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Prof Keith Macmillan from the University of Melbourne believes the fertility levels of high-producing dairy cows, given sufficient time to recover from the energy demands of early lactation, have scarcely declined over the years. The genetic selection for increased milk yield has increased energy partitioning towards the udder by altering hormone balance. Rapidly mobilising body tissue in early lactation comes at a price – fertility rates may be held up while the cow recovers from the early lactation energy demands.

Dairy cow health as shown by lactations/cow/lifetime is declining in most countries, along with milk yield increases. Major causes are associated with periparturient diseases, udder health and lower fertility. Summaries of reproductive records for herds of different levels of production have shown that milk production has negligible effects on reproductive efficiency. Improved reproduction in high-producing herds is probably owing to better feeding, healthier cows and better reproductive management.

Macmillan believes the fertility decline is genetically linked to changes in ‘modern’ cows associated with selection for higher milk yield. Fertility changes have been much less in non-lactating cows and heifers. Embryos from lactating dairy cows have been found to be inferior to those from dry cows as little as five days after ovulation. Research has shown that the average rate of embryo loss from days 27 to 45 in pregnant Holsteins is 13%. This same figure was found in research in Victorian (Australia) Holstein herds, where 13% of the cows not conceiving to the first insemination were found to have the same hormone concentrations as the pregnant cows for 6 to 12 weeks.

The most likely cause of the embryonic deaths is an abnormal uterine environment rather than abnormal genes. This raises the question of whether there is a conflict between the requirements of the uterine embryo and the udder demands for lactation. When the embryo dies after day 17, the cow does not show oestrus for a prolonged period.

Other health issues such as chronic or acute metritis, retained placenta, ketosis, lameness and ovulation dysfunction could all be linked to the decline in fertility. Reproductive performance can also be linked to milk composition. Some of the hormonal changes may become useful indicators or markers for yield and fertility.

How do you measure fertility?
The majority of people would probably answer that they use the conception rate. The conception rate on its own can be deceptive and needs to be looked at in combination with the number of cows serviced and the number of pregnancies, especially when comparing the work of two inseminators or comparing between farms. One inseminator (or farm) may have a high conception rate but only a few cows serviced and another inseminator (or farm) may have a lower conception rate, but owing to servicing, more cows have more pregnancies.

Another disadvantage is that it only includes pregnant cows, while cows that have left the herd are excluded. The advantages are that it measures the success of the services creating pregnancies; therefore, it is a well-known and commonly used measure of semen placement.

The intercalving period is another measure that is often used, but again only tells part of the story. It is a historical measure, as it can only be measured when a cow has had two successful calvings and only cows that remain in the herd are included.

A measure of fertility, which takes all the cows in the herd into account and is fast moving (effects of changes can be seen within weeks rather than months), is the core or 21-day pregnancy rate.

This measure is calculated by multiplying the heat detection rate by the conception rate. The heat detection rate is the number of cows inseminated in a 21-day period divided by the number of cows available to be bred in the same period.

The cows available for insemination are the cows that are past the voluntary waiting period, are not barren and are not yet pregnant. The conception rate is the number of cows pregnant divided by the number of cows inseminated.

This measure will tell you how well you are managing the herd. If necessary, it will allow you to take corrective action sooner rather than later. It also tells you how many pregnancies you are achieving. For this measure, and all others, solid and accurate data are needed.

Phone +27 (0)33 330 6698 or e-mail iqnutrition@iafrica.com for more information.