As beef producers either wrap up their spring joining, one vet has urged them to consider the pros and cons of how long the bulls remain with the cows.
Dr Stephen Laing, Bong Bong Veterinary Hospital, Bowral, was guest speaker at the recent Yallambee Angus field day at Berrima, where he explained the flow-on effects of joining periods and early pregnancy testing in cattle.
"I think people get advice from the wrong places about joining periods. They get advice from people that say, 'get more calves on the ground', not actually understanding the economics of leaving bulls in for far too long," Dr Laing said.
"When I pregnancy test, there can be such a spread of ages."
Dr Laing encouraged producers to join heifers for six weeks and their cow herd for nine weeks, with the heifers being joined three weeks before the cows.
This allowed the later calving heifers an extra three-week turnaround before they joined the main cow herd the following year.
"It is actually more profitable having 95 per cent of females pregnant in a nine-week joining for say cows, then it is to have 100pc pregnant of a 12 or 15 week joining," Dr Laing said.
"(Producers) think it's more calves, but just having that (tighter) age spread from a short joining you have a more even line of animals for sale.
"In particular, weaners reared from a shorter joining will be older and heavier on your weaning date, which therefore means your retained heifers will be fertile and start cycling earlier."
Weight and age were a big component when it came to conception rates in joined heifers, he said.
By using a six-week joining as opposed to a 12 week joining, home-bred heifers will be at least 10 kilograms heavier and 10 days older.
He said heifers joined at 15 months needed to reach a critical mating weight of 55 to 60 per cent of the heifer's predicted mature cow weight.
Shortening joining periods could not be carried out over a single year, he said. Under good conditions, it could only be reduced by one to two weeks a year to allow cows the time to adapt.
"Because in these herds that are joining for 12 weeks, trying to get that cow that calves in the last week of joining back to an eight or nine week joining, can be very difficult as she's only just calved as we're trying to put the bull back in," Dr Laing said.
"If you can get in five to six weeks after the bull's out and on an eight-week joining, the spread of ages of the calves is so much tighter and younger, we can split off those ones that are going to calve late and sell them as pregnancy-tested-in-calf, and that will help further tighten the herd.
"A cow that calves within the first cycle within a year will continue to do so. (Research has) shown that cows that calve early will calve again the following year."
Dr Laing said producers' main concerns were not having enough heifers in calf to replace the cast-for-age cows leaving the program that year.
"There is more risk with a short joining period. In a six-week joining they've only got two cycles, they come on heat every three weeks," Dr Laing said.
"They have to be cycling the day the bull goes in and that's where it's a good idea to retain at least 70 to 80 per cent of your heifers to join, as it reduces the risks of a short joining period.
"If something didn't go perfectly in those six weeks in your heifers, at least you've joined more."
Dr Laing said producers could also benefit from shorter joining periods in dry times, in conjunction with earlier weaning to give the cow a spell to regain condition.
However, this became more difficult if calving was spread across a larger window.
"If you're pulling calves off at five months but you have a 12-week joining, what are you going to do with the two-month-old calves?" Dr Laing said.
Health factors for the calves and their mothers were also more prominent in a longer joining period, with a prolonged high demand period for the cow.
Illnesses such as grassy tetany become more common in these situations.
As for the calves, illnesses such as scours were more frequent with a mixed-age population within the herd.
"What we start to see is, even though they are always there, is these pathogens and bugs that are shed in faeces are on higher volumes in a calving paddock," Dr Laing said.
"You've got a 12-week-old calf and then comes a one-week-old calf. We just find they are more susceptible to disease and I have definitely started to see that in late October, November here.
"Those herds that start calving in August, but are calving for too long, we've started to see calf scours and they're all in the younger-born calves.
"It depends on where you are, down around Wagga, Gundagai, Holbrook, you've got herds well and truly done and dusted calving by October, but as you start moving up around here (Bowral) people are only joining in November, December, so we still have calves being born in October, November."