Analysis of survivorship after displaced abomasum is diagnosed in dairy cows

The objective of this study was to analyze the survivorship of dairy cows after displaced abomasum was diagnosed. Survivorship from the calving preceeding diagnosis of displaced abomasum to removal from the herd was investigated for cows that were diagnosed with displaced abomasum compared with herdmates. For cows that were diagnosed with displaced abomasum, factors that affected survivorship from the diagnosis of the disease to removal were investigated. Data from a total of 135 cows that had been diagnosed with displaced abomasum and 373 controls matched by herd and lactation number were available from a database at the University of Guelph. The hazard rate of removal from the herd was estimated using proportional hazard regression adjusted for intraherd correlation. Diagnosis of displaced abomasum significantly increased the hazarad rate of removal from the herd. For cows that were diagnosed with displaced abomasum the hazard rate of removal increased as parity increased. However, the hazard rate decreased with recurrent displaced abomasum and with higher days in milk at diagnosis of the disease. Although the 305-d milk production of cows that were diagnosed with displaced abomasum was lower during the lactation when the disease was diagnosed, 305-d milk production was not associated with removal of cows with that disease from the herd. Milk production during subsequent lactations did not differ between cows that were diagnosed with displaced abomasum and control cows. Low milk production during the subsequent lactations increased the hazard of removal of cows with that disease from the herd.     

A Cox proportional-hazards model with time-dependent covariates to evaluate the relationship between body-condition score and the risks of first insemination and pregnancy in a high-producing dairy herd

The objective of this study was to determine the relationship between body-condition score and postpartum reproductive efficiency in dairy cattle. Data on body-condition score, reproduction, diseases, and production from 1404 lactations of 639 cows, calving between January 1984 and November 1996 on a commercial farm, were used. The Cox proportional-hazards model with time-dependent covariates was used to qualify the relationship between body-condition score on a scale of 1 to 5 and risks of first insemination and pregnancy. Cows with body condition at calving < 3 had lower first-insemination risks. Loss of body-condition score between calving and 45 days after calving was associated with increased days open and days-to-first insemination. Cows with body-condition scores < 2 after Day 45 and before first insemination were less likely to be inseminated and become pregnant compared with cows that had higher body condition. Milk yield, mastitis, lameness, milk fever and genital infection were associated with a reduction in reproductive efficiency. The results indicated that loss of body-condition and actual body-condition scores before conception or first insemination or both can be used as a prognosis for days open and days-to-first insemination in dairy herds.     

Variances of direct genetic effects, maternal genetic effects, and cytoplasmic inheritance effects for milk yield, fat yield, and fat percentage

Milk yield, fat yield, and fat percentage during the first three lactations were studied using New York Holsteins that were milked twice daily over a 305-d, mature equivalent lactation. Those data were used to estimate variances from direct and maternal genetic effects, cytoplasmic effects, sire by herd interaction, and cow permanent environmental effects. Cytoplasmic line was traced to the last female ancestor using DHI records from 1950 through 1991. Records were 138,869 lactations of 68,063 cows calving from 1980 through 1991. Ten random samples were based on herd code. Samples averaged 4926 dams and 2026 cytoplasmic lines. Model also included herd-year-seasons as fixed effects and genetic covariance for direct-maternal effects. Mean estimates of the effects of maternal genetic variances and direct-maternal covariances, as fractions of phenotypic variances, were 0.008 and 0.007 for milk yield, 0.010 and 0.010 for fat yield, and 0.006 and 0.025 for fat percentage, respectively. Average fractions of variance from cytoplasmic line were 0.011, 0.008, and 0.009 for milk yield, fat yield, and fat percentage. Removal of maternal genetic effects and covariance for maternal direct effects from the model increased the fraction of direct genetic variance by 0.014, 0.021, and 0.046 for milk yield, fat yield, and fat percentage; little change in the fraction was due to cytoplasmic line. Exclusion of cytoplasmic effects from the model increased the ratio of additive direct genetic variance to phenotypic variance by less than 2%. Similarly, when sire by herd interaction was excluded, the ratio of direct genetic variance to phenotypic variance increased 1% or less.     

Coping with intractable symptoms in collagen diseases--Behcet''s disease

OBJECTIVE: To determine associations between subclinical Mycobacterium paratuberculosis infection and milk production, milk components, and somatic cell counts of dairy cattle. DESIGN: Cross-sectional epidemiologic survey. ANIMALS: 23 dairy herds in Wisconsin containing 1,653 adult cows were studied. The herds had above average milk production and a history of bovine paratuberculosis in the herd within the previous 12 months. PROCEDURE: All adult cows in the herds were tested for paratuberculosis by use of an absorbed ELISA. Milk yield, fat, protein, and somatic cell count data were retrieved electronically from Dairy Herd Improvement Association records. RESULTS: 147 ELISA-positive and 1,506 ELISA-negative cows were identified. ELISA-positive cows had a mature-equivalent milk production of 376 kg (829 lb)/lactation less than that for ELISA-negative herdmates. Significant difference was not found in lactation average percent-ages of fat and protein, or somatic cell count linear score. When comparing ELISA-positive and -negative cow's current mature equivalent milk with all previous lactations, significant difference was found only from the immediate-preceding lactation. When this difference was examined by parity group, significant difference was confined to cows in the second lactation. CLINICAL IMPLICATIONS: Subclinical paratuberculosis infections, as determined by ELISA, are associated with a 4% reduction in milk yield and add to the already substantial costs of clinical M paratuberculosis infection in the dairy industry.     

Effect of accelerated postpubertal growth and early calving on lactation performance of primiparous Holstein heifers

Seventy postpubertal Holstein replacement heifers were assigned to one of four treatments to assess effects of accelerated postpubertal growth on performance during first lactation. Heifers were fed control or accelerated diets from 10 mo of age until parturition, and breeding was initiated at 14 or 10 mo of age, respectively. Heifers were also subdivided into two breeding groups (target or delayed) based on normal AI breeding efficiencies. Heifers fed accelerated diets had a higher average daily gain (933 vs. 778 g/d) and calved earlier (21.7 vs. 24.6 mo) than did heifers fed control diets. Heifers fed accelerated diets had similar prepartum (10 d) BW, but lower wither height and postpartum BW, than heifers fed the control. For heifers fed accelerated diets, milk fat and milk protein yields were lower, and milk fat percentage was higher, at mo 1 and 2; milk fat percentage was lower at mo 3 to 7 of lactation. Delayed bred heifers calved later (approximately 2 mo), had higher prepartum and postpartum BW, and were taller at the withers than target bred heifers. Delayed breeding resulted in higher body condition scores at calving and a greater incidence of dystocia, but lactation performance was not affected. Accelerated postpubertal growth resulted in earlier calving at similar prepartum BW, but data for wither height, pelvic area, and postpartum BW suggested that heifers fed accelerated diets were smaller. Accelerated postpubertal growth and early calving reduced performance during first lactation, but the exact mechanisms could not be determined.     

Effects of clinical ketosis on test day milk yields in Finnish Ayrshire cattle

A linear model for repeated measurements was used to estimate the effects of clinical ketosis on 722,198 test day milk yields collected from September 1, 1985 to January 31, 1988 on 60,851 Finnish Ayrshire cows of parity < 7. An index was created to differentiate among milk collected within 17 d following diagnosis of ketosis, milk collected before or > 17 d after diagnosis, and milk collected on nonketotic cows. For each parity separately, the statistical model included fixed effects (ketosis, calving season, year and season of milk sampling, and stage of lactation) and random effects (herd and permanent and temporary environments) on test day milk yields. The pattern underlying correlations between temporary environmental effects was accommodated in the statistical model. Compared with those for nonketotic cows, lactation curves of cows with ketosis showed a depression in early lactation; estimated milk loss was 44.3 kg for 17 d after diagnosis. The 305-d milk yield of cows diagnosed with ketosis was estimated to be 141.1 kg higher than that of cows free of ketosis. Although milk losses occurred after ketosis, ketotic cows yielded more milk over the entire lactation than did nonketotic cows; and yields would have been even higher if cows had not had ketosis.     

Administration of recombinant bovine somatotropin to dairy cows for four consecutive lactations

Effects of long-term administration of recombinant bovine somatotropin (bST) to dairy cows on complete lactational performance [60 (+/- 3) to 284 (+/- 3) d in milk (DIM)] were studied for four consecutive lactations. Beginning on d 60 (+/- 3) postpartum, Holstein cows received biweekly injections (500 mg) of bST (n = 39) or a placebo (control; n = 39) during the first lactation of the study. Cows either continued on the same treatment (n = 26) or were switched to the opposite treatment (n = 29) during the second lactation. Cows that changed treatments were injected for only 16 wk during the second lactation. Six cows per treatment completed four consecutive lactations. Treatment with bST during the first lactation did not have a residual effect on milk yields during the second lactation. Injections of bST during the second lactation increased milk yield 6.5 kg/d from 60 (+/- 3) to 172 DIM. For the four lactations, cows receiving bST yielded 3.7 kg/d (14%) more milk and gained 52 kg (37%) more body weight than did controls. Pretreatment (from 0 to 56 DIM) milk yields in yr 2, 3, and 4 were not affected by previous bST treatment. Milk yield, efficiency of feed utilization, and body weights were enhanced in cows injected with bST for four consecutive lactations. Previous bST treatment did not diminish milk yields in subsequent lactations.     

Effect of prepartum and postpartum dietary energy on growth and lactation of primiparous cows

Sixty-seven Holstein replacement heifers (19 mo) were fed a standard (59.7% TDN) or a high energy (69.3% TDN) diet until parturition. After parturition, primiparous cows were fed either 0 or 2.8% supplemental tallow for 150 d in a 2 x 2 factorial arrangement with prepartum treatments. High energy prepartum increased BW (693.5 vs. 663.7 kg) and body condition scores (3.72 vs. 3.55) at calving. Increased energy density of the diet prepartum did not affect milk yield or composition. Supplemental dietary fat postpartum increased milk yield approximately 1.5 kg/d, but the response was not observed until 7 wk postpartum. Heifers fed the standard diet prepartum and no supplemental fat postpartum had higher DMI than other treatments. Heifers fed high energy prepartum and supplemental fat postpartum lost the greatest BW and body condition from 1 to 5 wk postpartum. Heifers fed high energy diets prepartum had higher concentrations of blood NEFA, BHBA, and liver triglycerides. Increases in BW and body condition scores at calving above approximately 660 kg and 3.5, respectively, do not enhance lactation performance. When 2.8% supplemental fat was fed to primiparous cows, milk yield improved approximately 1.5 kg/d after wk 7 of lactation.     

Milk production and reproductive performance in dairy cows given bovine respiratory syncytial virus vaccine prior to parturition

OBJECTIVE: To assess the effect of vaccination against bovine respiratory syncytial virus on milk production, reproductive performance, and health in lactating dairy cows. DESIGN: Prospective randomized block design. ANIMALS: 385 Holstein dairy cows and heifers. PROCEDURE: Cows were grouped by lactation number, season of calving, and previous mature equivalent 305-day milk production (where appropriate). Prior to parturition, cows and heifers were randomly assigned to be vaccinated i.m. against infectious bovine rhinotracheitis, bovine viral diarrhea, and parainfluenza 3 viruses by use of a three-way vaccine, or to be vaccinated against those viruses as well as bovine respiratory syncytial virus, using a four-way vaccine. Milk production was measured daily through 305 days of lactation. Reproductive and medical records were reviewed to obtain insemination dates and record medical problems of cows in each vaccine treatment group. RESULTS: Compared with the three-way vaccine, administration of the four-way vaccine was associated with higher milk production (1.39 kg [3.06 lb] more milk/d) in first-parity cows during the first 21 weeks of lactation. Vaccination did not have any effect on milk production after the first 21 weeks of lactation in cows of any parity. Conception rates at first insemination were higher for four-way vaccinated first-parity cows than for three-way vaccinated first-parity cows (54.6 vs 32.7%). Compared with second-parity cows that received the three-way vaccine, first insemination conception rate was improved for second-parity cows vaccinated with the four-way vaccine (28.9 vs 47.8%, respectively). In cows of third or greater parity, first insemination conception rate was not different between the 2 vaccine treatment groups. CLINICAL IMPLICATIONS: Vaccination of heifers against bovine respiratory syncytial virus prior to partrition may increase milk production and first insemination conception rates.     

The effects of lactation on the fertility of dairy cows

Lactation has been negatively associated with fertility because pregnancy rates in maiden heifers exceed those obtained after first or subsequent calvings. The extent of this difference is less in pasture-fed dairy cows ( < 10%) than in American Holsteins ( > 20%) fed grain and conserved forages. The latter cows have pregnancy rates to first insemination and oestrus detection rates of only 40 to 45%. This suggests that the subsequent fertility of inherently fertile Holstein heifers may be severely compromised by high levels of milk production. International comparisons show that pasture-fed dairy cows may experience extended periods of anovulatory anoestrum but have normal fertility (60% pregnancy rate to first insemination) once cycling. The high-producing American Holstein may ovulate within 4 weeks postpartum but is more likely to continue ovulating without being detected in oestrus. Both situations are associated with negative energy balances (NEB) during early lactation. The severity and duration of this NEB may vary with body condition at calving, age or parity, ration formulation, production level and environmental factors. Relative daily milk yield is not an absolute indicator of NEB, because some lower producing cows within a herd have lower feed intakes and more severe energy deficits. NEB is not simple to measure; nonetheless, it is correlated with genetic improvement for milk yield. A positive energy balance, greater weight gain and higher body condition score have all been shown to be positively correlated with plasma progesterone concentrations in early lactation. No studies have investigated the possibility that the rapid increase in metabolic rate at this time may also alter steroid concentrations with consequent effects on oestrous behaviour and fertility. Studies to more precisely define the effects of increasing milk yields in early lactation, especially in Holsteins, may need to be completed in Australia and New Zealand. Oestrus detection rates and pregnancy rates for American Holsteins of less than 50% are accepted widely in the USA. Such low detection rates confound studies on fertility. The objective should be to increase these 2 rates to at least 80% and 60% respectively. This may involve the use of controlled breeding, especially if oestrous behaviour is less overt in high-producing Holstein cows.     

The association between previous 305-day milk yield and disease in New York State dairy cows

Logistic regression was used to examine the association between milk yield and disease. We studied 8070 cows of second or higher parity from 25 New York State Holstein herds, calving between June 1990 and November 1993. Previous 305-d mature equivalent milk yield was used because most of the disorders studied occurred too early in lactation for current milk yield to be considered. Seven disorders were studied: retained placenta, metritis, ovarian cyst, milk fever, ketosis, abomasal displacement, and mastitis. A separate model was used for each disorder. Each model included, in additional to milk yield, parity, calving season, and heard. Results showed that higher milk yield was not a risk factor for any disease except mastitis. However, the association between higher previous milk yield and mastitis does not necessarily imply causation.     

Use of ultrasonography to help to predict observed oestrus in dairy cows after the administration of prostaglandin F2 alpha

A total of 76 cows from seven herds with an ultrasonographically visible corpus luteum at routine herd fertility examinations were treated with prostaglandin F2 alpha. The sizes of the cavities of all the follicles on both ovaries with a diameter greater than 5 mm were measured. The cows were observed for signs of oestrus over the following six days and the time to onset of oestrus was recorded. Milk samples collected when the prostaglandin was administered and at oestrus were assayed for milk progesterone concentration. The herd of origin, lactation number, body condition score, days after calving at the time of examination and the total number of follicles when the prostaglandin was administered were not found to have any correlation with time to oestrus. Seventy-three of the 76 cows had milk progesterone concentrations consistent with active luteal tissue when the prostaglandin was administered, and 33 of them were observed in oestrus and had low milk progesterone concentration within six days. The mean time to the onset of oestrus was significantly and inversely related to the size of the cavity of the smallest follicle with a diameter of more than 5 mm.     

Parameters of test day milk yield and milk components for dairy ewes

A total of 3119 test day records for milk yield, somatic cell count (SCC), and contents of fat, protein, casein, serum protein, lactose, and total solids were obtained weekly from 155 lactations of Churra ewes during both the suckling and milking periods. All of the variables were significantly affected by stage of lactation, number of live lambs weaned, age at parity, and parity number within age. The lactation curves for SCC and for percentages of protein, casein, fat, and total solids were related inversely to the lactation curves for milk yield and lactose percentage. The SCC curve reached its lowest point at wk 5, which coincided with the maximum milk yield. At wk 3, lactose content reached its peak, and contents of fat, protein, casein, and total solids reached their lowest points. After weaning, monthly within-lactation correlations of the variables were simulated; values were 0.584 for yield, 0.468 for SCC, 0.371 for casein percentage, 0.360 for total solids percentage, 0.350 for protein percentage, 0.342 for fat percentage, 0.270 for lactose percentage, and 0.030 for serum protein percentage. Simplified procedures based on only one milking (a.m. or p.m.) could be effective in the testing programs for milk yield and SCC, thus, reducing the economic costs of milk recordings in mammary health and breeding programs.     

Event-time analysis of reproductive traits of dairy heifers

Data on the reproductive traits of dairy heifers were analyzed using event-time techniques. Traits analyzed were age at first calving (n = 4631), days to first breeding, and days open (n = 1992) during the first lactation. A proportional hazard model was used that included fixed effects of herd-year, year-season, breed type, herd weight, and heifer weight. Body weights were recorded at 390 d of age, on average. The model for days open and days to first breeding included two additional fixed effects of herd and heifer milk yield at 100 d. A significant effect of heifer weight category on age at first calving was found. The chance of calving was consistently higher for herds and heifers with higher body weight at 390 d and decreased linearly from the top to the lowest quartiles. The effects of herd weight category on days to first breeding and days open were significant. Heifers in herds with a higher average body weight were less likely to be bred, and heifers in herds with lower average body weight were less likely to get pregnant. The effect of heifer weight category on days to first breeding or days open was not significant. The effect of herd milk yield on days to first breeding was significant. Heifers in herds with lower yield were more likely to be bred. The effect of heifer milk yield category on days to first breeding and days open was significant, but no linear trend was found for the estimates of the hazard ratios. The chance of a heifer being bred and becoming pregnant was similar among the first three quartiles and was lower for heifers in the lowest quartile. The probability of a heifer reaching a first calving can be improved by increasing the body weight at 390 d. Body weight at 390 d did not appear to have a large effect on reproductive performance after first calving. High milk yield appears not to have a large negative effect on days open, at least for the milk yield levels analyzed in this study.     

Covariance functions across herd production levels for test day records on milk, fat, and protein yields

Multiple-trait BLUP evaluations of test day records require a large number of genetic parameters. This study estimated covariances with a reduced model that included covariance functions in two dimensions (stage of lactation and herd production level) and all three yield traits. Records came from all six states in Australia, were evenly distributed across the herd production levels, but decreased with increasing lactation stage from 9693 records for the 1st mo of lactation to 4199 records for the 10th mo. Using the variance component estimation package and a bivariate animal model, 1176 genetic (co)variances and 312 environmental (co)variances were estimated for 48 traits (1, 4, 7, and 10 mo of lactation; herd production levels of < 20, 20 to 22, 22 to 24, > 24 kg of milk/d; and milk, fat, and protein yields). The genetic (co)variances could be predicted by a multiplicative model that included 1) a term dependent on which yields (milk, fat, or protein) were involved in the covariance, 2) the covariance functions for month of lactation and herd production level, and 3) a covariance function for the interaction between these. This model required only 27 parameters instead of the 1176 (co)variances. For the environmental (co)variances, a model was fitted that contained several additional covariance functions. This model reduced the number of parameters from 312 to 71. For the same trait at the same production level, genetic correlations between test days ranged from 0.59 to 1, and environmental correlations ranged from 0.17 to 0.48. Genetic correlations between milk and fat, milk and protein, and fat and protein were 0.38, 0.83, 0.59, respectively, and correlations between the herd production levels ranged from 0.79 to 0.97. Failure to consider herd production level in a test day model evaluation might result, for instance, in overweighting of early lactation information from high production herds compared with information coming from bulls tested across all production levels.     

Short- and long-term production losses and repeatability of clinical mastitis in dairy cattle

Between 1985 and 1990, a study of 5313 lactations of 2477 Black and White cows was carried out. A stepwise least squares method was used to obtain unbiased estimates of milk, fat, and protein losses that were due to clinical mastitis and the carry-over effect from the previous lactation. Logistic regression was used to estimate the probability that a cow would have clinical mastitis in the next month. The effect of clinical mastitis on production within one lactation was estimated at 527 kg of milk (8.1%), 22.7 kg of fat (8.0%), and 13.7 kg of protein (6.2%) for > or = 3 cases of clinical quarters in the second lactation. One or 2 cases of clinical quarters in a lactation did not significantly affect the production in the next lactation. The negative carry-over effect of > or = 3 cases of clinical quarters was estimated at 381 kg of milk (5.9%), 23.7 kg of fat (8.4%), and 10.1 kg of protein (4.6%) up to and including mo 8 of the second lactation. The fat content in milk produced after the onset of mastitis decreased, and the protein content increased. The risk of clinical mastitis infection in the following month was influenced by month of lactation (a higher risk early in lactation), lactation number (risk increased with lactation number), production level (higher risk for high producing cows), number of clinical quarters in the previous lactation, number of clinical quarters in the previous months of the current lactation, and occurrence of clinical mastitis in the current month.     

Feed intake relative to stage of lactation for dairy cows consuming total mixed diets with a high or low ratio of concentrate to forage

This experiment examined the effect of feed quality on the relationship between intake and stage of lactation in dairy cows. Two total mixed diets composed of grass silage and concentrate were formulated. The high concentrate total mixed diet was designed to meet energy requirements, and the low concentrate total mixed diet was designed to limit intake. Twenty-four Holstein-Friesian cows were offered the total mixed diets in a full 2 x 2 change-over design with control treatments. The changeover was at 153 d in milk (DIM). For the statistical analyses, two periods of 13 wk, one period before and one period after the changeover, were used. Dry matter intake (DMI), milk yield, body weight, and body condition score were significantly greater for cows fed the high concentrate total mixed diet than for cows fed the low concentrate total mixed diet. Significant interactions between total mixed diet and period were observed for DMI and milk yield. However, no significant residual effects of changing from one total mixed diet to the other were observed. The interactions were due to substantially different slopes of DMI and milk yield relative to DIM for cows fed the two different total mixed diets. For cows fed the low concentrate total mixed diet, there was no effect of stage of lactation on DMI; the slope was 0. For cows fed the high concentrate total mixed diet, there was a significant decline in DMI as lactation progressed.     

Genetic evaluation of body weight of lactating Holstein heifers using body measurements and conformation traits

Genetic and phenotypic parameters of body weight (BW), hip height, heart girth, and linear conformation traits were estimated from field data for 7344 lactating Holstein heifers from 560 herds. Mean BW was around 540 kg at calving, decreased to a minimum of 514 kg by wk 6, and increased to > 590 kg by wk 45 of lactation. The statistical model for BW included month of recording, lactation stage, pregnancy stage, proportion of Holstein genes, parity of the dam, calving age, herd, and animal. Variation between herds accounted for 33% of the phenotypic variation of BW. Estimated heritability was 0.33 for BW and ranged from 0.32 to 0.54 for heart girth, hip height, and conformation traits. Genetic correlations of BW with heart girth, hip height, body depth, rump width, and muscularity ranged from 0.48 to 0.77. The accuracy of a selection index was 0.90 for direct selection and 0.83 for indirect selection using information on those conformation traits of 50 daughters. Genetic variation of BW is considerable, and genetic evaluation of BW can be based on data for conformation traits from a field recording system with only a limited loss of accuracy.     

Genetic evaluation for herd life in Canada

Methods were developed for the national genetic evaluation of herd life for Canadian Holstein sires. The genetic evaluations incorporate information from survival (direct herd life) and information from conformation traits that are related to herd life (indirect herd life) after adjustment for production in first lactation to remove the effect of culling for production. Direct genetic evaluations for herd life were based on survival in each of the first three lactations, which was analyzed using a multiple-trait animal model. Sire evaluations thus obtained for survival in each of the first three lactations were combined based on their economic weights into an overall sire evaluation for direct herd life. Sire evaluations for indirect herd life were based on an index of sire evaluations for mammary system, feet and legs, rump, and capacity. A multiple-trait sire model based on multiple-trait across country evaluation methodology was used to combine direct and indirect genetic evaluations for herd life into an overall genetic evaluation for herd life. Sire evaluations for herd life were expressed in estimated transmitting ability as the number of lactations and represent expected differences among daughters in functional herd life (number of lactations); the average functional herd life was set equal to three lactations. Estimated transmitting abilities were normally distributed and ranged from 2.31 to 3.43 lactations.     

Effect of time of artificial insemination on pregnancy rates, calving rates, pregnancy loss, and gender ratio after synchronization of ovulation in lactating dairy cows

In order to assess the optimal time of artificial insemination (AI) in relation to ovulation, lactating dairy cows (n = 732) from herds with rolling herd averages of 9980 to 11,800 kg from three milkings per day were randomly assigned to five groups by stage of lactation and parity. Ovulation was synchronized by administration of GnRH followed 7 d later with PGF2 alpha followed 2 d later with a second treatment with GnRH. Cows were inseminated at 0, 8, 16, 24, or 32 h after the second injection of GnRH (ovulation occurs between 24 and 32 h after GnRH). Pregnancy diagnoses were performed by ultrasound at 25 to 35 d post-AI. Pregnancy rates per AI were similar for the groups inseminated at 0, 8, 16, and 24 h and lower for the group inseminated at 32 h. A significant quadratic effect of treatment suggests that the middle time periods (8, 16, and 24 h) may produce the greatest pregnancy rate per AI. However, the group inseminated at 0 h had lowest pregnancy loss, and the group inseminated at 32 h tended to have the greatest pregnancy loss compared with that of the other groups. The calving rate was similar between the groups inseminated at 0, 8, 16, and 24 h and lower in the group inseminated at 32 h. The time of AI also appeared to affect gender of calf: cows bred at 0 and 32 h having a higher percentage of female offspring. In conclusion, there appears to be substantial flexibility in the time of AI after the second injection of GnRH, and lower reproductive rates were observed only when AI was after the time of ovulation.