Effect of beta-carotene and vitamin A on progesterone production by bovine luteal cells

The relationship between concentrations of plasma vitamin A and c-carotene and corpora lutea was studied using 52 Holstein cows. Bovine luteinizing hormone was added to incubation tubes in doses of 0, 10, or 100 ng/ml. Regression of progesterone secretion by luteal cells in vitro on plasma beta-carotene was positive and significant for corpora lutea collected during the winter months when plasma beta-carotene was low. The two were unrelated during the summer months when beta-carotene was higher. Similar regressions for in vitro progesterone production and vitamin A were not significant in either season. These results suggest that in vivo beta-carotene status is related to bovine luteal function in vitro.     

Relationship of plasma beta-carotene and vitamin A to luteal function in postpartum cattle

The influence of plasma concentrations of beta-carotene and vitamin A on in vivo progesterone production by bovine corpora lutea after gonadotropin-releasing hormone-induced LH release was assessed in 39 postpartum dairy cows. Thirty Holsteins and nine Jerseys were given 100 micrograms gonadotropin-releasing hormone on d 12 of an estrous cycle, which began from 30 to 49 d postpartum. Concentrations of beta-carotene and vitamin A in plasma and progesterone and LH in serum were determined prior to gonadotropin-releasing hormone injection (0 h); serum progesterone and LH concentrations were also determined 1, 2, and 3 h after injection of gonadotropin-releasing hormone. Serum concentrations of progesterone and LH were increased by gonadotropin-releasing hormone. Incremental progesterone production in an analysis of covariance was influenced by breed as well as the interactions of breed with vitamin A, of season with beta-carotene, and of season with vitamin A. The regression coefficients were positive for beta-carotene and negative for vitamin A in all cases. In conclusion, luteal function in the postpartum cow appears to be related to plasma concentrations of beta-carotene and vitamin A.     

Changes of luteinizing hormone and progesterone for dairy cows after gonadotropin-releasing hormone at first postpartum breeding

Gonadotropin-releasing hormone administered at breeding enhances fertility of dairy cows, so a study was designed to evaluate the mechanism for enhanced fertility following administration of gonadotropin-releasing hormone at first postpartum breeding. Twenty-four cows were assigned randomly to one of two treatments, 100 micrograms of gonadotropin-releasing hormone intramuscular or saline vehicle intramuscular at insemination. Blood samples for luteinizing hormone assay were taken at 2-h intervals prior to breeding and .5-h intervals for 3 h after insemination. Composite morning milk samples for progesterone assay were collected for 30 days after insemination or until next estrus. Cows given gonadotropin-releasing hormone had higher luteinizing hormone concentrations in blood serum following treatment than cows given saline, 13.2 versus 3.0 ng/ml. There was no relationship between luteinizing hormone and subsequent conception. Progesterone for cows that became pregnant was higher throughout sampling days. Mean progesterone concentrations were 4.6 versus 2.2 ng/ml in pregnant and nonpregnant cows during the first 4 days after insemination. Cows treated with gonadotropin-releasing hormone that conceived had higher progesterone than other cows, and that was evident at the first 4 days postbreeding.     

Effect of beta-carotene supplementation on reproductive performance of lactating Holstein cows

Fifty-four lactating Holstein cows were assigned by parity and calving date to a 2 X 3 factorial arrangement of a randomized block design to determine effects of beta-carotene supplementation and ration fiber content on reproduction. Because ration fiber treatments did not differ in their effects on reproductive performance, data for beta-carotene analyses were pooled over the three rations. From 3 to 98 d postpartum, cows were individually fed either 0 or 300 mg supplemental beta-carotene daily by adding it on top of their complete mixed ration. By wk 3 postpartum, concentrations of beta-carotene in blood serum were higher in cows fed supplemental beta-carotene and remained higher throughout the experimental period. Feeding supplemental beta-carotene had no effect on the interval from the time of parturition to uterine involution, ovulation, first observed estrus, or conception. Feeding beta-carotene did not influence incidence of cystic follicles based on palpation, ovarian cyclicity based on blood progesterone patterns, peak progesterone concentrations, or first service conception rate. Cows fed supplemental beta-carotene required fewer treatments for clinical mastitis. Reproductive performance in lactating Holstein cows was not improved by feeding supplemental beta-carotene.     

Reproductive and metabolic characteristics of dairy cattle supplemented with beta-carotene

Seventy-eight Holstein cows alternately were assigned at calving to receive beta-carotene supplementation or act as controls to determine effects of beta-carotene on reproduction and carotene, luteinizing hormone, progesterone, insulin, glucose, and glucagon concentrations in blood plasma. Cows were fed a corn silage-based complete ration. Biweekly jugular blood samples were collected beginning the week after parturition through 90 days. At day 30, supplemented cows received 600 mg synthetic beta-carotene daily for 60 days. Plasma carotene reached a peak of 2.45 micrograms/ml compared to 1.50 micrograms/ml in controls. Supplementation significantly increased plasma carotene but had no effect on luteinizing hormone, progesterone, insulin, glucose, glucagon, or reproductive measures. Days to first heat, days to first breeding, days open, and services per conception averaged 74, 74, 95, and 1.7 for supplemented cows and 64, 76, 102, and 1.9 for control cows. Progesterone increased as lactation progressed. Somatic cells were not different between supplemented and control cows. Supplementation of beta-carotene did not improve reproductive efficiency or alter luteinizing hormone, progesterone, insulin, glucose, or glucagon in blood plasma or affect somatic cells in milk.     

Effect of supplemental beta-carotene on incidence and responsiveness of ovarian cysts to hormone treatment

Sixty-two multiparous and 35 primiparous Holstein cows were assigned randomly at 10 days postpartum to receive a ration with or without 300 mg beta-carotene/cow per day. Multiparous and primiparous cows were grouped separately and group-fed complete rations once daily. Incidence of ovarian cysts (26% by rectal palpation) was not affected by beta-carotene fed. Multiparous cows had greater incidence (39%) of ovarian cysts than primiparous cows (11%). Fifty-seven percent of cysts were classified follicular by rectal palpation. Progesterone concentration of milk also was used for diagnosis of type of cyst. Cows with ovarian cysts and with progesterone concentrations in milk less than 1 ng/ml were classified follicular, and those having concentration greater than 1 ng/ml were classified luteal. As determined by milk progesterone, rectal palpation was more accurate for diagnosis of luteal cysts than for diagnosis of follicular cysts. Progesterone concentrations of milk for animals with luteal and follicular cysts were 10.66 +/- 1.29 and .37 +/- .07 ng/ml. beta-Carotene did not affect response or days to respond to treatment with human chorionic gonadotropin or gonadotropin-releasing hormone. Supplemental beta-carotene was not beneficial for reducing incidence of ovarian cysts in cows receiving an adequate supply of beta-carotene in their diet.     

Effect of dietary protein and exogenous gonadotropin-releasing hormone on circulating progesterone concentrations and performance of Holstein cows

Sixty-three Holstein cows were used in a 2 x 2 factorial arrangement of treatments to determine the effects of dietary protein and exogenous gonadotropin-releasing hormone on performance postpartum and on circulating concentrations of progesterone. The experimental diets, 14 or 20% CP (dry basis), were fed from parturition to 14 wk postpartum. Blood serum urea N was elevated in the cows fed the 20% versus 14% CP diet. Feed intake was increased, but yield and composition of milk were not affected by the higher protein diet. Circulating concentrations of progesterone were lower in cows fed 20% than 14% protein on d 12 of both the synchronized and subsequent estrous cycles. Exogenous gonadotropin-releasing hormone administered on d 12 of the preceding estrous cycle did not increase the concentration of progesterone on d 12 of the subsequent estrous cycle. Luteal phase concentrations of progesterone were reduced by high dietary protein, but were not affected by gonadotropin-releasing hormone given on d 12 of the preceding cycle.     

Progesterone responses to human chorionic gonadotropin in dairy cattle supplemented with beta-carotene

Twenty Holstein cows were assigned to one of four groups according to milk production and beta-carotene supplementation to evaluate effects of supplementation and milk production on reproductive performance and response of corpus luteum to human chorionic gonadotropin indicated by release of progesterone and secretion of luteinizing hormone in blood plasma. Cows were divided into groups of high and low production and divided further into beta-carotene supplemented and unsupplemented groups. Blood samples were taken 10 days postestrus at 10-min intervals from 1 h prior to injection of 5000 IU human chorionic gonadotropin until 5 h postinjection. Mean days to first detected estrus were 33, 37, 34, and 36 in supplemented, unsupplemented, high-production, and low-production cows. Treatment with beta-carotene and milk production did not affect days to first breeding (63, 67, 62, and 67), or services per conception (3.2, 5.1, 4.6, and 3.7). However, days open were reduced by supplementation (116 versus 186), but milk production had no effect (173 versus 129). Regressions of progesterone on sampling time within treatment and within milk production were significant. Regression of luteinizing hormone on time within production group was also significant. Neither beta-carotene nor milk production affected most reproductive measures, but differences were subtle in corpus luteum response to human chorionic gonadotropin, which may alter endocrine function.     

Early postpartum reproductive profiles in Holstein cows with retained placenta and uterine discharges

Sixty Holstein cows were allocated to three groups. Twenty cows had retained placenta. The remaining cows were examined on d 14 postpartum and those with purulent discharges (n = 22) were assigned to one group and the remaining (n = 18) to a control group. Within each group, cows were given randomly either gonadotropin-releasing hormone (i.m., 200 micrograms) or saline on d 15 postpartum to evaluate the effect on changes in ovarian structures and plasma progesterone through 50 d postpartum and fertility. Corpora lutea were found in control cows by d 21, cows with uterine discharge by d 28, and cows with retained placenta by d 27. Maximum progesterone production during the first luteal phase was higher in control cows than in cows with purulent discharge or retained placenta (4.66 ng/ml compared with 3.23 and 3.34 ng/ml, respectively). Duration of the first corpus luteum was affected by clinical condition. Only 6.2% of cows with retained placenta had corpora lutea of normal duration (12 to 16 d), whereas 43.8 and 50.0% of cows with uterine discharge and control cows had normal postpartum luteal phases. Measures of fertility were not affected by gonadotropin-releasing hormone. Control cows had less days to conception (97) and fewer services per conception (1.6) than cows with retained placenta (134 and 2.5, respectively). Clinical group affected reproduction more than gonadotropin-releasing hormone did, possibly by altering ovarian function.     

Effects of gonadotropin-releasing hormone and human chorionic gonadotropin on pregnancy rate in dairy cattle

This study was to determine if gonadotropin-releasing hormone, human chorionic gonadotropin, or their interaction would affect pregnancy rate or milk progesterone profiles in dairy cattle. Cows and heifers eligible for first, second, and third services were assigned to four treatments: 1) saline i.m. within 5 min after AI (d 0) and saline 15 d after AI (n = 222 AI); 2) gonadotropin-releasing hormone (100 micrograms) i.m. within 5 min after AI and saline on d 15 (n = 223); 3) saline after AI and human chorionic gonadotropin (3500 IU) i.m. on d 15 (n = 196); 4) gonadotropin-releasing hormone at AI and human chorionic gonadotropin on d 15 (n = 195). Progesterone was quantified in milk samples collected twice weekly for 5 wk from all cows. Neither gonadotropin-releasing hormone, human chorionic gonadotropin, gonadotropin-releasing hormone x human chorionic gonadotropin interaction, age, nor service number affected pregnancy rate, which averaged 46.2%. Pregnancy and human chorionic gonadotropin affected milk progesterone profiles. Pregnancy maintained progesterone concentrations and human chorionic gonadotropin on d 15 increased them. This study does not support the general use of gonadotropin-releasing hormone at AI or human chorionic gonadotropin 15 d after AI as methods for enhancing pregnancy rates of dairy cattle.     

Effect of high crude protein on pituitary and ovarian function in Holstein cows

Intact and ovariectomized cows were used to determine the effect of high amounts of dietary crude protein on pituitary regulation of luteinizing hormone. Estrus was synchronized with prostaglandin F2 alpha in 10 intact dry cows fed 15 or 25% crude protein, and serum luteinizing hormone profiles were evaluated during both follicular and luteal phases of the estrous cycle. Serum progesterone, pituitary luteinizing hormone content, and luteinizing hormone-releasing hormone receptors were also measured. Basal concentrations of serum luteinizing hormone tended to be lower during the follicular phase and were significantly higher during the luteal phase in cows fed 25% crude protein. Serum progesterone was not affected by dietary treatment. In 10 ovariectomized cows fed 24% crude protein, amplitude of luteinizing hormone response to luteinizing hormone-releasing hormone challenge tended to be smaller than that of 8 cows fed 16% crude protein, although total luteinizing hormone released in response to luteinizing hormone releasing hormone challenge was not different between dietary treatments. In both intact and ovariectomized cows, pulsatile luteinizing hormone patterns, pituitary luteinizing hormone content, and luteinizing hormone-releasing hormone receptors were not influenced by treatment. High dietary crude protein did not have a primary effect on luteinizing hormone or progesterone in nonlactating cows.     

Effects of progesterone intravaginal devices on synchronization of estrus in postpartum dairy cows

Effects of two intravaginal treatments of progesterone on the synchronization of behavioral estrus were compared in three groups of lactating dairy cows at 7 to 8 wk postpartum. Group 1 had progesterone-releasing intravaginal devices with attached estradiol benzoate capsules inserted for 12 d. Groups 2 and 3 had controlled internal drug release devices, containing progesterone alone, inserted for 9 or 12 d. Behavioral estrus was detected as early as 24, 32, and 36 h after removal of the devices in groups 1, 2, and 3, respectively. Estrus was better synchronized and the proportion of cows showing estrus after device removal was greater in group 1 than in groups 2 and 3. After removal of devices, mean milk progesterone of all cows that did not show estrus was higher than that of cows that showed estrus. In groups 1, 2, and 3, 1 of 7, 7 of 12, and 4 of 12 cows not showing estrus had high milk progesterone after removal of the devices, indicating the presence of functional corpora lutea. In cows not displaying estrus milk progesterone concentrations decreased, indicating these cows were cyclic, although behavioral estrus was absent.     

Effect of dietary vitamin A and beta-carotene on polymorphonuclear leukocyte and lymphocyte function in dairy cows during the early dry period

Vitamin A and beta-carotene improved mammary health in dairy cows around dry off. To define possible mechanisms, cows were fed 1) 53,000 IU vitamin A, 2) 213,000 IU vitamin A, or 3) 53,000 IU vitamin A plus 400 mg beta-carotene/cow per d (n = 10/treatment) from 6 wk before to 2 wk after dry off. Blood polymorphonuclear neutrophil function (phagocytosis, kill, and chemotaxis) and lymphocyte proliferation were measured at wk -6, 0 (dry off), and 2. Concentrations of vitamin A in serum did not differ across vitamin treatments. beta-Carotene in serum was elevated in cows fed beta-carotene. Treatment did not influence phagocytosis or kill. Kill ability increased after dry off in all treatment groups, but phagocytosis tended to decrease after dry off in cows fed vitamin A only. Lymphocyte blastogenesis stimulated by concanavalin A on wk 2 for cows fed 53,000 IU vitamin A but did not vary in the other two groups. Lipopolysaccharide-stimulated blastogenesis peaked at wk 0 and then decreased to pretreatment values by wk 2 in cows fed 213,000 IU vitamin A. These data indicate lymphocyte function is influenced by vitamin A supplementation and that beta-carotene supplementation seems to exert a stabilizing effect on neutrophil and lymphocyte function during the period around dry off.     

Stimulation of luteal cell progesterone production by lipoproteins from cows fed control or fat-supplemented diets.

Plasma lipoproteins from lactating dairy cows fed 0 or 7% supplemental fat were examined for their composition and ability to stimulate luteal cell progesterone production in vitro. Ultracentrifugation was utilized to isolate blood lipoproteins, and heparin affinity chromatography allowed separation of lipoprotein fractions based on the presence (low density lipoproteins) or absence of apolipoprotein B (high density lipoproteins). A portion of high density lipoproteins was fractionated by size, utilizing gel filtration chromatography. Slaughterhouse corpora lutea were dissociated, and plasma lipoproteins were added to the luteal cells on d 3 of culture and incubated for 48 h. In Experiment 1, blood was collected from heifers fed a diet that was not supplemented with fat. The addition of cholesterol from large, high density lipoproteins with a high cholesterol to protein ratio to luteal cultures increased progesterone production by an average of 17% compared with the addition of cholesterol from small, high density lipoproteins with a low cholesterol to protein ratio. In Experiment 2, electrophoretic mobility, apolipoprotein composition, and size of lipoproteins from control and fat-supplemented cows were similar. Lipoproteins from cows assigned to either a control or fat-supplemented diet showed no difference in their ability to stimulate progesterone production. Increased plasma progesterone concentration in lactating dairy cows fed supplemental fat does not appear to be mediated by alterations in lipoprotein composition.     

Effect of niacin supplementation on milk production and ketosis of dairy cattle

In two experiments with Holstein cows effects of dietary supplementation of niacin to periparturient cattle on subsequent milk production and ketosis were examined. In Experiment 1, 20 cows were assigned to two groups. The treatment group received niacin at 6 g per head per day for 2 wk prepartum and 12 g per head per day for 4 wk postpartum. The control group received no niacin. In Experiment 2, 40 cows were assigned to four groups of 10 cows. One group served as the control and the other three received 3, 6, or 12 g niacin per head per day for 10 wk postpartum. In both experiments niacin-supplemented groups produced slightly more milk than controls. There were no consistent changes of milk composition with treatment. Glucose concentration in blood serum was higher for cows receiving niacin. Concentrations of beta-hydroxybutyrate and nonesterified fatty acids in blood and plasma were generally lower for niacin-supplemented groups. Niacin supplementation had no effect on feed intake or body weight changes. In Experiment 1 there was a significant reduction of niacin concentration in red blood cells of postpartum cows not receiving supplemental niacin. Insulin concentration in blood serum in Experiment 2 was unaffected by niacin supplementation.     

Dynamics of gonadotropin release following ovariectomy and injection of gonadotropin-releasing hormone in early postpartum dairy cows

The influence of the ovary and of injection of low doses of gonadotropin-releasing hormone on secretion and gonadotropin-releasing hormone-induced release of gonadotropins in early postpartum cows was examined in a 2 x 2 factorial design (6 cows/group). Twelve cows were ovariectomized 2 d postpartum (calving = d 0; groups 1 and 2) and 12 cows were left intact (groups 3 and 4). Groups 1 and 3 received gonadotropin-releasing hormone (5 micrograms) every 120 min for 70 h from d 6 to d 9; groups 2 and 4 received saline by the same regimen. All animals received 50 micrograms gonadotropin-releasing hormone on d 9. Mean concentrations of FSH were higher following ovariectomy compared with those of intact cows 6 to 8 d postpartum, whereas no differences in FSH were observed between cows receiving gonadotropin-releasing hormone or saline during this period. Concentrations of LH were similar in intact and ovariectomized cows on d 6 to 8 but were increased in cows receiving gonadotropin-releasing hormone compared with cows receiving saline. Ovariectomy did not alter the gonadotropin-releasing hormone-induced release of gonadotropins on d 9. Injection of gonadotropin-releasing hormone on d 6 to 8 resulted in a decreased gonadotropin-releasing hormone-induced release of gonadotropins on d 9. It was concluded that removal of ovarian influence resulted in increased secretion of FSH but did not affect gonadotropin-releasing hormone-induced release of LH and FSH during the early postpartum period.     

Effect of adrenocorticotropin and cortisol on luteinizing hormone surge and estrous behavior of cows

In six cows, twice daily administration of 100 IU corticotropin for 3.5 days during the follicular phase delayed the preovulatory luteinizing hormone surge and onset of behavioral estrus. Corticotropin increased progesterone and decreased estradiol and basal luteinizing hormone concentrations of blood. Following corticotropin withdrawal, a shortened period of behavioral estrus (50% of control) was accompanied by an apparent luteinizing hormone surge and ovulation. To ascertain if these effects were caused by the elevated corticosteroid concentrations induced by corticotropin, we infused intravenously four heifers with cortisol sodium succinate. During this infusion, luteinizing hormone surge and estrous behavior were inhibited; however, estradiol and basal luteinizing hormone concentrations were not affected. Furthermore, there was no luteinizing hormone surge, ovulation, or behavioral estrus after cessation of cortisol treatment. These results are consistent with a role for corticosteroids in mediating inhibitory effects on reproduction produced by corticotropin administration.     

Peripartum changes of plasma and milk vitamin A and beta-carotene among dairy cows with or without mastitis

Over 12 mo we studied the relationship between peripartum concentrations of vitamin A and beta-carotene in blood plasma and milk of 93 Holsteins with or without subsequent mastitis. Blood was sampled daily from 7 days prepartum through 7 days postpartum and on alternate weeks through wk 10 of lactation. Milk samples were collected daily for 7 days postpartum and then biweekly for 10 wk. Somatic cell counts were on biweekly milk samples. Vitamin A and beta-carotene of blood plasma decreased rapidly prepartum to reach minimum concentrations at calving (vitamin A) or on day 4 to 6 postpartum (beta-carotene). Thereafter, both vitamin A and beta-carotene increased rapidly through 10 wk postpartum. Concentrations of vitamin A and beta-carotene in colostrum were higher than concentrations in milk. Cows with mastitis (somatic cells greater than 500,000 cells/ml milk) had lower vitamin A in blood plasma during days 0 to 7 and wk 2 and 4 postpartum than cows without mastitis. When data were analyzed with loge of somatic cell count as an independent regression variable, results were similar. In contrast to vitamin A, peripartum beta-carotene in blood plasma was higher among mastitic cows and was related to higher loge of somatic cell count. No significant difference was observed between mastitic and non-mastitic cows for vitamin A and beta-carotene in milk. Lower concentrations of plasma vitamin A and higher concentrations of beta-carotene during the immediate postpartum period were associated with higher milk somatic cell counts among dairy cows during lactation.     

Effect of duration and level of supplementation of diets of lactating dairy cows with selenized yeast on selenium concentrations in milk and blood after the withdrawal of supplementation

Cows’ milk containing elevated concentrations of Se provides a rich nutritional source of this essential element for meeting daily nutritional requirements or providing health benefits in humans with low immune function or at risk of cancer. An experiment involving either 2 or 6 wk of dietary supplementation with Se yeast (with the yeast supplying about 30, 40, and 60 mg of Se/d for cows supplemented for 2 wk, and about 20, 30, 40, and 60 mg of Se/d for cows supplemented for 6 wk), and 21 wk of monitoring of Se status after the withdrawal of supplementation, was undertaken between September 2008 and April 2009 using 35 multiparous Holstein-Friesian cows. Using milk and blood Se concentrations as surrogates, the research examined the time taken for Se build-up in tissue due to supplementation of lactating dairy cows with Se yeast to dissipate back to normal levels. At the end of Se supplementation, a significant relationship was found between milk Se concentration and Se intake, whereby milk Se concentration had increased by 4.5 μg of Se/kg of milk for each mg of Se eaten per day, but no effect of duration of supplementation on this relationship was observed. At the same time, both Se intake and duration of supplementation affected blood Se concentration; it increased by 3.6 μg of Se/kg of blood for each mg of Se eaten per day, and was 86 μg of Se/kg higher after 6 wk compared with 2 wk of supplementation. After the withdrawal of Se supplementation, milk Se concentrations responded quickly to the change in the quantity of Se consumed, and again, duration of supplementation had no effect on the response, but any effect that Se intake had on milk Se had completely dissipated by 4 wk. In contrast to milk, blood Se concentrations continued to be affected by both amount and duration of Se supplementation for at least 4 mo after the withdrawal of supplementation, although by 5 mo the effects of the previous supplementation treatments had virtually disappeared. The slow decline in blood Se concentrations after the withdrawal of supplementation would most likely be due to the protracted clearance of Se from the various tissues that had accumulated Se during supplementation and the rate of erythrocyte turnover. When undertaking an on-farm Se enhancement program to generate milk for the manufacture of Se-enriched milk products, post-supplementation milk Se concentrations are unlikely to create any problems at the milk factory beyond 4 wk, but the high residual blood/tissue Se concentrations that take considerably more time to dissipate may provide the potential for possible unintended consequences at the food chain/farm environment level.     

Gonadotropin-releasing hormone and conception of Holsteins

Effects of gonadotropin-releasing hormone (100 micrograms) and time of artificial insemination on fertility, were examined in lactating dairy cows at first, second, and third services. Inseminations were either soon after detected estrus (0 h) or 12 h later, and cows were given gonadotropin-releasing hormone or saline after inseminations. Conception at first service was not improved by hormone treatment. But conception rates at repeat services were improved by nearly 21% when cows were treated with gonadotropin-releasing hormone within 30 s after insemination. Inseminations soon after detection of estrus (0 h) followed by hormone treatment resulted in the highest conception rates for all services. Time of the insemination (0 versus 12 h) had no effect on conception. Administering gonadotropin-releasing hormone at repeat services should improve conception rate of lactating dairy cows.