Control of estrus in bos indicus and bos taurus heifers with prostaglandin F2 alpha

Lutalyse^*, a commercial preparation of prostaglandin F_2a was injected intramuscularly into nulliparous heifers of the German Brown, Holstein—Friesian and White Fulani cattle. Two injections, 25mg/animal each time, were given at intervals of 12 days. All the heifers were inseminated 80h after the second injection. Heifers that were cycling responded well to the treatment and had a higher conception rate than the controls.     

Seasonal LH profile in ovariectomized cattle

Two experiments were conducted to examine seasonal changes in circulating LH concentrations in ovariectomized heifers. In experiment 1, four Holstein heifers were ovariectomized in April 1977 during middiestrus. Blood samples were collected daily for 30 days surrounding each equinox and solstice for one year to examine changes in plasma LH levels at the time of seasonal photoperiod changes. The LH concentrations were highest during the winter solstice period and lowest during the summer solstice period. In addition, samples taken at two-week intervals indicated a distinct LH profile with maximal LH concentrations during November-April and minimal concentrations during May-October. In experiment 2, eight Holstein heifers were ovariectomized in June-July, 1979 and given an estradiol or a control implant in October. A distinct LH profile for the interval extending from January, 1980 to February, 1981 was found in the heifers that were not treated with estradiol. Concentrations were maximal during December-April and minimal during May-November. The LH profile followed a similar pattern in the estradiol-treated heifers; however, the overall profile was at a higher level. These data indicate that underlying seasonal reproductive mechanisms are present in cattle even though the species ovulates and breeds throughout the year.     

Thermoregulatory responses of Holstein and Brown Swiss Heat-Stressed dairy cows to two different cooling systems

Thirty-seven Holstein and 26 Brown Swiss dairy cows were used to evaluate the effect of two different cooling systems on physiological and hormonal responses during the summer. A control group of cows had access only to shade (C). A second group was cooled with spray and fans (S/F) and the third group was under an evaporative cooling system called Korral Kool (KK). The maximum temperature humidity index during the trial was from 73 to 85. Rectal temperatures and respiration rates of the C group were higher (P < 0.05) than those of the S/F and KK groups in both Holstein and Brown Swiss cows. Triiodothyronine levels in milk were higher (P < 0.05) in the KK group than in the S/F and C groups, while cortisol levels were lower (P < 0.05) in the C group than in S/F and KK. There was no significant difference in the hormonal response of the two breeds. These results demonstrate that both cooling systems may be used increase the comfort of Holstein and Brown Swiss cows during summer in hot, dry climates.     

Plasma progesterone concentration in Bos taurus and Bos indicus heifers

Progesterone concentrations were determined by radioimmunoassay in the plasma of heifers of German Brown Swiss, Holstein-Friesian and White Fulani cattle during two consecutive estrous cycles in the dry and wet seasons in a tropical climate. Seasonal differences in the levels were not significant but peak levels were significantly higher in the $\text{Bos}$$\text{taurus}$ than in the $\text{Bos}$$\text{indicus}$ heifers. The low levels at estrus and the gradual rise five days after estrus were similar among the heifers. The levels on the 1st and 2nd days preceding estrus were significantly higher in the $\text{Bos}$$\text{taurus}$ than in the $\text{Bos}$$\text{indicus}$ heifers, and suggest a possible tendency for the former to be more susceptible to infertility arising from high progesterone levels 1 and 2 days preceding estrus and insemination. The genital tract and ovaries at the mature luteal phase were generally smaller in the $\text{Bos}$$\text{indicus}$ than in the $\text{Bos}$$\text{taurus}$ heifers.     

Genetic diversity among Canadienne, Brown Swiss, Holstein, and Jersey cattle of Canada based on 15 bovine microsatellite markers.

The genetic diversity among Canadienne, Brown Swiss, Holstein, and Jersey cattle was estimated from relationships determined by genotyping 20 distantly related animals in each breed for 15 microsatellites located on separate chromosomes. The Canadienne, Holstein, and Jersey cattle had an average of six alleles per loci compared with five alleles for Brown Swiss. Furthermore, a number of potentially breed-specific alleles were identified. The allele size variance among breeds was similar, but varied considerably among loci. All of the loci studied were equally heterozygous, as were Brown Swiss, Canadienne, and Holstein cattle (0.68-0.69) whereas Jersey cattle showed lower heterozygosity (0.59). The within-breed estimates of genetic distance were greater than zero and significant. The genetic distance between Canadienne and Holstein (0.156), Brown Swiss (0.243), and Jersey (0.235) was negligible, suggesting close relationship. Concurrently, Brown Swiss and Holstein (0.211) cattle also demonstrated close relationship. In contrast, the Jersey breed was genetically distant from the Brown Swiss and Holstein cattle (0.427 and 0.320, respectively). The characterization of Canadienne cattle, as part of the genetic resource conservation effort currently underway in Canada, underscores the difficulty in scientifically establishing unique breeds. Therefore, the need to consider all relevant morphological characteristics and production performance in combination with available cultural, historical, pedigree, and molecular information becomes relevant when identifying breeds for conservation.     

A pilot study on the foraging behaviour of heifers in intensive silvopastoral and monoculture systems in the tropics

Intensive silvopastoral systems (ISS) are a sustainable alternative to monoculture systems (MS). The presence of trees and legumes improves animal welfare due to the increased food quality and quantity and the presence of shade while providing a variety of environmental services. As cattle behaviour is greatly affected by environmental conditions, knowledge on the behavioural trade-offs that cattle make to meet their demands while foraging in different grazing systems is important, as this will help us understand the perceived advantages of ISS. This pilot study assessed the behaviour of heifers in an ISS (n=8 heifers) and MS (n=8 heifers) in the Mexican tropics during the dry and rainy seasons, and its relationship with forage availability, mean travelled distance and the temperature humidity index (THI). In both seasons, daily foraging times were longer in the MS than the ISS (P<0.01). The duration of rumination was higher for ISS (P<0.01) and the duration of lying was higher for the dry season (P<0.05). The decrease in foraging times in relation to THI was significantly higher in the ISS than in the MS (mean slope±SE: ISS=−4.64±0.34; MS=−2.34±0.22; t=−14.20, P<0.001). The results suggest that the forage availability and access to shade in the ISS allow cattle to rest longer and increase rumination, whereas cattle in MS spend more time searching for food and foraging at times of the day were the temperatures were higher as a compensatory strategy, which potentially decreases cattle’s welfare and production qualities when compared with the ISS. In conclusion, ISS are likely to generate positive behavioural trade-offs that result in better welfare conditions and higher productive potential.     

Effect of clitoral stimulation after artificial insemination on conception in Zebu-crossbred heifers in the tropics

In order to increase conception rates to AI, an experiment was conducted to determine the effect of clitoral stimulation on pregnancy rate. Seven hundred and thrity-five Zebu-cross heifers were assigned to either of the following treatments: 1) control without clitoral stimulation (n=351) or 2) clitoral stimulation after IA (n=384). The data were analyzed using a linear model procedure. Clitoral stimulation increased pregnancy rates (57 vs 45 % for treated and control groups, respectively; P<0.02). Holstein x Zebu and Brown Swiss x Zebu heifers attained similar rates (P>0.05). There was a significant treatment-by-season interaction (P<0.01), with pregnancy rates being higher using clitoral massage during the rainy season (54.7 and 25.5% for the treated and control group respectively). In this study, manual clitoral stimulation increased conception rates in Zebu cross heifers under tropical conditions.     

The relative influence of temperature and humidity on cutaneous function in Bos indicus and Bos taurus females

The relative importance of dry- and wet-bulb temperatures on cutaneous function inBos indicus andBos taurus females under humid tropical climatic conditions was evaluated. The parameters investigated were sweating rate and skin temperature, while the species utilised were zebu White Fulani (Bos indicus) and German Brown and German Black and White (Bos taurus).The sweating rate, irrespective of breed, differed with the site of sampling and was more influenced by dry-bulb (59%) than by wet-bulb temperature (41%). Skin temperature responded more to wet-bulb temperature in White Fulani and German Black and White cattle, but not in German Brown cattle.It is concluded that the response of the animals, with respect to sweating, was similar but that the efficiency of sweating, judged by the lowering of skin temperature, was higher inBos indicus than inBos taurus. This, in part, may explain the higher degree of comfort demonstrated byBos indicus under tropical conditions.     

Quantitation of glucocorticoid receptors in bovine skeletal muscle: Topographical distribution, sex effect and breed comparisons

The concentration of the cytosolic glucocorticoid receptor (GR) was determined in skeletal muscles of calves in order to study possible differences in individual muscles from different parts of the body as well as the influence of sex and breed. In male and female Simmental calves the topographical distribution of GR was similar: the lowest concentrations were seen in abdominal muscle, whereas in neck, shoulder and hindleg the GR concentrations were higher; this difference was more pronounced in male than in female calves. In general, female calves had about 2-fold higher GR concentrations than males. The cytosolic cortisol concentrations were differing neither between individual muscles nor between sexes. The cortisol secretion during a 24-h sampling period 1 week prior to slaughter showed no sex difference. GR concentrations in neck muscle of female calves of four different German cattle breeds (Holstein Friesian, Brown Swiss, Simmental and German Gelbvieh) were rather similar; however, when Brown Swiss with the highest GR levels were compared to Holstein Friesian calves with the lowest concentrations, a significant difference was evident (P < 0.05).     

Effects of calf breed on milk production and other economic traits of Holstein dams

Production of purebred or crossbred feeder calves for beef, especially HolsteinxJapanese-Black (HxJB) and Japanese-Black (JB), from dairy cattle using artificial insemination or embryo transfer have been used widely in Japan. However, dairy farmers feel uneasy about the effects of calf breed on the economic traits of dams. In this study, those effects were investigated in 798 Holstein heifers bearing Holstein, HxJB, JB or other breed calves. The results of the least-squares ANOVA indicated the effects of calf breed to be significant for gestation length (P<0.01) and calf birth weight (P<0.01) but not for milk yield, fat yield, protein yield, peak yield, day of peak, number of artificial inseminations per pregnancy or days open. Frequency of dystocia was lower in dams bearing HxJB calves than in those bearing Holstein calves (P<0.05). There were no significant differences for frequencies of still birth, retained placenta and subsequent pregnancy. The present data suggest that the effects of calf breed do not place a serious problem on the economic traits of Holstein dams. In conclusion, it is indicated that the production of JB and HxJB feeder calves from Holstein dams does not result in a decrease in dam productivity.     

Influence of season and microclimate on fertility of dairy cows in a hot-arid environment

Records were obtained over a 3 year period from six Holstein dairy farms of 300 to 500 cows each in the Phoenix, Ariz. area. Dairies were selected on the basis of similar management practices, herd size, milk production and facilities (with the exception of cooling systems). Microclimatic modifications (two dairies each) were shade only (approximately 3.7 m²/cow), evaporative-cooled shades and low-pressure water foggers under the shades. Data were categorized by season of calving (spring, Feb.–May; summer, June–Sept.; and fall, Oct.–Jan.). Traits evaluated were calving interval, days open and services/conception. Calving interval was shortest for cows calving in the spring (378 days), intermediate in fall (382 days) and longest in summer (396 days). Similar seasonal trends were observed for days open (103, 103 and 119 days, respectively) and services/conception (1.54, 1.81 and 1.93, respectively). All differences between spring and summer were significant (P < 0.05). Calving interval and days open were less for evaporative-cooled groups (374 and 98 days, respectively), with no difference between shade only and foggers (391 and 392 days, 112 and 116 days, respectively). Services/conception were similar for all groups (1.72 to 1.79). A significant interaction between microclimate and season for services/conception could be interpreted as (i) smaller season differences for evaporative-cooled groups than for shade or foggers, or (ii) a change in the ranking of control and fogger groups during summer versus fall. Evaporative cooling was more effective than fogging for reducing the detrimental effects of seasonal high temperatures on fertility.     

Influence of summer heat stress on pregnancy rates of lactating dairy cattle following embryo transfer or artificial insemination

Lactating Holstein cows were used to determine if pregnancy rate from embryo transfer (n = 113) differed from contemporary control cows (n = 524) that were artificially inseminated (AI). Holstein heifers (n = 55) were superovulated with FSH-P (32 mg total) and inseminated artificially during estrus and subsequently managed under shade structures. On Day 7 post estrus, embryos were recovered, and primarily excellent to good quality embryos (90.3%) were transferred to estrus-synchronized lactating cows. Cows were managed under conditions of exposure to summer heat stress. Pregnancy status was determined by milk progesterone concentrations at Day 21 and palpation per rectum at 45 to 60 d post estrus. Pregnancy rates of cows presented for AI (Day 21, 18.0%; Days 45 to 60, 13.5%) were typical for lactating cows inseminated during periods of summer heat stress in Florida. Pregnancy rates of embryo recipient cows were higher (P<0.001) than those of control cows (Day 21, 47.6%; Days 45 to 60, 29.2%). Summer heat stress had no adverse effect on heifer superovulatory response, but it increased (P<0.05) the incidence of retarded embryos (相似文献   

Intake,performance, and feeding behavior of Holstein and Holstein × Gyr heifers grazing intensively managed tropical grasses during the rainy season

Holstein × Gyr and Holstein are the primary dairy breeds used in tropical systems, but when rearing under pasture, feed intake, behavior, and performance might differ between them. This study aimed to evaluate the voluntary intake, nutrient digestibility, performance, and ingestive behavior of Holstein and Holstein × Gyr (½ Holstein × ½ Gyr) heifers managed in a rotational system of Guinea grass (Panicum maximum Jacq. cv. Mombaça). The experiment was conducted during the summer season throughout four periods of 21 d. Two 8-heifers (four Holstein and four Holstein × Gyr) groups, averaging 258.6 ± 24.79 kg and 157.1 ± 24.99 kg BW, were used. Each group grazed a separate set of 16 paddocks, and all heifers received a concentrate supplement daily. Heifers were weighed at the beginning and end of the experiment. Fecal, forage and concentrate samples were evaluated for their DM, CP, crude fat, ash, NDF, and indigestible NDF. Feeding behavior was evaluated through 24 h of live observation for 48 h of each experimental period. Grazing, ruminating, resting, and intake of concentrate times were recorded, and rumination criteria, bout criteria, mealtime, meal frequency, and meal duration were calculated. There was no difference in total dry matter intake (DMI), but forage DMI of Holstein × Gyr was 11.70 % greater than the Holstein heifers. The Holstein × Gyr heifers had greater NDF intake and feed efficiency tended to show greater CP and NDF digestibilities, consequently, they had greater average daily gain (ADG). Holstein grazed less than Holstein × Gyr heifers in the afternoon. Ruminating time was 18.43 % lower for Holstein than Holstein × Gyr heifers, and rumination criteria (i.e. longest non-feeding interval within a rumination event) were greater for Holstein heifers. Holstein heifers presented more prolonged rumination bouts and resting time than Holstein × Gyr heifers. Holstein × Gyr can ingest and ruminate greater amounts of fibrous material, and Holstein heifers needed to spend more time ruminating the cud. Overall, even though the behavior was not markedly different between breeds, rearing young Holstein heifers in tropical pasture conditions is less suitable than Holstein-Gyr because of their lower ADG. Therefore, this management condition seems appropriate for Holstein × Gyr but inappropriate for Holstein dairy heifers.     

Grazing, social and comfort behaviour of Ankole and crossbred (Ankole × Holstein) heifers on pasture in south western Uganda

The aim of this study was to assess the grazing, social and comfort behaviour of the indigenous purebred Ankole cattle breed and crossbred (Holstein × Ankole) animals under typical management conditions in south western Uganda. Twelve focal animals in each of four groups (two groups per genotype) were observed regarding their grazing, social and comfort behaviour on pasture.No significant differences in grazing behaviour patterns (eating, walking, standing) were found between the genotypes. Resting occurred only very rarely in both genotypes. Walking distances of Ankole and Ankole × Holstein crosses were also similar. There was no difference in the occurrence of agonistic interactions between the two genotypes. However, Ankole cattle engaged in more non-agonistic social interactions than their crossbred counterparts. Individual distances were lower in Ankole heifers and more herd mates were found within a radius of 5 m around the Ankole animals. The most important comfort behaviour pattern in both genotypes was self-licking, which occurred to similar frequency in Ankole and crossbred heifer groups. Crossbred animals scratched themselves and rubbed on objects more often than Ankole heifers.Although Ankole cattle and their Holstein crosses did not differ in grazing, distances walked and agonistic behaviours, the significant differences between the two genotypes in herd cohesion and comfort behaviour may pose challenges on the management of crossbred animals under extensive open grazing conditions as present in south western Uganda. Thus, apart from (re)productive performance traits, behavioural traits of both genotypes may also be taken into account for breeding decisions and management under current production conditions.     

Effects of shade on physiological changes,oxidative stress,and total antioxidant power in Thai Brahman cattle

The purpose of this study was to assess the effects of artificial shade, tree shade, and no shade on physiological changes, oxidative stress, and total antioxidant power in Thai Brahman cattle. Twenty-one cattle were divided into three groups: cattle maintained under artificial shade, under tree shade, and without shade. On days 1, 7, 14, 21, and 28 of the experimental period, after the cattle were set in individual stalls for 2 h, physiological changes, thiobarbituric acid reactive substances (TBARS), and total antioxidant power were investigated. The results revealed that the respiratory rate, heart rate, sweat rate and the neutrophil/lymphocyte ratio of the no-shade cattle were significantly higher than those of cattle maintained under artificial shade and tree shade (P < 0.05). During the early period of heat exposure, the total antioxidant power of the no-shade cattle was lower than those of cattle maintained under artificial shade and tree shade, but the total antioxidant power of cattle maintained under artificial shade and tree shade were not different (P > 0.05). However, rectal temperature and packed cell volume of the cattle in all groups did not differ (P > 0.05). These results showed that artificial shade and tree shade can protect cattle from sunlight compared to no shade, and that the effectiveness of tree shade for sunlight protection is at an intermediate level.     

Two-spotted spider mite reared on resistant eggplant affects consumption rate and life table parameters of its predator,Typhlodromus bagdasarjani (Acari: Phytoseiidae)

The blue tick, Rhipicephalus microplus, threatens cattle production in most tropical and subtropical areas of the world. Delayed skin hypersensitivity reactions are thought to cause Nguni cattle to be more resistant to R. microplus than Bonsmara cattle yet the cellular mechanisms responsible for these differences have not been classified. Tick counts and inflammatory cell infiltrates in skin biopsies from feeding sites of adult R. microplus ticks were determined in 9-month-old Nguni and Bonsmara heifers to determine the cellular mechanisms responsible for tick immunity. Nguni heifers (1.7 ± 0.03) had lower (P < 0.05) tick counts than the Bonsmaras (2.0 ± 0.03). Parasitized sites in Nguni heifers had higher counts of basophils, mast and mononuclear cells than those in the Bonsmara heifers. Conversely, parasitized sites in Nguni heifers had lower neutrophil and eosinophil counts than those in the Bonsmara heifers. Tick count was negatively correlated with basophil and mast cell counts and positively correlated with eosinophil counts in both breeds. In the Bonsmara breed, tick count was positively correlated with mononuclear cell counts. Cellular responses to adult R. microplus infestations were different and correlated with differences in tick resistance in Nguni and Bonsmara cattle breeds. It is essential to further characterise the molecular composition of the inflammatory infiltrate elicited by adult R. microplus infestation to fully comprehend immunity to ticks in cattle.     

The use of embryo transfer to produce pregnancies in repeat-breeding dairy cattle

Repeat breeding is an important factor affecting economic success in dairy management. The objective of this study was to investigate the effectiveness of transfer of frozen-thawed IVF embryos in establishing pregnancy in repeat-breeding Holstein cattle. Cumulus oocyte complexes were collected by aspiration of 2-5 mm follicles from ovaries obtained at two local abattoirs. After IVF, days 7 and 8 blastocysts were frozen either in 1.5M ethylene glycol with 0.1M sucrose, or in 1.4M glycerol with 0.1M sucrose. Holstein recipients (122 heifers and 410 cows) included those that had not conceived after 3-21 inseminations. Embryos frozen in ethylene glycol were transferred directly, and embryos frozen in glycerol were transferred after dilution of the cryoprotectant in sucrose into recipients 7 or 8 days after estrus (without-AI group), or following AI (with-AI group). Pregnancy rates were compared by the Chi-square test. Significantly higher pregnancy rates were achieved by embryo transfer following AI (with-AI group) than by embryo transfer alone (without-AI group) in both heifers (49.2 and 29.5%, respectively) and cows (41.5 and 20.4%, respectively; P<0.05). Pregnancy rates were not significantly different between heifers and cows. However, pregnancy rate decreased as the number of inseminations prior to embryo transfer increased in the with-AI group, but not in the without-AI group. Therefore, transfer of frozen-thawed IVF embryos during the same cycle in which AI was done improved pregnancy rates in repeat-breeding Holstein heifers and cows, and suggested that embryo transfer is an alternative in the treatment of repeat breeding.     

Influence of season on sexual development in heifers: age at puberty as related to growth and serum concentrations of gonadotropins, prolactin, thyroxine and progesterone

An experiment was done to test the hypothesis that seasonal changes in environment during the first and second 6 months of life influence age at puberty in heifers. Twenty-eight Angus X Holstein heifers, born in March (M) or September (S), were reared under natural conditions until 6 months of age. From 6 to 12 months of age, heifers were reared in environmental chambers programmed to simulate seasonal changes in temperature and photoperiod characteristic of spring, summer and early autumn (Sp-F chamber) or autumn, winter and early spring (F-Sp chamber). S were younger (P less than 0.06) at puberty than M, and Sp-F were younger (P less than 0.08) than F-Sp for both M and S. Mean ages at puberty were 295 for S, Sp-F; 319 for S, F-Sp; 321 for M, Sp-F and 346 days for M, F-Sp. Average daily gain (ADG) between 6 and 9 months of age [1.03 kg/day (S) vs. 0.91 kg/day (M)] and mean concentrations of serum luteinizing hormone (LH) between 6 and 7 months of age [3.45 ng/ml (S) vs. 0.47 ng/ml (M)] were greater (P less than 0.01) for S than M, suggesting an association between these traits and date of birth effects on age at puberty. Differences in these traits did not seem to be involved in the chamber effect on age at puberty, since ADG from 6-9 months of age was greater (P less than 0.05) for F-Sp heifers and chamber did not generally affect LH concentrations. Serum concentrations of follicle-stimulating hormone (FSH) were not significantly influenced by month of birth or chamber, but concentrations tended to decrease with age. Serum concentrations of thyroxine (T4) were higher in M than S at 6 months of age (7.8 micrograms/dl vs. 6.3 micrograms/dl) but not at other times, and chamber did not have a significant affect. Prolactin (Prl) concentrations paralleled patterns of temperature and day length and did not appear to be related to age. Although cattle are not seasonal breeders, these results demonstrate that season of birth and season of attainment of puberty influence age at puberty in heifers. Season may have influenced age at puberty by affecting serum concentrations of LH or Prl, or growth rate.     

Risk factors for stillbirth in Holstein heifers under field conditions in France: a prospective survey

Risk factors for stillbirth, defined as birth of a dead calf or a calf dead within 24 h after parturition, were studied in Holstein heifers. Data came from an overall 4-yr prospective survey conducted in French dairy herds. Only heifers that delivered a single calf were included in the study. The stillbirth incidence was 6.9%. The predictive indicators of stillbirth risk were: gestation length, prepartum body condition and dirtiness scores, biochemical and hematological blood parameters measured during the last 2 mo of gestation, and calving conditions. Multiple logistic regressions were run using herd, calving year, calving season, blood sampling-to-parturition interval, and body scoring-to-parturition interval as the fixed effects. The results, expressed as the ratio of the odds of disease occurrence in the exposed and non-exposed subgroups (OR), indicated that dystocia and a body condition score (BCS) higher than 4 before calving were significant risk factors for stillbirth (OR=14.6, P<0.0001 and OR=2.98, P<0.05, respectively). Prepartum circulating neutrophil counts higher than 1950/mm3 (OR=0.50, P<0.05) were associated with a lower risk of stillbirth. A higher occurrence of placental retention, lower fertility and a lower 305-d milk yield were significant consequences of stillbirth.     

Serum progesterone concentrations in pregnant and nonpregnant heifers and after gonadotropin releasing hormone in luteal phase heifers

Serum progesterone (P₄) concentrations were quantitated in 18 Holstein heifers from days 5 to 16 after estrus in an effort to ascertain the effects of pregnancy on circulating levels of this hormone. The P₄ concentration rose faster between days 5 and 10 in the pregnant heifers compared to P₄ levels in both the non-pregnant heifers and the killed sperm inseminated group. It was found that serum P₄ levels were significantly (P<.002) elevated across the entire interval in the pregnant heifers compared to the other experimental groups. It was concluded that pregnancy had a stimulatory effect on P₄ levels and that pregnancy failure is not simply due to the inability of corpora lutea to secrete sufficient P₄.The administration of 250μg gonadotropin releasing hormone (GnRH) to 10 Holstein heifers on day 7 after estrus resulted in a significant (P<.025) elevation of serum luteinizing hormone concentrations from 15 min to 3 h postinjection. Similarly this treatment resulted in a significant (P<.05) elevation of serum P₄ levels from .5 to 4 h post-injection. It is concluded that GnRH at this dosage has a transient effect on P₄ levels in luteal phase heifers.