T. G. Dunn

Effect of norgestomet on peripheral levels of progesterone and estradiol-17β in beef cows

Peripheral levels of progesterone and estradiol 17beta were quantified in 27 cycling cows following administration of a single Hydron ear implant (G. D. Searle and Co.) containing 2, 4 or 6 mg norgestomet or controls which received no implant. Implants were inserted subcutaneously in the ear on day 15 of the estrous cycle (day of estrus = day 0) and removed 9 days later. The 4 mg (seven of seven cows) and 6 mg (six of six cows) implants suppressed estrus; however, three of eight cows in the 2 mg group exhibited estrus prior to implant removal. The 6 mg implant group had a significantly longer interval from implant removal to estrus than either the 2 or 4 mg group. Failure to detect differences in the rate at which progesterone declined indicated norgestomet treatment did not affect normal corpus luteum regression. Estradiol levels rose at a similar rate approaching estrus in all treatments. There was no indication of increased endogenous estradiol levels due to norgestomet treatment.

Negative Feedback Control of Luteinizing Hormone Secretion in Prepubertal Beef Heifers at 60 and 200 Days of Age

Prepubertal beef heifers at 60 and 200 d of age, born in the fall or spring, were assigned randomly to one of three treatment groups: (1) intact = 1; (2) bilateral ovariectomy (OVX); or (3) OVX plus estradiol-17 beta(E2) administered in silastic implants (OVX + E2). Luteinizing hormone (LH) was measured in serum samples collected at 20-min intervals for 4 h from heifers on -1, +7, +21, +35 and +49 d after OVX. Luteinizing hormone concentrations increased in the serum by 7 d after OVX in heifers at both 60 and 200 d of age (P less than .001; time X treatment). Prior to OVX, the LH patterns were characterized by low levels and infrequent episodic pulses. By 49 d after OVX, the mean LH concentrations increased and the pattern changed to one of rhythmic LH pulses with a periodicity of 1 h (P less than .001; time X treatment). Estradiol-treated OVX heifers did not exhibit a postovariectomy rise in serum LH concentrations. Serum E2 concentration 49 d after OVX in OVX heifers was threefold greater than in 1 or OVX heifers, thus demonstrating that E2 exerted negative feedback on pituitary LH secretion in prepubertal heifers. There was no measurable difference in serum E2 concentrations between I and OVX heifers; however, the contrast in the concentration and pattern of serum LH between the two groups was dramatic and suggested gonadal factors in addition to E2 are involved in controlling LH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and identification of metabolites of 14C‐labeled estradiol in cattle

A total of six steers and six heifers received three daily injections containing either 200 muCi (1 mg) of [4-14C] estradiol-17beta or 312 muCi (2.16 mg) of [4-14C] estradiol 17beta 3-benzoate. Major metabolites of the administered estradiol-17beta and estradiol-17beta 3-benzoate were identified in muscle, fat, liver, and kidney samples obtained 3 hr after the final injection. Estradiol benzoate was nondetectable in the tissues analyzed, suggesting rapid hydrolysis of the benzoate ester. Consequently, the relative proportions of the various metabolites were similar for both the injected estrogens. Estradiol-17beta and estrone, which together accounted for 80-90% of the total extracted radioactivity, appear to be the major metabolites in both muscle and fats. In contrast, the major metabolites present in liver and kidney appear in the conjugate fraction. Most of the conjugated metabolites were glucuronates, which represent 85-95% of the total recovered conjugate radioactivity.

Comparison of serum cortisol and prolactin in sheep blood sampled by two methods

Abstract The objective was to compare serum cortisol and prolactin levels in sheep following remote blood sampling with levels after the animals were handled. Twelve ovariectomized ewes were studied during two consecutive years. Each year, six ewes were kept in metabolism crates inside a building for 1 month. During this period they were accustomed to being handled. Prior to sampling, sheep were assigned to two groups (A and B) of three. Group A animals were fitted with an iv-catheter in one jugular vein (conventional sampling) and a 4 m long polyvinyl cannula in the other jugular vein (remote sampling). Group B animals were fitted only with the iv-catheter. In the first part of the study, group A sheep were sampled remotely and undisturbed for 3 hr at 10-min intervals and immediately afterwards were sampled conventionally for an additional 3 hr at 10-min intervals. In the second part of the study, group A animals were sampled remotely while group B animals were sampled conventionally for 3 hr at 10-min intervals. During this sampling period, group A animals, although not handled, were aware of disturbances in the sampling room. Serum cortisol (28.6 ± 0.9 vs. 11.8 ± 0.7; x ± SEM; ng/ml) and prolactin (58.1 ± 3.6 vs. 32.7 ± 2.9; x ± SEM; ng/ml) levels were higher (P

Estrous response and pregnancy rates following calf removal in beef cows treated with prostaglandin F2α

Five hundred fifty-four suckled beef cows in three herds were allotted within postpartum interval to one of four treatments. All cows received two injections of prostaglandin F2α (PGF2α) 11 days apart. Treatment I served as a control. Calves were removed for 48 hr following the first injection of PGF2α in treatment II. Calves were removed similarly after the second injection of PGF2α in treatment III and after both injections of PGF2α in treatment IV. Pregnancy rates at the synchronized service, by 24 days and by 45 days of breeding were not (P>0.05) affected by treatment. Similarly, the treatments had no significant (P>0.05) effect on percentage of animals exhibiting estrus following the first and second injections of PGF2α.

LH response to GnRH infusion in postpartum, fall-lambing ewes subjected to prepartum energy restrictions.

Fall-lambing western range ewes were fed either a high-or low-energy ration the last two months of gestation and were fed protein and energy in excess postpartum. GnRH was infused for 10 hours on day 5 or 26 postpartum. Blood samples were collected at 20-minute intervals during infusion and radioimmunoassayed for LH. Net weight change from day 60 prepartum to day 1 postpartum was 0.67 +/- 1 kg vs. -6.9 +/- 1.5 kg (mean +/- SE) for the high-and low-energy groups, respectively (P < 0.05). Nutritional treatment had no effect on LH response at either day 5 or day 26 postpartum, althoughthe day 26 LH response to GnRH was greater (P < 0.05) than the day 5 LH response. LH response was greater (P < 0.05) in ewes that gained weight prepartum vs. ewes that lost weight prepartum, but only in ewes infused on day 5 postpartum.