Fat = Fertility

We all know that cattle in poor condition are hard to get in calf. What may be surprising is that the body condition score (BCS) at calving time has a far greater influence on conception rates than condition score at joining.

After calving there is an infertile period called the postpartum interval where a cow’s body recovers from her pregnancy and prepares for her next breeding cycle. The length of this period is strongly influenced by how much body fat she has at calving. Studies show cows with a BCS of 3 at calving take 89 days to return to heat, while cows with a BCS of 7 take 31 days. The table 1 demonstrates the difference. (American BCS ranges from 1 to 9 were as the Australian range is 0 to 5.) (Houghton et al 1990.)

Table 1

This obviously has big implications if you are trying to have a cow calve every 365 days. Chart 1 is a graphic demonstration of how this interval can affect conception rates, particularly for cattle calving late in the calving season and in poor condition.

Chart 1: How body fat at calving time affects female fertility at joining time

If we look at the effect this interval has on pregnancy rates (Chart 2), we can see how dramatically fat levels at calving affect fertility. Studies show conception rates as low as 15% for very lean cattle to about 90% conception for cattle in good condition. (Selk et al 1988.)

Chart 2: Pregnancy Rates

While adequate nutrition and BCS at joining also makes a difference, BCS at calving has a greater effect. Also prepartum is a more efficient time to increase an animal’s fat levels due to her lower energy requirements (Chart 3). Some breeders think that if their cows are a bit lean or losing weight at calving, they can turn it around after calving and regain their fertility before the breeding season begins. There are a couple of reasons why this doesn’t work.

First, the infertility period begins on the day of calving so any delay in reaching the optimal BCS will increase the infertility period.

The second reason is the first week’s post calving are the highest energy requirements of a cow’s year (Chart 3). This means it is the most difficult and potentially expensive time to be trying to increase the cows BCS especially if the time of calving doesn’t coincide with a time of high feed availability.

Chart 3: Energy Requirements for Maintenance, Lactation, and Pregnancy

At Bannitup this information influences both our time of calving and our sire selection. We have pushed calving back to May and will consider delaying further if required. This is so the cows have their lowest energy requirements during the period of low food availability and calve down after the break of the season, although this is not guaranteed with the changing climate. We also avoid sires with extremely negative fat EBV’s as it will be harder and more expensive to get an adequate BCS on their daughters, partially at the notoriously difficult time after their first calf.