Most farmers are now at the six-week stage of breeding. Many will be turning out bulls to clean up the rest of the repeats as it gets much harder to spot cows in heat when there are fewer cows bulling. On the other hand, some farmers will use vasectomised bulls and continue to artificially inseminate until the end of the breeding season. If using mop-up bulls, you should match the number of bulls to the number of cows not yet pregnant. As a rule of thumb and depending on herd fertility, about 55% of the cows served in the first round should be in calf. If 90/100 cows were served this means that 50/100 cows held in the first three weeks. If 40 cows were served in the second three-week period and if 60% of these held, this is another 24 cows in calf. So at this point around 74/100 cows should be pregnant, leaving 26 cows to be served over the next six weeks.