Effect of factors associated with insemination on calving rate in dairy cows

First Service Calving Rates In Irish Dairy Herds: Trends From 1991-1996

In Ireland, surveys in the 1970's have shown calving rates to first service of 60-69%. Since then genetic merit and milk yield per cow have increased significantly. The objectives of this study were to determine calving rates in Irish dairy herds for the period 1991-1996 and if these had declined over time. Breeding records for 58 Spring calving commercial dairy herds maintained on the computerised DAIRYMIS System were analysed. Inseminations were categorised as either DIY AI (DIY), commercial AI (COM) or Natural Service (NAT). Between 1991 and 1996 there was a significant trend towards DIY (P<0.01) and away from NAT (P<0.01) with no consistent trend in COM usage. In 1991, roughly one third of services were to each of the three service methods but by 1996 DIY accounted for nearly 50% of all services. A subset of 34 herds, which had been present for at least five of the six years between 1991 and 1996, was used in the calving rate analysis. Calving rates to first service for DIY (48.4%) and NAT (46.0%) were not significantly different although both were significantly lower (P<0.001) than those for COM (54.5%). For small (<65), medium (65-128) and large herds (˃128) the respective proportion of services in each category were 22%, 44% and 34%. The respective calving rates of 55.2%, 51.6% and 45.7% declined significantly as herd size increased For all service types, there was a significant (P<0.001) decline in first service calving rate over time. The increased usage of DIY, combined with its lower fertility rates, would appear to be one factor responsible for the reduction in calving rate for 1991-1996 period.

Predicting the time of ovulation in dairy cows using on-farm progesterone kits

A protocol of infrequent, but strategically timed milk-sampling was established for predicting the time of ovulation, and thus the optimum time for insemination, in lactating dairy cows. In the experimental group (no. = 49), the time of ovulation was predicted on the basis of a fall in milk progesterone concentrations, which was identified by the use of ‘on-farm’ progesterone assay kits. Reproductive performance in these animals was compared with that in control cows (no. = 45) that were inseminated solely on the basis of oestrous detection by stockmen. The accuracy of ovulation prediction, and of oestrous detection, was assessed from progesterone profiles based on milk samples collected three times weekly from all 94 cows. These milk samples were analysed by laboratory enzyme-immunoassay after the end of the trial.

Over the period during which milk samples were monitored with ‘on-farm’ progesterone kits, a total of 88 ovulations occurred in the experimental group. The progesterone-testing protocol accurately predicted 87 (99%) of these. Over the same period, there was a total of 81 ovulations in the control group and 63 (78%) of these were associated with correct oestrous detection. Conception rates to correctly timed insemination did not differ significantly between groups, but by the end of the three-cycle experimental period, significantly more of the cows in the experimental group (21·8%) than in the control group (4·8%; P < 0·05). In contrast, the use of ‘on-farm’ progesterone results to confirm oestrus avoided any mistimed inseminations (13% of inseminations in the control group). By using the milk-sampling and ‘on-farm’ progesterone-testing protocol, only 1% of ovulations were not accompanied by a correctly timed insemination. This compared with 22% of ovulations in the control group not associated with an insemination because oestrus was not detected. By ensuring that all ovulations are associated with a correctly timed insemination, herd reproductive performance can be significantly improved.

Effect of genetic merit, milk yield, body condition and lactation number on in vitro oocyte development in dairy cows

The effects of milk yield, body condition score (BCS) and lactation number on the number of oocytes recovered and blastocysts formed were studied following in vitro maturation, fertilization and culture of bovine oocytes collected from 48 high and 46 medium genetic merit dairy cows in their first and third lactation. The cows were slaughtered between 125 and 229 d post partum. Ovaries were recovered, and 2- to 10-mm follicles were aspirated. Cleavage rate and number of blastocysts were determined at 44 h and 7 d after insemination, respectively. Oocytes from high genetic merit cows formed fewer blastocysts and had lower cleavage and blastocyst formation rates than those from medium genetic merit cows (0.36 +/- 0.19, 70.4 and 6.8% vs 0.85 +/- 0.22, 77.4 and 11.4%, respectively). The effect of milk production was tested by grouping cows in their third lactation into high and low groups. There was no difference in number of oocytes recovered and subsequent development into blastocysts between the cows in the high milk production group (4559 to 5114 kg, n = 20) and cows in the low yield (3162 to 3972 kg, n = 20) group (6.9 +/- 1.34 vs 8.9 +/- 1.32, respectively). The effect of BCS was tested by grouping cows in their first or third lactation into high and low groups. Cleavage and blastocyst formation rates were greater for oocytes from cows with a high BCS (3.3 to 4.0, n = 20) than a low BCS (1.5 to 2.5, n = 20) (75.7 vs 61.9% and 9.9 vs 3.0%, respectively). Cows in the first lactation yielded fewer oocytes (5.7 +/- 1.24) than cows in the third lactation (7.8 +/- 0.79). Thus, the quality of oocytes probably contributes to reduced fertility, often evident in high genetic merit dairy cows.