Reproductive performance of lactating dairy cows treated with cloprostenol at the time of insemination

The effect of intravenous cloprostenol treatment at the time of insemination on reproductive performance was consecutively evaluated in three different subpopulations of high producing lactating dairy cows: Study (1) early postpartum synchronized and fixed-time inseminated (about 50 days in milk) cows (n = 379: 187 control and 192 treated cows); Study (2) presumed high fertility cows first inseminated between 90 and 120 days postpartum (n = 248: 124 control and 124 treated cows); and Study (3) heat stressed repeat breeder cows (n = 183: 93 control and 90 treated cows). Data were analyzed using multiple regression methods. Study 1: Parity (primiparous versus multiparous), milk production, body condition score at AI, insemination season (cool versus warm period) and treatment were included in the analysis as potential factors affecting ovulation, double ovulation, return to estrus, and pregnancy to first AI and to second AI (first AI plus return AI) rates. Logistic regression analysis indicated that the final model for ovulation rate only included the interaction (P = 0.002) between insemination season and treatment. Cloprostenol treatment at insemination led to a 4.2-fold increase in the ovulation rate in cows inseminated during the warm period. There were no significant effects of treatment, parity, milk production, body score or the insemination season on the return to estrus rate. The only variables included in the final logistic model for double ovulation and pregnancy to first AI rates were treatment and season, respectively. Treatment led to a 2.6-fold increase (P = 0.001) in the double ovulation rate, whereas cows inseminated in the warm period were 2.1 times less likely (P = 0.007) to become pregnant at first AI compared to those inseminated in the cool season. The variables included in the final logistic model for the pregnancy rate to second AI were treatment and season. Cloprostenol given at AI increased the risk of pregnancy 1.9 times (P = 0.002), and cows inseminated during the warm season were two times less likely to become pregnant (P = 0.003). No significant interactions were found among these three dependent variables (double ovulation and pregnancy to first and to second AI rates). Study 2: Logistic regression analysis of all the dependent variables: return to estrus, and pregnancy to first and to second AI (first AI plus return to AI) rates indicated no significant effects of treatment, parity, days in milk, milk production or body score at AI. No significant interactions were found. Study 3: The final model for the pregnancy rate only included the interaction between parity (primiparous versus multiparous) and treatment. Days in milk, milk production and insemination number showed no significant effect on pregnancy rate. Cloprostenol treatment at insemination increased the pregnancy rate in primiparous repeat breeder cows (odds ratio: 3.6). The treatment group and parity showed significant (P < 0.0001) interaction. This interaction suggests that cloprostenol treatment of primiparous cows at insemination might enhance pregnancy yet have no effect in multiparous cows. Our findings indicate that cloprostenol administered at insemination promotes ovulation and double ovulation in lactating dairy cows. Cloprostenol treatment showed no benefit in cows with acceptable reproductive performance, suggesting that cloprostenol treatment at AI may only be useful in cows in which stress factors affect ovulation and in repeat breeder cows.     

Is fertility declining in dairy cattle? A retrospective study in northeastern Spain

The past few decades have seen a dramatic increase in infertility and reproductive disorders associated with a rising milk yield. Herein, we present a 10-year (from 1991 to 2000) retrospective survey of these factors using data from a reproductive management program performed on high-yielding dairy herds in north-east Spain. The data series included 12,711 lactations. The year was divided into warm and cool periods. Data were obtained from cows first inseminated or examined 45-70 days postpartum. The reproductive tract of each animal was examined by palpation per rectum within 43-48 days postpartum. Cows were then weekly examined until insemination or until postpartum Day 70. Cows with a corpus luteum were synchronized for estrus and timed AI. Cows showing natural estrus in the same period were also inseminated. All inseminations were pooled as a single group and inseminated cows were considered cyclic. Inactive ovaries and ovarian cysts were recorded as ovarian disorders. Incomplete uterine involution, endometritis and pyometra were grouped as uterine disorders.Overall cyclicity and pregnancy rates of all AI cows, and cyclicity and pregnancy rates corresponding to the warm period significantly decreased over the 10-year period, yet remained practically constant during the cool period. The incidence of inactive ovaries in the entire population and that corresponding only to cows examined during the warm period significantly increased with time but was similar during the cool period. Ovarian cysts were more frequent during the warm (12.3%) than during the cool (2.4%) period, though these proportions were maintained throughout the study. The incidence of uterine disorders significantly decreased with time and did not differ between cool and warm periods. Milk yield per cow and year increased from 7800 kg in 1991 to 10,200 kg in 2000. Regression analysis revealed that, for all cows and those inseminated or examined in the warm period respectively, each 1000 kg increase in average milk yield was related to decreases of 3.2 and 6% in pregnancy rate, 4.4 and 7.6% in cyclicity, and to increase of 4.6 and 8% in the incidence of inactive ovaries. The rate of uterine disorders decreased 1.1% regardless of season. Our overall results clearly reflect the increased infertility and incidence of reproductive disorders over the study period. This occurred simultaneous to increasing average milk yield. Nevertheless, under our study conditions, a cool environment appears to preserve fertility and reduce the risk of reproductive disorders irrespective of the milk yield. These findings prompt the need to improve management practices by attempting to reduce the effects of factors provoking stress.     

Compared to dinoprost tromethamine, cloprostenol sodium increased rates of estrus detection, conception and pregnancy in lactating dairy cows on a large commercial dairy

Using two PGF_2α treatments 14 days apart as a way to enhance estrus detection rate following the 2nd treatment is a reproductive management tool that continues to be used on large dairy farms. In one study, in cows with a functional CL and a dominant follicle, treatment with cloprostenol vs. dinoprost resulted in greater peripheral estradiol concentrations. The objective of the present study was to determine if cloprostenol could enhance pregnancy rates of cows in a large dairy herd using a PGF_2α program for 1st artificial insemination (AI). Lactating dairy cows (n = 4549) were randomly assigned to receive two treatments of either 500 μg cloprostenol or 25 mg dinoprost 14 days apart, with the 2nd treatment on the 1st day of the voluntary waiting period (57 DIM). Cows detected in estrus within 5 days after the 2nd treatment were inseminated. There was no effect of treatment on day of estrus detection, with 78% of cows inseminated on Days 3 or 4 following treatment. Cloprostenol increased (P < 0.01) estrus detection rates in 1st parity cows compared to dinoprost, 42.4 vs. 34.0%. In cows inseminated on Days 3 or 4 after treatment, cloprostenol increased (P = 0.05) conception rates compared to dinoprost, 38.3 vs. 34.4%. When treatments and parities were combined, conception rates increased (P < 0.02) with interval after treatment (27.0, 36.4, and 44.5% for Days 1 or 2, Days 3 or 4, and Day 5, respectively). Cloprostenol increased (P = 0.02) overall pregnancy rate compared to dinoprost, 14.4 vs. 12.2%. In summary, cloprostenol increased fertility in 1st parity cows inseminated on Days 3 or 4 following treatment and subsequently enhanced pregnancy rates of 1st parity lactating dairy cows compared to dinoprost. Fertility appeared greater in cows expected to have had a young antral ovarian follicle at treatment.     

Side of gestation in dairy heifers affects subsequent sperm transport and pregnancy rates after deep insemination into one uterine horn

Effects of side of previous gestation on sperm transport and pregnancy rates after deep cornual insemination were evaluated in 1686 Friesian cows in their first lactational period. Only single ovulating animals were used. At insemination, semen was deposited deep into the uterine horn ipsilateral or contralateral to the preovulatory follicle. A total of 876 cows (52%) ovulated in the ovary ipsilateral to the postgravid horn, and 810 cows ovulated in the contralateral ovary. Semen was deposited into the previously nongravid uterine horn of 832 cows, and into the gravid horn of 854 cows. The pregnancy rate was higher (P < 0.00001) for semen deposition into the previously nongravid horn (46.6%) than for semen deposition into the gravid horn (35.7%). For inseminations ipsilateral to the side of impending ovulation, pregnancy rates were higher (P = 0.0004) when ovulations occurred on the opposite side to the postgravid horn than on the same side. Pregnancy rates were higher (P = 0.002) for contralateral inseminations when ovulations occurred on the same side to the postgravid horn than on the opposite side; they were higher (P = 0.0001) for total ipsilateral than for total contralateral inseminations. There was no difference between ipsilateral and contralateral inseminations (P = 0.64) when ovulation occurred ipsilateral to the postgravid horn, but pregnancy rates were higher (P < 0.00001) when ipsilateral insemination was carried out into the nonpostgravid horn. Results indicate that the side of gestation in dairy heifers affects subsequent pregnancy rates after deep insemination into one uterine horn, possibly by affecting sperm transport.     

Intrafollicular insemination for the treatment of infertility in the dairy cow

Intrafollicular insemination (IFI) is based on direct introduction of a sperm suspension into a preovulatory follicle. To our knowledge, the first cases of IFI resulting in pregnancy of the cow are reported here. The experiment was performed on a dairy herd with low fertility during the warm season of the year. Following the sequence of estrus detection, one in three inseminations was intrafollicular (n = 17), whereas deposition of semen was performed into the uterine body (IUI) in the remaining cows (n = 33). Approximately 0.06 ml of a seminal dose containing five million spermatozoa (one quarter of a commercial seminal dose in a 0.25 ml French straw) was injected trans-vaginally into the preovulatory follicle in the IFI group. Four (23.5%) and 3 (9%) cows of the IFI and IUI groups, respectively, became pregnant. On days 8-11 post-insemination, ovulation failure was registered in 3 (18%) and in 4 (12%) cows of the IFI and IUI groups, respectively. Our results suggest that IFI could be used as an alternative procedure to the usual deposition of semen into the uterus in cows of low fertility.     

Associations between the manipulation of patterns of follicular development and fertility in cattle

The wave-like patterns of ovarian follicular development in cattle can be manipulated by shortening the luteal phase with prostaglandin F2alpha (PGF), lengthening the period of follicle dominance with progesterone or curtailing follicle development with GnRH or oestradiol as 17beta, benzoate or cypionate. These hormones can also be used to synchronise ovulation allowing timed inseminations without detected oestrus. Progesterone, PGF, GnRH and oestradiol benzoate have each been used to increase conception rates in some situations, but their use has reduced them in others. For example, inseminations made within 96 h of a single injection of PGF administered during the luteal phase were associated with increased conception rates in dairy cows whereas double injection protocols reduced conception rates. The three forms of oestradiol and GnRH have greater effects on follicular development following divergence and dominance than following wave emergence. This can mean that follicles of differing maturity will be present about 7 days later and can result in varied intervals to the onset of oestrus following a PGF injection. The consequent variation in ovulation time can be reduced by injecting GnRH or an oestradiol during pro-oestrus. This means that some less mature follicles will ovulate, forming corpus luteum (CL) associated with a slower rise in plasma progesterone and lower mid-luteal concentrations. The lower conception rates recorded with single timed inseminations with synchronised ovulations have been associated with increased prevalences of short cycles in lactating dairy cows (with GnRH), with long luteal phases in cows and heifers (with oestradiol benzoate) and with embryo loss following positive pregnancy diagnosis (as with Ovsynch in lactating Holstein cows). Extensive Canadian studies have demonstrated that these same hormones can be successfully used without these limitations and reliably obtaining conception rates over 50% and up to 70% in beef cattle that have been supplemented with a progestin during the period of ovarian follicle synchronisation. The inherently lower fertility of Holstein cows during early lactation may be contributing to the reduced effectiveness of hormonal treatments for synchronised follicle development and ovulation. The role of reduced dose rates of GnRH in compromising this effectiveness needs to be determined if the potential of these treatments realised with beef cattle is to be achieved with lactating Holstein cows.     

Effect of insemination with doses of 2 or 15 million frozen-thawed spermatozoa and semen deposition site on pregnancy rate in dairy cows

The effects of low-dose artificial insemination (AI) on pregnancy rates have seldom been studied in lactating dairy cows. We evaluated the pregnancy results after AI with doses of 2 and 15 million frozen-thawed spermatozoa and the effect of semen deposition in lactating dairy cows. A total of 284 first inseminations with 2 million spermatozoa and 312 first inseminations with 15 million spermatozoa were performed on 480 dairy farms. Low-dose inseminations (2 million spermatozoa) under field conditions in commercial dairy herds, without estrus synchronization, generally resulted in significantly reduced pregnancy rates compared with normal doses (15 million spermatozoa). The bull x technician effect on fertility was statistically significant. This finding indicates that there is a high variability in fertility among bulls using 2 million spermatozoa per dose. The semen deposition site did not influence pregnancy rates. It is concluded that a dose of 2 million frozen-thawed spermatozoa is probably too low for most bulls to achieve acceptable pregnancy rates in dairy cows.     

Effect of insemination time of frozen semen on incidence of uterine fluid in mares

Ninety five mares were inseminated with frozen semen either within 12 h before ovulation or within 8 h after ovulation. The effect of preovulatory versus postovulatory insemination (AI) on the subsequent detection of uterine fluid was studied. The overall pregnancy rate was 43% and this was not significantly influenced by preovulatory or postovulatory insemination. When mares were first examined 12 h after AI, 18 of 52 mares (35%) had accumulated uterine fluid. However, when mares were first examined 18 to 24 h after AI, only 6 of 43 mares (14%) had uterine fluid. Presence of intrauterine fluid significantly lowered pregnancy rates. Timing of insemination did not affect incidence of uterine fluid. Serum concentrations of estrogen and progesterone at time of insemination did not influence uterine clearance or pregnancy rates, but both hormones were higher at preovulatory than at postovulatory inseminations. We concluded that there was no evidence that postovulatory inseminations would predispose mares to persistence of uterine fluid after AI.     

Follicular development during early pregnancy and the estrous cycle of the sow

The objective of this study was to monitor and compare follicle populations and follicular development in pregnant and nonpregnant sows from Day 3 to Day 20 after breeding. Twenty-four sows were paired within parity on the day of artificial insemination and were randomly allocated within pair for insemination with either killed (n=12) or live spermatozoa (n=12). All the sows were artificially inseminated with the pooled ejaculate of the same boar. From Day 3 through Day 20 post estrus, ovarian follicles were scanned daily by ultrasonography. Ultrasound images were recorded on videotape and were retrospectively analyzed. Follicles were mapped to indentify the existence of follicular waves. The follicles were then classified as small (< 3 mm), medium (3-5 mm), or large (>/=5 mm). Pregnancy diagnosis was performed on Day 21 by ultrasonography. Pregnant sows maintained a constant proportion of the follicle population in the small, medium and large follicle categories. However, in the nonpregnant sows, the proportion of follicles in the various size categories remained constant until Day 15. Thereafter, the proportion of small follicles decreased (P < 0.05) from Day 15 to 20, and the proportions of medium and large follicles increased (P < 0.05). The predictability of pregnancy status on Day 20 based on follicle populations in any of the 3 follicle categories was low. Moreover, there was no evidence of follicular waves during the estrous cycle or early pregnancy. In conclusion, the proportion of small follicles decreased while medium and large follicle increased from Day 15 through Day 20 of the estrous cycle, but not during a similar stage of pregnancy. This latter finding concurs with follicle recruitment from the pool of small follicles for ovulation following PGF2alpha secretion to induce luteolysis, which reduces progesterone concentrations and thereby allows for the stimulation of the pool of small follicles by gonadotropins.     

Comparing early embryo mortality in dairy cows during hot and cool seasons of the year

The objectives of this study were to identify the level and stage of embryonic mortality that occur in dairy cows during hot and cool seasons of the year. Experimental dairy cows, of varying ages, were artificially inseminated with frozen-thawed semen from proven Holstein sires. Females on each dairy unit were then randomly allocated to one of three experimental groups after partitioning by day of artificial insemination, days post partum, parity, and current milk production level. In Group I and Group II, nonsurgical embryo collection was performed on each cow using Dulbecco's phosphate-buffered saline as the flushing medium. Embryos from cows in Group I were collected on Days 6 or 7 post insemination during the hot (n=93) and cool (n=64) seasons. Embryos from cows in Group II were collected on Days 13 or 14 post insemination during the hot (n=97) and cool (n=63) seasons. In Group III, contemporary control cows were also inseminated during the hot (n=106) and cool (n=106) seasons, and fetal heart beat was evaluated via ultrasound between Days 25 and 35 following insemination. Embryo viability decreased (P<0.05) from 59% at Day 7 to 27% at Day 14 in the hot season, but was not decreased during the cool season (52 vs. 60%). Pregnancy rate at Days 25 to 35 was 21% in the hot season, which was less (P<0.05) than the 36% in the cool season. The percentage of unfertilized ova collected in both the hot and cool seasons suggests that fertilization failure was not affected by season of breeding. In summary, embryonic loss after Day 7 of pregnancy appears to be a problem in this hot, dry climate.     

Low dose insemination in cattle with the Ghent device

A new artificial insemination device for semen deposition near the uterotubal junction (UTJ) in cattle (Ghent device) was developed at Ghent University (Belgium). In this study, UTJ insemination of dairy cows with the Ghent device was compared with the conventional insemination technique to evaluate the effect on pregnancy rates after insemination with different doses of semen. In each of three field trials, the cows (n=795, 659, 360) and heifers (n=253, 182, 231) were randomly assigned to receive 12 million sperm deposited in the uterine body using conventional techniques (control) or a reduced sperm dose (RSD) deposited in the same manner as the control or bilateral deposition near the uterotubal junction using the Ghent device (Ghent). Sperm dosages for RSD and Ghent inseminations were 8, 4, and 2 million sperm for field trials 1-3, respectively. In the multivariable analysis, the pregnancy rates were significantly affected by the parity of the cow (p相似文献   

Effect of insemination timing on the fertilizing capacity of frozen/thawed equine spermatozoa

A breeding trial was conducted to evaluate the effect of insemination timing on the fertility of mares bred with frozen/thawed equine semen. One stallion and 60 reproductively sound, estrous-synchronized mares were included in the study. Mares were assigned to one of three groups (n = 20): 1) insemination with fresh semen every other day during estrus from detection of a 35-mm follicle until ovulation, 2) insemination with frozen/thawed semen every day during estrus from detection of a 35-mm follicle until ovulation or 3) insemination with frozen/thawed semen once, within 6 h after ovulation. Single-cycle 18-d pregnancy rates resulting from insemination with fresh semen (70%), preovulation insemination with frozen/thawed semen (60%) and postovulation insemination with frozen/thawed semen (55%) were not different (P > 0.05). Possibly, equivalent pregnancy rates could be achieved with frozen/thawed semen using either daily inseminations until ovulation occurs or frequent ovarian palpations with a single post-ovulation insemination. Further studies regarding the effect of insemination timing on stallion fertility are needed since the present investigation included only one stallion and a small number of mares.     

Factors affecting the fertility of high producing dairy herds in northeastern Spain

Infertility has been often correlated to a rising milk yield in high producing dairy cattle. The aim of the present study was to evaluate, using logistic regression procedures, the effects of several management indicators on the fertility of four dairy herds in northeastern Spain. Data derived from 10,965 artificial insemination (AI). The factors examined were: herd, milking frequency (three versus two milkings per day), lactation number, previous twinning and disorders such as placenta retention and pyometra, milk production at AI, the inseminating bull, season (warm versus cool period) and year effects, AI technician and repeat breeding syndrome (cows undergoing four or more AI). Our findings indicated no effects on fertility of the herd, year of AI, previous twining, placenta retention and pyometra and milk production at AI. Based on the odds ratios, the likelihood of pregnancy decreased: in cows milked three times per day (by a factor of 0.62); for each one unit increase in lactation number (by a factor of 0.92); for inseminations performed during the warm period (by a factor of 0.67); in repeat breeder cows (by a factor of 0.73); and when 3 of the 45 inseminating bulls included in the study were used (by factors of 0.35, 0.43 and 0.44, respectively). Of the 13 AI technicians participating in the study, 3 were related to a fertility rate improved by odds ratios of 1.86, 1.84 and 1.30, respectively, whereas 2 technicians gave rise to fertility rates reduced by odds ratios of 0.64 and 0.49, respectively. Under our study conditions, management practices were able to compensate for the effects of previous twining and reproductive disorders such as placenta retention and pyometra. However, fertility was significantly affected by the factors milking frequency, AI technician, inseminating bull, repeat breeding syndrome, lactation number and AI season.     

Equine sperm-oocyte interaction: results after intraoviductal and intrauterine inseminations of recipients for oocyte transfer.

Insemination of recipients for oocyte transfer and gamete intrafallopian transfer (GIFT) in five experiments were reviewed, and factors that affected pregnancy rates were ascertained. Oocytes were transferred into recipients that were (1) cyclic and ovulated at the approximate time of oocyte transfer, (2) cyclic with aspiration of the preovulatory follicle, and (3) noncyclic and treated with hormones. Recipients were inseminated before, after, or before and after transfer. Intrauterine and intraoviductal inseminations were done.Pregnancy rates were not different between cyclic and noncyclic recipients (8/15, 53% and 37/93, 39%). The highest numerical pregnancy rates resulted when recipients were inseminated with fresh semen from fertile stallions before oocyte transfer or inseminated with cooled transported semen before and after oocyte transfer. Oxytocin was administered to recipients before oocyte transfer when fluid was imaged within the uterus. Administration of oxytocin to recipients at the time of oocyte transfer resulted in significantly higher pregnancy rates than when oxytocin was not administered (17/26, 65% and 28/86, 33%). Intraoviductal and intrauterine inseminations of recipients during oocyte transfer resulted in similar embryo development rates when fresh semen was used (12/22, 55% and 14/26, 55%). However, embryo development rates significantly reduced when frozen (1/21, 5%) versus fresh sperm were inseminated into the oviduct.Results suggest that insemination of a recipient before and after transfer could be beneficial when semen quality is not optimal; however, a single insemination before transfer was adequate when fresh semen from fertile stallions was used. Absence of a preovulatory follicle did not appear to affect pregnancy rates in the present experiments. The transfer of sperm and oocytes (GIFT) into the oviduct was successful and repeatable as an assisted reproductive technique in the equine.     

Secretion of platelet-activating factor by periovulatory ovine follicles

Secretion of platelet-activating factor (PAF) in vitro by ovine follicles and ovarian interstitium obtained at various times before, during and after the endogenous preovulatory surge of luteinizing hormone (LH) and ovulation was quantified by radioimmunoassay. Release of PAF by the preovulatory follicle increased within 2 h after initiation of the surge of LH. Capacity for secretion of PAF was greatest at the time of ovulation, then declined thereafter. Production of PAF by ovarian interstitium throughout the periovulatory period was relatively low and did not change with time. It appears that PAF could act as an intrafollicular mediator in the mechanisms of ovulation and(or) luteinization.     

Walking activity at estrus and subsequent fertility in dairy cows

Poor detection of estrus, still a major problem in the dairy industry, has prompted the development of electronic estrous detection technologies. One of the features of estrous behavior is a marked increase in walking activity. The objectives of the present study were to evaluate the effects of various management factors on walking activity increase at estrus, and the relationship between this trait and fertility. Data from 5883 artificial inseminations (AI) conducted in two high-producing dairy herds were analyzed. Detection of estrus was performed using a pedometer system. Of the total AI investigated, 2072 (35.2%) resulted in pregnancy. The following data were recorded for each animal at AI: herd, lactation number, milk production (average for the 3 days prior to AI), lactation stage (early, mid, and late lactation), previous estrous synchronization (cloprostenol or progesterone releasing intravaginal device [PRID] for animals showing estrus within 7 days of treatment), season (warm versus cool period), insemination number following parturition, inseminating bull, inseminator, and pedometer measurements. Variables were screened for associations with walking activity by analysis of variance (ANOVA) through generalized linear model procedures (PROC GLM). Increased parity and milk production, and insemination during the warm period were associated with lower pedometer measurements. No significant effects of the herd, estrous synchronization, and lactation stage were observed. The link between walking activity and fertility was determined by applying logistic regression models. We detected no significant effects of herd, milk production, estrous synchronization, lactation stage, and inseminator on pregnancy rate. A higher lactation and insemination number, and insemination during the warm period were negatively correlated with the pregnancy rate. The likelihood of pregnancy was greater when semen from one of the bulls was used and when physical activity at estrus was increased. Our findings indicate that cow and management factors contribute intensely to walking activity at estrus, and also reveal a close link between increased walking activity and fertility.     

Pregnancy success of lactating Holstein cows after a single administration of a sustained-release formulation of recombinant bovine somatotropin

Background

Results regarding the use of bovine somatotropin for enhancing fertility in dairy cattle are variable. Here, the hypothesis was tested that a single injection of a sustained-release preparation of bovine somatotropin (bST) during the preovulatory period would improve pregnancy success of lactating dairy cows at first service.

Results

The first experiment was conducted in a temperate region of Mexico. Cows inseminated following natural estrus or timed artificial insemination were given a single injection of bST or a placebo injection at insemination (n = 100 cows per group). There was no significant difference between bST and control groups in the proportion of inseminated cows diagnosed pregnant (29 vs 31% pregnant). The second experiment was performed during heat stress in Florida. Cows were subjected to an ovulation synchronization regimen for first insemination. Cows treated with bST received a single injection at 3 days before insemination. Controls received no additional treatment. As expected, bST did not increase vaginal temperature. Treatment with bST did not significantly increase the proportion of inseminated cows diagnosed pregnant although it was numerically greater for the bST group (24.2% vs 17.8%, 124–132 cows per group). There was a tendency (p = 0.10) for a smaller percent of control cows to have high plasma progesterone concentrations (≥ 1 ng/ml) at Day 7 after insemination than for bST-treated cows (72.6 vs 81.1%). When only cows that were successfully synchronized were considered, the magnitude of the absolute difference in the percentage of inseminated cows that were diagnosed pregnant between bST and control cows was reduced (24.8 vs 22.4% pregnant for bST and control).

Conclusion

Results failed to indicate a beneficial effect of bST treatment on fertility of lactating dairy cows.

     

Carry-over effect of summer thermal stress on characteristics of the preovulatory follicle of lactating cows

Low fertility of cattle during the cool autumn is mostly associated with exposure to thermal stress in the previous summer. This study examined carry-over effects of summer heat stress and induction of enhanced follicular turnover, on the characteristics of preovulatory follicles during autumn. Holstein cows were treated with PGF₂ and GnRH to induce six successive 9-day follicular waves during autumn. Intact cows served as controls. Follicular fluid concentrations of androstenedione were higher in treated cows, but those of IGF-I and progesterone did not differ between groups. The concentration of estradiol at the end of the treatment was higher, and the content per follicle increased with time and was numerically higher in treated cows. The volume of follicular fluid increased during the autumn and was greater in treated cows. A delayed effect of heat stress on preovulatory follicles was noted, and induction of follicular waves by GnRH was able to improve follicular characteristics in autumn.     

The relationship between periovulatory endocrine and follicular activity on corpus luteum size, function, and subsequent embryo survival

The objectives of this study were to examine the relationships between periovulatory endocrine events, ovarian activity, and embryo survival after artificial insemination (AI) in cattle (Bos taurus). Eighty-four reproductively normal beef heifers were estrus synchronized using a prostaglandin-based regimen. Artificial insemination was performed between 5 and 21 h after heat onset. Ultrasonic examination of ovarian structures began 12 h after the onset of heat and continued every 6 h until confirmed ovulation. Blood samples were collected for measurement of estradiol, progesterone, and insulin-like growth factor-1 (IGF-1). Pregnancy diagnosis was conducted on Days 30 and 100 after AI. Embryo survival was defined as the presence of an embryo with a detectable heartbeat in a clear amniotic sac at Day 30 postinsemination. There was no effect of the intervals from the onset of heat to AI or ovulation or from AI to ovulation on embryo survival (P > 0.10). There was a tendency (P = 0.09) of an inverse relationship between preovulatory follicle size and embryo survival that was unrelated to concentrations of estradiol or IGF-1 during the periovulatory period (P > 0.05). There was evidence (P = 0.08) of a positive association between embryo survival and concentrations of progesterone on Day 7; however, this relationship was independent (P < 0.05) of hormonal and follicular measurements during the periovulatory period. This study shows that heifers could be inseminated up to 31.5 h before ovulation without compromising the probability of embryo survival. This study suggests that there is an optimum range of follicle size within which high embryo survival rates can be achieved.