Prediction of calving date in beef cows by real-time ultrasonic scanning

Three hundred pregnant beef cows between 35 and 125 days of gestation were scanned ultrasonically and their calving dates were predicted from measurements of fetal parts. The mean difference between the actual and predicted calving dates was 0.9 day with a standard deviation of 9.0 days. The accuracy and precision of the prediction of calving date were sufficient to be of benefit in the management of cows in late pregnancy and at calving.     

Calving patterns in seasonal dairy herds

Two field studies examined the calving patterns of cows in seasonal dairy herds in the Waikato (Field Study 1) and South Taranaki regions (Field Study 2). The first study examined patterns for cows commencing their second or subsequent lactation in herds which had used an inseminating service during the previous season. The second study included first lactation heifers only in 15 herds where animals had been naturally mated, and in 15 herds in which they had been synchronised and then artificially inseminated at the synchronised oestrus. The parameters describing calving patterns were based on the date for each herd's planned start of calving (PSC), which was 282 days from the date on which breeding commenced in the preceding season. The average interval from PSC to mean calving date for the 35 herds in Field Study 1 was 22 days, with individual herds ranging from 15 to 30 days. In herds with heifers which had been naturally mated (Field Study 2), it was 17.6 days compared to 11.0 days for previously synchronised animals. Calculating the intervals from PSC to median calving date and separately for the last two quartiles more effectively described a herd's calving pattern. The duration for the last quartile of the calving pattern was influenced by the extent and timing of induced calving. In Field Study 1, 88.6% of the 35 herd owners induced premature parturition in at least one cow. In these herds, 11.3% of cows were treated and calved prematurely. Only 61.7% of heifers which had previously been naturally mated calved by 3 weeks after PSC. Their calving dates were not evenly distributed over this 3-week period, with 9.8% in the first week and 25.6% in the third week. The calving pattern for heifers which had been previously synchronised showed several distinct peaks. Calvings to the synchronised mating were completed 15 days after PSC, by which time 64.7% of animals had calved. By 3 weeks after PSC, 72.9% of these heifers had calved. The results showed that there was considerable variation in calving patterns in seasonal dairy herds. This variation would have been due to differences in conception pattern, and the way induced calving had been applied. The calving pattern in heifers which had been naturally mated was less concentrated than had been expected. Synchronisation can significantly concentrate the calving pattern of these first lactation animals. The parameters used to describe calving patterns may be less applicable in herds in which a high proportion of animals is induced to calve prematurely, or where a whole herd is synchronised. Nonetheless, they do serve as an illustrative example of the variation in calving patterns among herds.     

Calving patterns in seasonal dairy herds

Two field studies examined the calving patterns of cows in seasonal dairy herds in the Waikato (Field Study 1) and South Taranaki regions (Field Study 2). The first study examined patterns for cows commencing their second or subsequent lactation in herds which had used an inseminating service during the previous season. The second study included first lactation heifers only in 15 herds where animals had been naturally mated, and in 15 herds in which they had been synchronised and then artifically inseminated at the synchronised oestrus.

The parameters describing calving patterns were based on the date for each herd's planned start of calving (PSC), which was 282 days from the date on which breeding commenced in the preceding season. The average interval from PSC to mean calving date for the 35 herds in Field Study 1 was 22 days, with individual herds anging from 15 to 30 days. In herds with heifers which had been naturally mated (Field Study 2), it was 17.6 days compared to 11.0 days for previously synchronised animals. Calculating the intervals from PSC to median calving date and separately for the last two quartiles more effectively described a herd's calving pattern. The duration for the last quartile of the calving pattern was influenced by the extent and timing of induced calving. In Field Study 1, 88.6% of the 35 herd owners induced premature parturition in at least one cow. In these herds, 11.3% of cows were treated and calved prematurely.

Only 61.7% of heifers which had previously been naturally mated calved by 3 weeks after PSC. Their calving dates were not evenly distributed over this 3-week period, with 9.8% in the first week and 25.6% in the third week. The calving pattern for heifers which had been previously synchronised showed several distinct peaks. Calvings to the synchronised mating were completed 15 days after PSC, by which time 64.7% of animals had calved. By 3 weeks after PSC, 72.9% of these heifers had calved.

The results showed that there was considerable variation in calving patterns in seasonal dairy herds. This variation would have been due to differences in conception pattern, and the way induced calving had been applied. The calving pattern in heifers which had been naturally mated was less concentrated than had been expected. Synchronisation can significantly concentrate the calving pattern of these first lactation animals.

The parameters used to describe calving patterns may be less applicable in herds in which a high proportion of animals is induced to calve prematurely, or where a whole herd is synchronised. Nonetheless, they do serve as an illustrative example of the variation in calving patterns among herds.     

Evaluation of a fixed-time artificial insemination protocol for postpartum suckled beef cows

Treatment with melengestrol acetate (MGA), an oral progestin, prior to administration of gonadotropin-releasing hormone (GnRH) and prostaglandin F2alpha (PG) effectively synchronizes estrus and maintains high fertility in postpartum beef cows. The objective of this experiment was to determine whether treatment with MGA prior to a GnRH-PG-GnRH protocol would improve pregnancy rates resulting from fixed-time artificial insemination (AI). Multiparous crossbred beef cows at two University of Missouri-Columbia farms (n = 90 and n = 137) were assigned by age and days postpartum to one of two treatments. Cows were fed carrier (1.8 kg x animal(-1) x d(-1)) with or without MGA (0.278 mg x kg(-1)) for 14 d. All cows were administered GnRH (100 microg; intramuscularly) on d 12 after MGA or carrier withdrawal and 7 d before PG (25 mg; intramuscularly). All cows received a second injection of GnRH and AI 72 h after PG. Mean days postpartum for MGA and control cows at the initiation of treatment were 39.6 and 38.9 d for herd 1; and 51.9 and 50.9 d for herd 2, respectively (P > 0.70 within herds). Blood samples were collected from all cows at 10 and 1 d before the feeding of MGA or carrier began and at the times GnRH and PG were administered. Concentrations of progesterone in serum at the initiation of treatment were elevated (>1 ng/mL) in 0% of MGA and 7% of control cows in herd 1, and 54% of MGA and 49% of control cows in herd 2 (P > 0.05 within herds). Pregnancy rates to fixed-time AI were determined by transrectal ultrasonography 50 d after AI. Pregnancy rates in herd 1 were 58% (26/45) and 51% (23/45) for MGA-treated and control cows, respectively (P = 0.52), and 63% (44/70) and 45% (30/67) for MGA-treated and control cows in herd 2, respectively (P = 0.03). Differences in pregnancy rates to fixed-time AI were significant (P = 0.04) when data from the two herds were combined (with MGA = 70/115 [61%]; control = 53/112 [47%]). There was no difference (P > 0.20) in final pregnancy rates (timed AI plus 45 d exposure to bulls) between treatments, within herds, or when herds were combined. In summary, pregnancy rates resulting from fixed-time AI may be improved with treatment of MGA prior to a GnRH-PG-GnRH protocol.     

Failure of a single postpartum prostaglandin treatment to improve the reproductive performance of dairy cows

A study was conducted to evaluate the effects of postpartum prostaglandin treatment on reproduction in 3 seasonal calving dairy herds. Recently calved lactating dairy cows were paired on herd, age, calving date and previous production index. One cow in each of the 196 pairs received a single intramuscular injection of 25 mg of the prostaglandin analogue, dinoprost, between 14 and 28 days after calving. Subsequent reproduction was monitored. Within each herd and overall, there was no significant effect of treatment on the intervals from calving to first service, mating start date to first service, calving to conception, mating start date to conception and first service to conception. Treatment also had no significant effect on 21-day submission and pregnancy rates, on the proportion of each group not pregnant at the end of mating, and on first service pregnancy rates. Responses to treatment did not vary between cows calving within 50 days of mating start date and earlier calving cows or between cows aged less than 5 years and older cows.     

Days to calving in artificially inseminated beef cows: Comparison of potential traits

Four fertility traits were compared for artificially inseminated (AI) beef cows: A) for cows that calved to the AI sire (from either the initial or follow-up inseminations that season), the number days from initial AI to calving; B) for cows calving either by AI or to a backup bull, the number days from initial AI to calving; C) As trait B for cows that calved, otherwise the maximum of trait B for the contemporary group plus a penalty of 21 days; D) Define the ‘start date’ for a contemporary group as the date the first cow in the group was AI'd. For cows that calved, trait D was the number of days from the ‘start date’ to calving, otherwise the maximum of trait D for cows in the group that calved plus a penalty of 21 days.The vast majority of cows received only one insemination in a season, so trait A resembled gestation length and had estimated heritability of 12%. Traits B, C and D had estimated heritabilities of 3.2%, 3.5% and 5.2% respectively; estimated genetic correlations of traits A–D with naturally mated days to calving were 0.48, 0.60, 0.80 and 0.74 respectively. Trait D is therefore the recommended female fertility trait for AI cows. It has a similar frequency distribution to days to calving from natural mating and should be included in a joint analysis with days to calving of naturally mated cows.     

Effects of periparturient diseases and conditions on the reproductive performance of New Zealand dairy cows

AIM: To determine the effect of retained foetal membranes (RFM), periparturient recumbency, calf mortality, dystocia, twin births and induction of parturition on submission and pregnancy rates of pasture-fed, seasonally-calving dairy cows in New Zealand. METHODS: Data were retrospectively collected for 2652 cows from 11 herds on the occurrence of periparturient diseases and condition, calving date, age and treatment for anoestrus prior to the planned start of mating (PSM). The effects of each disease or condition on submission and pregnancy rate by 28 days after PSM, final pregnancy rate, and the proportion of cows that conceived more than 50 days after PSM were examined using separate logistic regression models. Results are expressed as odds ratios (OR). Additionally, the effect of each disease or condition on the interval from PSM to conception was examined using survival analysis. Age, herd and late calving (i.e. cows calved < or =40 days before PSM) were included as factors in the models examined. RESULTS: Induction of parturition increased the risk of RFM (OR=3.3, p<0.01). The 28-day submission rate was affected by age and herd but not by any of the periparturient conditions examined. The 28-day pregnancy rate was reduced by RFM (OR=0.04, p<0.05), induction of parturition (OR=0.67, p<0.01), assisted calving (OR=0.61, p<0.01), late calving (OR=0.55, p<0.05) and anoestrus (OR=0.27, p<0.05). The final pregnancy rate was reduced by RFM (OR=0.013, p<0.01), induction (OR=0.25, p<0.05), assisted calving (OR=0.30, p<0.005) and anoestrus (OR=0.32, p<0.05), and was increased if the cow was submitted for mating within 28 days after PSM (OR=2.72, p<0.05). Effects of recumbency and twinning were not significant in any of the models. CONCLUSIONS: Cows that had had RFM or were assisted, induced or late to calve, had significantly lower pregnancy rates or took longer to conceive than unaffected herd-mates. CLINICAL RELEVANCE: Intervention programs aimed at the examination and treatment of cows with periparturient conditions may improve the reproductive performance of New Zealand dairy herds. Investigations of poor reproductive performance of a herd should include analysis of the prevalence and effects of periparturient diseases and conditions.     

Effects of breeding season length and calving season on range beef cow productivity

A 5-yr study was conducted beginning in 1983 with 460 cows to evaluate the effects of three breeding seasons (30, 45, and 70 d in length) and two times of spring calving, March (early) and April (late), on cattle production under Nebraska Sandhills range conditions. Criteria evaluated included pregnancy and weaning percentages, calving date and distribution, cow weights and body condition at four intervals, calf birth and weaning weights, and cow productivity. The 30-d breeding season included a 10-d estrus synchronization and AI period; in the other breeding seasons only natural breeding was used. The same sires were used over the entire study period. Percentage of cows pregnant and percentage of calves weaned were lower (P less than .01) for cows bred for 30 d than for cows bred for 45 or 70 d. Average calving dates were similar among the breeding groups within the early and late calving herds. Pregnancy rates from AI were higher (P less than .01) for the cows calving in April (64%) than for the cows calving in March (41%). Cows calving in April lost less weight between precalving and prebreeding and were heavier (P less than .05) at prebreeding time than the cows calving in March. Calf weaning weights were not different (P greater than .10) among any of the breeding season groups or between the two calving herds when calves were weaned at a similar age. Cow productivity (calf weaning weight per breeding female) was highest (P less than .05) for the cows bred for 70 d (186 kg), intermediate for the cows bred for 45 d (172 kg), and lowest for cows bred for 30 d (162 kg).(ABSTRACT TRUNCATED AT 250 WORDS)     

Estimation of sensitivity and specificity of pregnancy diagnosis using transrectal ultrasonography and ELISA for pregnancy-associated glycoprotein in dairy cows using a Bayesian latent class model

AIMS: To determine the sensitivity (Se) and specificity (Sp) of pregnancy diagnosis using transrectal ultrasonography and an ELISA for pregnancy-associated glycoprotein (PAG) in milk, in lactating dairy cows in seasonally calving herds approximately 85–100 days after the start of the herd’s breeding period.

METHODS: Paired results were used from pregnancy diagnosis using transrectal ultrasonography and ELISA for PAG in milk carried out approximately 85 and 100 days after the start of the breeding period, respectively, from 879 cows from four herds in Victoria, Australia. A Bayesian latent class model was used to estimate the proportion of cows pregnant, the Se and Sp of each test, and covariances between test results in pregnant and non-pregnant cows. Prior probability estimates were defined using beta distributions for the expected proportion of cows pregnant, Se and Sp for each test, and covariances between tests. Markov Chain Monte Carlo iterations identified posterior distributions for each of the unknown variables. Posterior distributions for each parameter were described using medians and 95% probability (i.e. credible) intervals (PrI). The posterior median estimates for Se and Sp for each test were used to estimate positive predictive and negative predictive values across a range of pregnancy proportions.

RESULTS: The estimate for proportion pregnant was 0.524 (95% PrI?=?0.485–0.562). For pregnancy diagnosis using transrectal ultrasonography, Se and Sp were 0.939 (95% PrI?=?0.890–0.974) and 0.943 (95% PrI?=?0.885–0.984), respectively; for ELISA, Se and Sp were 0.963 (95% PrI?=?0.919–0.990) and 0.870 (95% PrI?=?0.806–0.931), respectively. The estimated covariance between test results was 0.033 (95% PrI?=?0.008–0.046) and 0.035 (95% PrI?=?0.018–0.078) for pregnant and non-pregnant cows, respectively. Pregnancy diagnosis results using transrectal ultrasonography had a higher positive predictive value but lower negative predictive value than results from the ELISA across the range of pregnancy proportions assessed.

CONCLUSIONS AND CLINICAL RELEVANCE: Pregnancy diagnosis using transrectal ultrasonography and ELISA for PAG in milk had similar Se but differed in predictive values. Pregnancy diagnosis in seasonally calving herds around 85–100 days after the start of the breeding period using the ELISA is expected to result in a higher negative predictive value but lower positive predictive value than pregnancy diagnosis using transrectal ultrasonography. Thus, with the ELISA, a higher proportion of the cows with negative results will be non-pregnant, relative to results from transrectal ultrasonography, but a lower proportion of cows with positive results will be pregnant.     

Pregnancy loss in dairy cattle in the Waikato region of New Zealand

AIM: To define the incidence rate of pregnancy loss and risk factors for those losses in pasture-fed dairy cattle in the Waikato region of New Zealand. METHODS: Cows (n=2,004) from 10 pasture-fed, spring-calving dairy herds in the Waikato were enrolled following confirmation of pregnancy 29-45 days after insemination, for inseminations that occurred within the first 16 days of the seasonal breeding period. Transrectal ultrasonographic examinations for pregnancy were conducted at approximately 6, 8, 10, 14 and 22 weeks gestation, and subsequent calving data were recovered. Pregnancy loss was defined as having occurred when a confirmed pregnancy was not re-diagnosed, when gross abortion was detected, or when a cow calved <265 days after the confirmed conception date. Data were analysed using reverse stepwise logistic regression and Cox's proportional hazards analysis. RESULTS: A total of 128 (6.4%) pregnancy losses were detected. The incidence rate was higher in early compared to late gestation (10.9 vs 2.8 losses/10,000 cow-days between Weeks 6-10 vs Weeks 10-14, respectively; p<0.001). Higher rates of loss were associated with the occurrence of clinical mastitis (Hazards ratio (HR)=1.57; p=0.071), being treated for anoestrus (HR=1.69; p=0.007), and in cows that had calving-to-conception intervals < or =63 days compared with those that had calving-to-conception intervals >92 days (HR=2.49; p=0.06). In addition, the rate of pregnancy loss differed between herds (p=0.05). CONCLUSIONS: The highest rate of pregnancy loss occurred in early gestation. Clinical mastitis, anoestrus and calving late in the calving season were risk factors for pregnancy loss. CLINICAL RELEVANCE: Pregnancy diagnosis using ultrasonography can be undertaken from 28 days post-insemination. However, due to the high rate of pregnancy loss at this stage of gestation, herdowners need to be warned of possible losses, and cows should be re-examined to confirm pregnancy before certification of pregnancy status is given.     

Measuring dairy herd fertility.

The use of the calving index as a measure of herd fertility ignores the proportion of the herd that is culled, generally for failing to conceive. It is more important to consider the total cost of long calving intervals, high culling rate and even low pregnancy rates in an integrated index that reflects inefficient management, than to have to cope with balancing a number of separate physical indicators. In a study of 91 herds containing 14,524 cows a full range of physical indices was examined. The average herd calving interval was 380.3 days, with a culling rate of 23.1 per cent. Of the cows calving, 76.9 per cent recalved, a figure which when adjusted for the calving interval (CIA calving rate) became 73.8 per cent. In quartiles split on the basis of CIA calving rate, the top quartile achieved 82 per cent with a calving index of 375.2, and a culling rate of 16.7 per cent. These standards were achieved by serving 91.9 per cent of the cows after calving, at an interval to first service of 67.2 days. The submission rate for artificial insemination in the first 24 days after the earliest service date was 57.5 per cent and the overall pregnancy rate was 51.2 per cent. As a result 92.1 per cent of the cows served, and 85.3 per cent of those which calved, conceived again, with an average of 1.9 services per conception. Assessing fertility on a financial basis, with costs attributed to calving interval, culling rate and pregnancy rate to give a fertility index, the average herd was losing pounds 62/cow/year, compared with target levels.(ABSTRACT TRUNCATED AT 250 WORDS)     

Direct effects of induced parturition on subsequent reproductive performance of dairy cows from commercial herds in south-western Victoria

SUMMARY Reproductive performance was compared between cows whose previous parturition was induced and non-induced cows with similar calving dates, in 49 winter-calving, pasture-fed, commercial dairy herds in south-western Victoria. Parturition was induced in winter when most cows were between 27 and 35 weeks of pregnancy. Reproductive performance was assessed during the next mating period after induction which was mainly in spring of the same year. Percentages of cows in induced and untreated groups that were not pregnant after the mating period (9.0% and 7.2%, respectively) did not differ significantly. Induction tended to increase the percentage of cows of unknown pregnancy status. Mean percentages for induced and untreated groups were 11.5% and 7.9%, respectively. Induced and untreated groups calved at similar intervals after the planned start of calving in the following year, and the percentages of groups that required induction in that year did not differ significantly. The direct effects of induced parturition on reproduction were therefore concluded to be minimal. In seasonal calving herds, improvements in reproductive performance could be expected among cows whose calving dates were altered substantially by induction, due to increased intervals from calving to mating start date .     

Detection of a corpus luteum in apparently anoestrous cows by manual palpation, transrectal ultrasonography and plasma progesterone concentration

AIMS: To determine the level of agreement between transrectal ultrasonography, manual palpation and measurement of progesterone in the detection of corpora lutea in post-partum cows. To evaluate the reproductive performance of cows not detected in oestrus prior to mating, but detected with a corpus luteum at the start of mating. METHODS: Cows from seven herds which were not detected in oestrus were examined between 5 and 7 days before the herd's planned start of mating. Ovaries were examined by palpation and ultrasonography in 160 cows and plasma progesterone concentrations were also measured in 103 cows. Insemination dates and pregnancy data were recorded in 159 cows not detected in oestrus but having a corpus luteum and 1405 cycling herd mates. RESULTS: The level of agreement between ultrasonography and measurement of progesterone (kappa = 0.74) was higher than with manual palpation (kappa = 0.67) although it was significant for both techniques (p <0.001). Cows which were not detected in oestrus, but detected with a corpus luteum had a longer interval to first service (12 v. 10 days, p <0.05) and conception (19 v. 15 days, p = 0.01) than cycling herd mates and had a higher empty rate (9.7% v. 3.8%, p <0.001). CONCLUSIONS: These data indicate that ultrasonography may provide a better diagnostic tool for examining cows not detected in oestrus than manual palpation or measurement of progesterone concentration. Cows not detected in oestrus but having a corpus luteum were shown to have significantly poorer reproductive performance than cycling herd mates.     

Fertility of dairy cows in Northern Ireland

A comprehensive database was established on the milk production and reproductive performance of dairy cows in 19 selected herds in Northern Ireland, varying in size, management system and genetic merit. Data were obtained for 2471 cows, 1775 of which calved in a second year, and 693 were culled from the herd for specific reasons. The estimated mean rate of heat detection (assessed by the interheat interval during the main breeding season) in all the herds was 71 per cent, with a range from 53 to 92 per cent The average conception rate to first insemination was 37.1 per cent (range 21 to 66 per cent). The average calving interval for the retained cows was 407.2 days (range 359 to 448 days). Twenty-eight per cent of the cows that calved were culled, with infertility being the largest single reason (26.8 per cent of the cows culled). There were major differences in reproductive performance between the herds, but heat detection rate, conception rate and calving interval did not appear to be affeded by a herd's genetic merit. The herds with shorter calving intervals were characterised by better heat detection efficiency (83 v 61 per cent, P<0.01), a shorter interval from calving to first insemination (74 v 97 days; P<0.05), a higher conception rate to first insemination (45 v 34 per cent, P>0.10) and a lower removal rate (23 v 37 per cent, P<0.01). Furthermore, the cows in these herds had lower body condition scores (BCS) in the dry period (3.0 v 3.3; P<0.05) but lost less body condition in early lactation (0.3 v 0.6 BCS units, P<0.05). These results show that dairy herd fertility in Northern Ireland is generally low and similar to that previously reported for England and the USA, but that in some herds changes in herd management practices improved the cows' fertility.     

Effect of prostaglandin F2 alpha on the fertility of dairy cows after calving

Three hundred and thirty-one dairy cows in six herds were used to study the effect of a single injection of 25 mg dinoprost, a prostaglandin F2 alpha derivative, administered eight days after calving on several indices of fertility. A detailed comparison was made between 113 treated cows and 113 control cows of the same parity, which had calved within seven days of each other on the same farm and had experienced the same degree of dystocia. In cows which had calved without assistance, the treatment reduced the mean interval between calving and first heat from 40 days to 37 days but increased the mean interval between calving and conception from 83 days to 85 days. In cows which had required assistance at calving, the treatment reduced the mean interval between calving and first heat from 44 days to 34 days and reduced the mean calving to conception interval from 86 days to 68 days. The cows which benefited most were those which had required assistance during their second, third or fourth calvings; their calving to conception intervals were shortened by 22.5 days. An average of 1.39 services was required to establish conception in the treated cows which calved unaided, with 70 per cent conceiving to the first service, compared with an average of 1.34 services per conception and a 72 per cent conception rate to first service for their untreated herdmates.(ABSTRACT TRUNCATED AT 250 WORDS)     

Improved Prediction of Ovulation Time may Increase Pregnancy Rates to Artificial Insemination in Lactating Dairy Cattle

A prospective observational study was conducted in two Australian dairy herds to assess the potential for improving pregnancy rates (proportions of inseminations that result in pregnancy) to artificial insemination (AI) if the time of ovulation could be predicted with more certainty. Herd 1 calved year‐round and inseminations were performed during two periods each day. Herd 2 calved during autumn–winter and inseminations were performed only after the morning milking each day. In both herds, the AI to ovulation interval of enrolled cows was determined by trans‐rectal ovarian ultrasonography approximately 0, 12, 24 and 36 h after AI, and pregnancy was assessed by palpation per rectum 35–56 days after AI. Also, in Herd 1 vaginal electrical resistance (VER) measurements were taken at approximately 0, 12, 24 and 36 h after AI, and in Herd 2 cows were fitted with neck mounted activity meters that monitored cow activity count in 2‐h periods. There was substantial variation in the intervals from AI to ovulation within and between herds (mean ± SD 21.2 ± 10.7, n = 102; 14.7 ± 10.4, n = 100 in herds 1 and 2, respectively). Pregnancy rates were higher for inseminations close to, but preceding, ovulation. Using combined herd data (n = 202), the highest pregnancy rate (50.8%) was observed for inseminations between 0 and 16 h before ovulation, a period in which only a modest proportion of inseminations (31.2%) occurred. In contrast, pregnancy rate was significantly lower (28.7%; risk ratio 0.6; 95% CI 0.4–1.0; p = 0.039) for inseminations between 16 and 32 h before ovulation, a period where the highest proportion of inseminations (53.2%) occurred. Thus pregnancy rates could potentially be improved if a greater proportion of inseminations were conducted shortly before ovulation. In Herd 1, mean VER during the peri‐ovulatory period varied with time from ovulation. Lowest values (mean ± SEM, VER = 64.8 ± 1.2, n = 55) occurred approximately 18 h before ovulation and were significantly lower than measurements approximately 6 h before ovulation (67.4 ± 1.0; n = 73; p = 0.003). Further work is required to determine if VER can be used to identify ovulation time and hence the optimal time to inseminate in individual animals. In Herd 2 a modest proportion of inseminations (26.9%) occurred between 24 and 40 h after the onset of increased cow activity where the highest pregnancy rate (67.9%) was observed, whereas a significantly lower pregnancy rate (42.4%; risk ratio 0.6; 95% CI 0.4–0.9; p = 0.036) was observed for inseminations between 8 and 24 h after the onset of increased cow activity where the highest proportion of inseminations (56.7%) occurred. Thus cow activity monitoring may be useful to identify the optimal time to inseminate cows. Results from this study indicate that improved methods of ovulation prediction may allow better insemination timing relative to ovulation and consequently increased pregnancy rates.     

Sources of variation in reproductive performance in selected herds of beef cattle in north-eastern Australia

This paper contains observations of several sources of variation in reproductive performance on 13 herds of beef cattle in north-eastern Australia. Nearly 15,000 cows were examined, man, on more than one occasion. Variation between herds in pregnancy rate in June or September 1964, was 44 to 75 per cent. Lactating heifers had lower pregnancy rates than lactating cows, which in turn had lower rates than non-lactating animals. Within classes, cows in poor body condition had lower fertility than those in forward-store or fat condition. Losses of calves between pregnancy tests and branding ranged from 6 to 40 per cent in a sample of herds. In herds where year-to-year data were avail-able, the majority of cattle had one calf every two years, or two calves every three years. In one herd, cows that calved before February reared more calves than those that calved later. The main conclusion was that improved nutrition of lactating heifers and cows would be expected to increase pregnancy rates substantially.     

Effects of diets fed to dairy cows before and after calving on their plasma progesterone profiles after calving

Four weeks before their predicted mean second lactation calving date, 60 spring-calving Holstein-Friesian cows were blocked into groups of six on the basis of their predicted calving date and body condition score, and allocated at random to one of six dietary treatments in a factorial design: ad libitum grass silage, ad libitum grass silage plus barley straw or ad libitum grass silage plus 3 kg of concentrates, was offered for four weeks before the expected calving date, and after calving they were offered either 4 kg or 8 kg of concentrates plus ad libitum grass silage for eight weeks. On average, the first luteal activity occurred in all the groups at 29 days after calving. Seventeen of the cows had an atypical first plasma progesterone profile; 12 were anovulatory, three had prolonged luteal phases and two became anovulatory after having been cyclic. The cows offered grass silage only before calving had a significantly shorter mean (se) interval between calving and second luteal activity (44.9 [2.1] days), than the cows offered grass silage and straw (53.5 [1.9] days) or grass silage and concentrates (51.5 [3.2] days) (P<0.05). After calving none of the 28 cows offered grass silage and 4 kg of concentrates started cycling before day 21, whereas five of the 30 cows offered grass silage and 8 kg of concentrates cycled before day 21 (P<0.05).     

Pregnancy loss in dairy cattle in the Waikato region of New Zealand

AIM: To define the incidence rate of pregnancy loss and risk factors for those losses in pasture-fed dairy cattle in the Waikato region of New Zealand.

METHODS: Cows (n=2,004) from 10 pasture-fed, spring-calving dairy herds in the Waikato were enrolled following confirmation of pregnancy 29–45 days after insemination, for inseminations that occurred within the first 16 days of the seasonal breeding period. Transrectal ultrasonographic examinations for pregnancy were conducted at approximately 6, 8, 10, 14 and 22 weeks gestation, and subsequent calving data were recovered. Pregnancy loss was defined as having occurred when a confirmed pregnancy was not rediagnosed, when gross abortion was detected, or when a cow calved <265 days after the confirmed conception date. Data were analysed using reverse stepwise logistic regression and Cox's proportional hazards analysis.

RESULTS: A total of 128 (6.4%) pregnancy losses were detected. The incidence rate was higher in early compared to late gestation (10.9 vs 2.8 losses/10,000 cow-days between Weeks 6–10 vs Weeks 10–14, respectively; p<0.001). Higher rates of loss were associated with the occurrence of clinical mastitis (Hazards ratio (HR)=1.57; p=0.071), being treated for anoestrus (HR=1.69; p=0.007), and in cows that had calving-to-conception intervals ≤63 days compared with those that had calving-to-conception intervals >92 days (HR=2.49; p=0.06). In addition, the rate of pregnancy loss differed between herds (p=0.05).

CONCLUSIONS: The highest rate of pregnancy loss occurred in early gestation. Clinical mastitis, anoestrus and calving late in the calving season were risk factors for pregnancy loss.

CLINICAL RELEVANCE: Pregnancy diagnosis using ultrasonography can be undertaken from 28 days post-insemination. However, due to the high rate of pregnancy loss at this stage of gestation, herdowners need to be warned of possible losses, and cows should be re-examined to confirm pregnancy before certification of pregnancy status is given.     

Abortion in dairy cattle with advanced twin pregnancies: Incidence and timing

This study examines incidences and timing of abortion in Holstein Friesian dairy cows with advanced twin pregnancies in north‐eastern Spain. The study population consisted of 1,194 twin pregnancies in two herds recorded from 2010 to 2015: 522 bilateral and 672 unilateral. The presence of live twins was determined by transrectal ultrasonography between 55 and 61 days post‐AI and pregnancy confirmed 60 days later. Abortion was recorded in 278 (23.3%) cows before Day 260 of pregnancy: 7 (1.3%) in bilateral and 271 (40.3%) in unilateral twin pregnancies. Using binary logistic regression procedures the following were excluded as risk factors for abortion: herd, year, heat stress (temperature‐humidity index values >72), lactation number and right versus left side for unilateral twins. Based on odds ratios, cows carrying unilateral twins were 49.9 (right unilateral twins) and 49.3 (left unilateral twins) times more likely to suffer abortion than cows with bilateral twins. The average time of abortion for unilateral twins was 173 ± 32 days and ranged from 135 to 249 days, 43% of abortions occurring between 135 and 154 days of gestation. The authors suggest prevention or reduction of unilateral twin pregnancies by follicular puncture and drainage of subordinate follicles at AI or manual rupture of the amniotic vesicle of one of the two twins at the time of pregnancy diagnosis.