Use of 1alpha-hydroxycholecalciferol in the prevention of bovine parturient paresis.

A synthetic biologically active derivative of vitamin D (350 microgram of 1alpha-hydroxycholecalciferol [1alpha(OH)CC]) was injected into 2 nonlactating 7-year-old Israeli-Friesian cows. Plasma calcium values increased after 24 hours, peaked at 48 hours, and returned to base-line values 120 hours after injection. An injection of 350 microgram of 1alpha(OH)CC was given to 23 parturient-paresis-prone Israeli-Friesian cows from 7 days to 6 hours prepartum; 13 cows were injected once, 6 were injected twice, and 4 were injected 3 times, all at 48-hour intervals. Parturient-paresis-prone cows (n = 23) of the same breed were used as controls. Within 0 to 36 hours postpartum, plasma calcium concentrations were found to be higher in cows injected with 1alpha(OH)CC than in the control cows. The increase was highly significant (P less than 0.01) in cows injected at least twice. None of the cows injected with 1alpha(OH)CC, within 72 to 24 hours prior to calving developed parturient paresis; but 9 of 23 control cows developed parturient paresis. Prior to calving, none of the injected cows developed hypercalcemia and there was no local or systemic clinically detectable signs of toxiosis. When given at the right time prepartum, 1alpha(OH)CC is considered to be an improvement over previous methods of preventing bovine parturient paresis.     

Use of a beta 2-adrenergic stimulant (clenbuterol) for eliminating night-time calving.

To determine whether beta 2-adrenergic stimulant (clenbuterol) would exhibit beneficial effects upon the elimination of night-time calving, 42 Holstein Friesian cows and heifers at the first stage of labour were injected intramuscularly twice with clenbuterol; 300 micrograms at 18:00 and 210 micrograms at 22:00. Other 26 cows and heifers of the same breed at the first stage of labour at 18:00 were not treated and served as controls. All of the treated animals, but one, calved after 5:00 in the next morning, showing a peak of parturition between 5:00 and 10:00. In contrast to this, 42% of the control animals calved at night between 22:00 and 5:00. Thus, the double administrations of clenbuterol were shown to be effective for eliminating night-time calving. The cows treated with clenbuterol tended to show lower incidences of dystocia and retained placenta and a higher first insemination conception rate than the control animals. No adverse effects of the treatment on the viability of newborn calves and milk yields were observed.     

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.     

Reductions in milk production after induced parturition in dairy cows from commercial herds in south-western Victoria

SUMMARY The milk production of cows after induced parturition was compared with that of non-induced cows with similar calving dates, in 37 winter-calving, pasture-fed, commercial dairy herds in south-western Victoria. Parturition was induced when most cows were between 27 and 35 weeks of pregnancy. Milk production was compared for the lactation that commenced immediately after induction. The percentage of cows whose lactations were terminated less than 120 days after calving was increased by induction. Mean percentages for untreated and induced groups were 3.0% and 5.4%, respectively . For cows with lactations of at least 120 days duration, lactation length was not affected by the prior induction of parturition. However, milk production during the subsequent lactation was 9.0% less in induced groups. Fat and protein concentrations were not substantially affected. The reduction in milk production was not affected by stage of pregnancy when cows were induced, but tended to be greater in herds that delayed milking of induced cows .     

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.     

Pregnancy rate of cows given synthetic gonadotrophin-releasing hormone at the time of service

A trial involving 25 herds was conducted in the Maffra area of Victoria to study pregnancy rates after injection of synthetic Gn-RH at service. Of 3502 cows having a first service, 674 cows were injected by the farmer at first service with a single intramuscular injection of 250 micrograms of synthetic Gn-RH, while the remaining 2828 cows acted as untreated controls. First service pregnancy rates were 58.8% in the group treated with Gn-RH and 54.1% in the untreated group, a 4.7% increase or an odds ratio of 1.21 (P less than 0.05). However, adjustment for the potentially confounding variables calving to first service interval and herd reduced the odds ratio to 1.07 (P less than 0.4). Treatment with Gn-RH at second or third service did not alter pregnancy rate to that service compared to untreated services. There was no interaction of treatment with Gn-RH at first service with calving to service interval. It is concluded that injection of 250 micrograms of synthetic Gn-RH at the time of service does not increase pregnancy rate to that service.     

Use of pre-partum urine pH to predict the risk of milk fever in dairy cows

The objective of this study was to evaluate the correlation between serum calcium (Ca) and inorganic phosphorus (IP) values and urine pH of cows fed common rations without the addition of anionic salts in late pregnancy. One hundred and seven Holstein cows, having completed two or more lactations and with an expected calving date within the next seven days were selected from two herds. In order to determine levels of serum Ca and IP and urine pH, blood and urine samples were collected seven to one days before parturition. Of the 107 sampled cows, 17 developed recumbency after calving and were considered to be affected by milk fever. There were significant ( p<0.01 ) negative correlations between urine pH and serum Ca, IP and the ratio of Ca to IP, The urine pH, and levels of serum Ca and IP measured within 48 h prior to parturition differed significantly ( p<0.001 ) between recumbent and non-recumbent cows. The sensitivity, specificity, positive and negative predictive values of urine pH test 48 h prior to parturition, using a cut off level of above pH 8.25, were 100%, 81%, 55%, and 100%, respectively. These signify that monitoring urine pH within 48 h prior to parturition is a sensitive method to assess the risk of parturient paresis. The results of this study emphasize the importance of acid-base status of the animal in the pathophysiology of milk fever.     

Effect of Supplemental Feeding with Glycerol or Propylene Glycol in Early Lactation on the Fertility of Swedish Dairy Cows

The aim of this field study was to evaluate the effect of supplemental feeding with glycerol (GLY) or propylene glycol (PG) during early lactation on the fertility of Swedish dairy cows. Within 17 commercial dairy herds, 798 cows were randomized to three groups that were daily fed supplements with 450 g GLY, 300 g PG or nothing (control, C). The supplements were given twice daily during 0–21 days in milk as a top dress on concentrates. Data on calving date, insemination dates, gynaecological examinations, as well as breed, parity and monthly milk yield were collected. From a subset of 308 cows in seven herds, milk samples for progesterone analysis were taken twice weekly and used to determine the time for onset of luteal activity. The effects of supplements on the intervals from calving to first luteal activity (FLA), first AI (FAI) and conception (CON), respectively, were analysed using semi‐parametric survival models (Cox proportional hazards models) controlling for the effect of parity, breed, calving season, milk yield and the clustering effect of herd. There was no difference in time to FLA between the cows in group C and in group GLY or PG. No differences in time to FAI or in time to CON were found between cows in group PG and group C. However, cows in the GLY group tended to get their FAI later compared with cows in the control group but without at subsequent delaying of time to CON.     

Impact of iodine supplementation of dairy cows on milk production and iodine concentrations in milk

AIM: To determine the effect of iodine supplementation on milk production and iodine concentrations in milk for pasture-fed, seasonally calving dairy cows. METHODS: The study was run over two consecutive seasons on the same dairy farm. In Trial One, 294 Friesian dairy cows were either untreated or injected intramuscularly three times with iodised oil (2,370 mg iodine/dose) at the start of lactation (Day 1) and at about 100-day intervals thereafter. Iodine concentrations in milk were determined prior to each injection on Days 1, 98 and 210, and milk, fat and protein yields measured on Days 43, 98, 176 and 238. In Trial Two, 234 Friesian dairy cows were either untreated or injected intramuscularly with iodised oil (2,370 mg iodine/dose) in early lactation (Day 1) and 99 days later. Iodine concentrations in milk were determined prior to treatment on Days 1, 55 and 161. Serum thyroxine (T4) and triiodothyronine (T3) concentrations were determined in the latter two samples and milk, protein and fat yields were determined on Days 25, 119, 182 and 221. In both trials, pasture was sampled at 1- or 3-monthly intervals and iodine concentrations in herbage measured. RESULTS: Iodine concentrations in pasture averaged 0.24 mg/kg dry matter (DM) in both seasons and varied little over the experimental period. Iodine concentrations in milk from unsupplemented cows were <20 microg/L in both trials. Three iodine injections at about 100-day intervals increased iodine concentrations in milk to 58 microg/L for at least 98 days after each treatment. Two iodine injections at about 100-day intervals raised milk iodine concentrations to 160 microg/L and 211 microg/L at least 55 days after each treatment, but had no effect on serum thyroid hormone concentrations. Iodine supplementation had no significant effect on milk, milkfat or milk protein yield. CONCLUSIONS: Pastures in New Zealand that contained 0.24 mg iodine/kg DM appeared to provide an adequate iodine intake for dairy cows. Intramuscular injections of iodine resulted in significant increases in iodine concentrations in milk. CLINICAL SIGNIFICANCE: Increasing iodine concentrations in milk via intramuscular injection of iodine could provide a method for significantly increasing iodine intakes of humans, in particular, children.     

A novel nutritional strategy to prevent milk fever and stimulate milk production in dairy cows

AIMS: To assess the efficacy of a feed supplement designed to reduce the dietary availability of calcium when fed during the last 2-3 weeks of pregnancy, on the incidence of postparturient hypocalcaemia (milk fever) and on milksolids (MS) production in pasture-fed dairy cows in early lactation. METHODS: In Experiment 1, late-pregnant Holstein/Friesian cows, 4-10 years old, grazing pasture, were either fed the supplement (1 kg/cow/day) for 2-4 weeks prior to calving (n=11) or no supplement (n=10). Plasma calcium concentrations were measured immediately before and after a 20 h fast intended to induce hypocalcaemia at the end of this period, 1-7 days before the cows calved. In Experiment 2, mixed-breed dairy cows, 3-10 years old in 7 commercial dairy herds were fed the same supplement (1 kg/cow/day) for 1-3 weeks immediately prior to calving (n=565) or no supplement (n=614), and incidences of clinical milk fever and MS production at 6-9 weeks post calving were compared between groups. RESULTS: In Experiment 1, plasma calcium concentrations were higher (p=0.01) in supplemented than in non-supplemented cows after fasting, indicating reduced susceptibility to fasting-induced hypocalcaemia. In Experiment 2, the incidence of milk fever was lower (6.4% vs 17.1%, p=0.001) and MS production was higher (p=0.01) in supplemented than in non-supplemented cows. CONCLUSIONS: These preliminary results demonstrate that feeding a supplement designed to reduce dietary calcium availability for 2-4 weeks immediately prior to calving reduced the susceptibility of cows to fasting-induced hypocalcaemia, reduced the incidence of clinical milk fever and increased MS production in early lactation. CLINICAL RELEVANCE: The concept and product reported here have potential to provide dairy farmers with a practical means to prevent hypocalcaemia and improve milk production during early lactation.     

Production responses of lactating dairy cows and heifers given thiabendazole at parturition

A field trial was conducted to examine the effects on productivity of dairy cattle in 6 herds given thiabendazole at parturition. Criteria examined were milk and fat production, calving interval, services per conception, culling rate, and (in a subset of cows) hematologic response. Analysis of the data for all cows as a group did not indicate a significant effect. For first-calf heifers in 1 herd, a significant (P less than 0.01) and economically important increase in milk production was observed; why the effect was seen was not determined.     

Induction of parturition in dairy cattle and its effects on health and subsequent lactation and reproductive performance

OBJECTIVE: To determine the effect of induction of parturition on health, milk production and reproductive performance of dairy cows. DESIGN: A prospective cohort study in 62 dairy herds. PROCEDURE: Health, milk production and fertility indices were documented for 1449 dairy cows treated with dexamethasone trimethylacetate, with or without prostaglandin to induce calving. Equivalent data was collected for 603 untreated herd mates that calved at approximately the same time. RESULTS: The median interval from initial treatment to calving was 11 days. Induction was associated with a substantially lower calf survival and commercial value of surviving calves. Calf viability and value was lower when induced cows were at an earlier stage of pregnancy. Retained foetal membranes, photosensitisation and other problems were significantly more frequent in the induced group compared to the untreated group. Milk production of induced cows was approximately 4% lower than untreated ones, but the majority of reproductive indices were not significantly different between the two groups. CONCLUSIONS: The practice of induction of parturition in seasonal calving dairy herds is a reliable way of shortening the gestation period of cows. Costs associated with morbidity and mortality of induced cows and losses in lactation and calf production are offset by benefits of improved reproductive performance and more efficient management of the herd. The welfare aspects of induction on calf survival must be considered.     

Effect of prostaglandin treatment on the fertility of problem cows

Four autumn-calving dairy herds were selected to investigate the effect of an injection of prostaglandin in the period 14 to 28 days (mean 22 days) after calving on subsequent fertility. The cows were selected on the basis of having a condition likely to affect their fertility, including assisted calving, endometritis, retained fetal membranes, milk fever, cows with five or more lactations, cows having twins, or a combination of any of these conditions. They were assigned to treatment or control groups and paired as closely as possible on the basis of their condition and date of calving. Milk progesterone concentrations were measured on the day of treatment and then three and 10 days later. The trial ran for four months and involved 90 treated and 90 control cows. The combined data from all the animals in the trial failed to show any difference between the calving to conception interval, the first service conception rate or the numbers of services per conception of the treated and control groups. A Student's paired t test for groups of cows with a particular condition, both within individual herds and in all the herds, failed to show any significant effect of treatment (P greater than 0.05). Milk progesterone data showed that the presence of a corpus luteum did not influence the outcome of prostaglandin treatment. There was no evidence for excessive failure of luteolysis. It was concluded that there was no benefit in a routine injection of prostaglandin to dairy cows in the period 14 to 28 days after calving when re-breeding commenced more than 70 days after calving.     

1alpha hydroxycholecalciferol in the prevention of milk fever

During the 12 months commencing July 1976 the animals in a herd of 400 Friesian cows which were carrying their second or later calf were injected intramuscularly with either microgram of 1alpha-hydroxycholecalciferol in 5 ml aqueous carrier or with 5 ml of carrier. The injections were randomised so that 135 of the 269 injected cows received the drug and 134 received the carrier solution. Ten of the experimental group (7.4 per cent) and 22 of the control group (16.4 per cent) had to be treated for milk fever, (P less than 0.08). The protective effect was greatest, (P less than 0.001), when the cows calved between one and eight days after injection of the drug.     

Effects of four hormone treatments after calving on uterine and cervical involution and ovarian activity in cows

Uterine and cervical involution after calving was measured sequentially in 25 parous commercial Friesian cows by using electronic calipers and real-time ultrasound imaging, transrectally. Ovarian activity was monitored by using real-time ultrasound imaging and by the assay of milk progesterone concentrations. Peripheral plasma prostaglandin F2 alpha metabolite concentrations were assayed during this period. Five groups of five cows were treated intramuscularly, approximately 48 hours after calving, with either 100 mg progesterone in oil, 25 mg dinoprost tromethamine, 5 mg oestradiol benzoate, 1.2 mg of the long-acting oxytocin analogue carbetocin or 5 ml sterile water. There were no statistically significant differences between the intervals from calving to the completion of involution or between the intervals from calving to the first ovulation for the cows in the different groups. The mean PGFM concentrations in the peripheral circulation were significantly higher in the carbetocin-treated group, presumably owing to the stimulation of endogenous prostaglandin F2 alpha secretion. This study provides no evidence that a single treatment with any of the four hormones significantly affected the reproductive function of the cows after calving.     

The effects of calcium supplementation of dairy cattle after calving on milk, milk fat and protein production, and fertility

AIM: To determine the effect of calcium supplementation of dairy cattle after calving on serum concentrations of calcium, magnesium, and phosphorous after calving, and on reproductive and productive performance in the following lactation. METHODS: This was a double-blind randomised clinical trial using 356 dairy cows of parity 3 or greater from six seasonally calving commercial dairy herds. Cows were randomly assigned to a treatment and control group. The treatment group received two doses of 380 ml of calcium chloride in oil (49 g of elemental calcium); the first dose was given as soon as practical after calving and the second 12 hours later. Control cows received 380 ml of a placebo using the same treatment regime. Treatment and control cows had blood samples taken during the first 7 days after calving to determine serum calcium, magnesium and phosphorous levels. Treatment and control cows were monitored throughout the lactation to measure production and reproductive performance. RESULTS: Treatment had no effect on PSM to first service intervals. Treatment significantly reduced PSM to conception intervals in only one of the four herds with data available for analysis (p = 0.09). There was no difference between treatment groups for adjusted corrected milk yield calculated from the first herd test milk yield (p = 0.93) or 305-day milk solids production (p = 0.19). CONCLUSION: Further studies are required to identify reasons for the between-herd effectiveness of calcium chloride treatment in improving fertility.     

Coxiella burnetii: Serological reactions and bacterial shedding in primiparous dairy cows in an endemically infected herd—impact on milk yield and fertility

Coxiella burnetii (C. burnetii) is the causative agent of Q fever both in humans and animals. The objectives of this study were to investigate seropositivity and bacterial shedding in heifers and primiparous cows in an endemically infected herd and to assess the effects on post‐partum diseases, fertility and milk production. At the age of 9 months, 96 Holstein heifers were included. Sampling was performed reproduction‐orientated: at the beginning of the study, at detection of first pregnancy, 3 weeks before expected calving date (blood serum), at parturition and after 21, 42, 100 and 150 days in milk (DIM) (blood serum, vaginal swabs and milk). Serum samples were investigated by a commercial ELISA for the presence of specific antibodies and vaginal swabs and milk samples by PCR to detect C. burnetii DNA. Individual animal data (calving ease, stillbirth, retained foetal membranes, puerperal metritis, endometritis after 42 DIM, presence of corpus luteum after 42 DIM, interval calving‐first service, interval calving‐conception, number of inseminations until 150 DIM, proportion of pregnant cows until 100 and 150 DIM, proportion of pregnant cows after first service and data of the dairy herd improvement test) were documented. All heifers were seronegative at the age of 9 months and 3 weeks before the expected calving date. Subsequently, the proportion of seropositive animals and the antibody score increased significantly towards 42 and 100 DIM, respectively. Vaginal C. burnetii shedding was highest at parturition (30.9%), while the most positive milk samples were detected after 100 DIM (15.3%). Coxiella burnetii seropositivity and shedding had no impact on parameters of reproduction. However, milk fat yield was declined in puerperal vaginal shedders and cows which seroconverted during their first 42 DIM, respectively.     

Risk factors for abomasal displacement in dairy cows.

OBJECTIVE: To determine whether various periparturient events or 305-day milk production during the previous lactation period were associated with abomasal displacement in dairy cows. DESIGN: Retrospective, case-control study. ANIMALS: 75 pairs of case and control cows from 3 university-owned and 3 commercial dairy herds. PROCEDURE: Cows with abomasal displacement were matched with control cows on the basis of herd of origin, breed, age, and calving date. Frequency of specific periparturient events during the period from 2 weeks prior to parturition to diagnosis of abomasal displacement, as well as milk production during the preceding lactation period, were compared between case and control cows. RESULTS: Multivariate analyses indicated that case cows were significantly more likely to have had retained placenta, ketosis, a stillborn calf, metritis, twins, or parturient paresis than were control cows. Dystocia, mastitis, and milk production during the previous lactation period were not associated with abomasal displacement. CLINICAL IMPLICATIONS: Results indicated that a variety of periparturient events were associated with development of abomasal displacement among dairy cows.     

Comparison of cholesterol levels among lipoprotein fractions separated by anion-exchange high-performance liquid chromatography in periparturient Holstein–Friesian dairy cows

Changes in lipoprotein profiles occur in dairy cows during the periparturient period and in cows with transition cow disease. Here, the lipoprotein profiles of Holstein–Friesian dairy cows during the periparturient period were obtained by anion-exchange, high-performance liquid chromatography to evaluate the usefulness of lipoprotein profile evaluation during the periparturient period and in cows with fatty liver and milk fever. Lipoprotein levels (including total and high- (HDL-C) and low-density lipoprotein (LDL-C) cholesterol) in 10 healthy cows were low 4 weeks prepartum, with the lowest values at calving or within 1 week of calving; the values increased at 8 weeks postpartum. The lipoprotein levels were measured in 16 cows diagnosed with fatty liver (n=10) or milk fever (n=6) and compared to 10 healthy dairy cows. A significant difference was observed in HDL-C between healthy cows (at calving and 1 week postpartum), and the fatty liver and milk fever cows. Cows with fatty liver and milk fever had a lower mean HDL-C than the 10 healthy dairy cows at calving and 1 week postpartum. HDL-C might be a good indicator of energy balance for differentiating healthy cows from those with transition cow disease.     

Effects of sodium monensin on reproductive performance of dairy cattle. I. Effects on conception rates, calving-to-conception intervals, calving-to-heat and milk production in dairy cows

SUMMARY A total of 1061 lactating dairy cows in six different herds were randomly allocated to treatment and control groups. One herd was lot-fed on total mixed rations; three herds were fed on pasture with significant amounts of supplementary concentrates, and two herds were primarily pasture fed. Treated cows received a slow-release, intraruminal capsule containing 32 g sodium monensin within 7 days of calving. Conception rates at first service, days to first oestrus and calving-to-conception interval did not differ significantly between untreated and monensin-treated cows in the 5 herds, 3 herds and 4 herds, respectively, in which these outcomes were examined. Treatment of lactating cows immediately after calving may not be the optimal method to achieve fertility responses with capsules containing sodium monensin. Monensin treatment significantly increased milk yield in one of the six herds. Milk fat or milk protein production was not significantly affected by treatment.