of passage were determined using nylon bag in situ techniques and external markers in ruminally cannulated calves of the same age. Fecal output by calves increased as body weight and age increased. Milk intake was higher (P less than .05) in 1985 than in 1984, but FO was higher (P less than .01) in 1984 than in 1985. Fecal output by calves was negatively correlated to MI in July (r = -.62; P less than .05) and August (r = -.56; P less than .05). No significant correlations were detected between MI and ADG (P greater than .10). Forage intake estimates were derived from FO, rate of passage, and in situ digestibility values. During July, calves consumed .3 kg more forage for each kilogram of reduction in fluid MI (P less than .05). In both August and September, calves consumed .6 kg more forage for each kilogram of reduction in fluid MI (P less than .10). Calves maintained similar digestible energy (DE) intake both years, although the source of DE varied.