- Around 55 successful pregnancies will be established for every 100 cows inseminated during the first round of AB.
- Pregnancy will fail in the first week after insemination for 34 of these 100 cows.
- Fertilisation failure and impaired embryo development are the greatest contributors to pregnancy losses up to 70 days after insemination.
We know from industry statistics that the odds of dairy cows establishing a viable pregnancy to any single insemination are little more than half a chance1. Improving these odds requires answers to key questions, such as: Were the cows actually on heat at insemination? Were the eggs fertilised? Did the embryo die between fertilisation and 21 days after insemination, or did the embryo die later, perhaps explaining why some cows have long-return intervals? These are examples of early embryonic mortality and international data suggests this represents the largest source of reproductive failure in dairy cattle, with the greatest loss occurring in the first three weeks after insemination. Fertilisation of the egg is not considered to be a major problem as 80-100 percent of the eggs are fertilised when cows are inseminated at the correct time2. New Zealand research has shown that six percent of cows pregnant to a single insemination will experience late foetal loss between 42 and 154 days after insemination3. The early embryonic mortality rate has not been determined in New Zealand dairy cattle and there is no information regarding the partitioning of embryo losses during critical developmental stages up to Day 35 of pregnancy. Science solutions for improving pregnancy rates are somewhat hampered by this lack of knowledge.
Accordingly, a two-year study was undertaken to address the issue of early embryonic mortality following the first insemination in the New Zealand dairy cows. The primary objectives of this experiment were to: (1) validate the premise that fertilisation of the egg occurs more than 80 percent of the time and is not the major source of conception failure to Day 35; and (2) measure and partition when losses are occurring between fertilisation and Day 35 of pregnancy. Secondary objectives were to: (1) account for cases where insemination did not coordinate with ovulation (i.e. fertilisation was not possible from the outset); (2) quantify likely pregnancy losses between Day 35 and final pregnancy testing 10-12 weeks after the first insemination; and (3) examine some of the possible risk factors for conception failure, such as calving date, cow condition, pre-mating cycling status, age, and milk yield.
Four farms located in Taranaki, Waikato and Northland were involved over two consecutive breeding seasons with 1890 cows enrolled in the study. Cows were randomly allocated into one of four groups at Artificial Breeding; 8, 16, 35 and 70 days after insemination. Determining embryo losses between 8 and 16 days required removing the embryo from the cow’s uterus by flushing it out. This is possible because the bovine embryo floats unattached in the uterus until implantation at approximately 20 days after insemination. Recovered embryos were evaluated based on stage of development, quality of the embryo (based on the probability of producing a viable pregnancy if transferred into a suitable recipient), and serum progesterone levels. For the remaining two groups, pregnancy rates were determined by ultrasonography 35 and 70 days after insemination.
Submission rates averaged 77 percent and varied between farms, ranging from 67 to 89 percent. The majority of cows (96 percent) were submitted at the correct stage for insemination. We found that 1 percent of the cows had an acute uterine infection and another 4 percent did not produce adequate levels of serum progesterone to support a pregnancy, causing the cow to short cycle or return to heat at approximately 21 days. Recovering embryos eight days after insemination indicated that 87 percent of the eggs were fertilised. However, 13 percent of the flushed cows had embryos that had either arrested in development, or were of poor quality, and so had little or no chance of establishing a pregnancy. The percentage of flushed cows having an embryo that had a high probability of establishing a pregnancy was 66 percent. Thus, within the first week, 34 cows per 100 inseminated had no, or little, chance of establishing a successful pregnancy.
The fate of embryos recovered 16 days after insemination indicated that a further 5 percent of pregnancies had failed, leaving 61 percent of the cows with a good chance of going on to maintain a successful pregnancy. Pregnancy diagnosis by ultrasonography determined that 58 percent of the cows were pregnant 32 to 37 days after the first insemination, using the presence of a foetal heartbeat as the criteria for a viable pregnancy. The final pregnancy rate 70 days after first insemination was 55 percent. Preliminary results demonstrated a greater risk of embryo loss in the first 16 days following insemination for cows that had a body condition score (BCS) less than 4 at the planned start of mating. Cows that were three and eight years of age experienced greater embryo loss, compared with cows in other age groups.
Based on these results, 100 New Zealand dairy cows submitted for their first insemination would result in four cows inseminated at the incorrect time, one cow having an acute uterine infection and three cows not producing adequate progesterone to establish or maintain a pregnancy. Of the remaining 92 cows, 13 cows would have unfertilised eggs and another 13 cows would experience embryo loss. Eight days after insemination, 66 cows would remain pregnant. Sixteen days following insemination, 61 cows would remain pregnant. Pregnancy diagnosis at 70 days following the first insemination would determine that 55 of the original 100 cows remained pregnant. An additional three cows would experience late foetal loss after 70 days which would leave 52 cows calving3. These results demonstrate that fertilisation failure and impaired embryo development in the first week are the greatest contributors to pregnancy failure.
These findings indicate a huge opportunity to improve reproductive performance through science-based solutions for improving egg quality and the early maternal nurturing of a fertilised egg in dairy cows.
- New Zealand Dairy Statistics 2014-15. www.dairynz.co.nz/dairystatistics
- Diskin, M. G., S. M. Waters, M. H. Parr, and D. A. Kenny. 2016. Pregnancy losses in cattle: potential for improvement. Reproduction, Fertility and Development 28: 83-93.
- McDougall, S., F. M. Rhodes, and G. A. Verkerk. 2005. Pregnancy loss in dairy cattle in the Waikato region of New Zealand. New Zealand Veterinary Journal 53: 279-287.
This work was funded by the Ministry of Business, Innovation and Employment, New Zealand dairy farmers through DairyNZ and AgResearch Core Funding.
This article was originally published in Technical Series September 2016