Winter holidays were cut short and transport had to be arranged early to bring back cows from runoffs as some calving dates on reports were earlier than expected last year. As well, cows started calving before they were due.
LIC, although it doesn’t select for or against gestation length in its fresh semen bull teams, admits that its gestation lengths are getting shorter, probably due to short gestation giving more days in milk and so a higher breeding worth (BW). The KiwiCross daughter proven team for 2012 has an average gestation length breeding value (BV) of -3.55. As traits are usually halved on farm (due to the genetic influence of the dam), cows mated with the team will calve on average almost two days earlier than the farmer planned.
“Aside from the issue of the team average, there is natural variation. Most bulls fall within a range of plus or minus six days which is three days on farm, but a few will vary up to BV 9 or even more,” LIC’s general manager, genetics Peter Gatley said.
Compounding the problem, cows on farms may have significant short gestation lengths themselves, of up to nine days.
“For example, if you put a -8 BV bull over a -8 BV cow, MINDA would know the value of the bull and would predict calving four days early. On average, though, the calving would be eight days early, but there’s variation around the date so it could be even earlier than that, 10-12 days early.”
Although MINDA calving reports show the bull’s influence on the calving date, it doesn’t take into account the cow’s gestation length.
“Each cow has relatively little data underpinning its gestation length BV so reliability is low,” Gatley said. “For this reason only bull BVs are including in the calculation of calving date, and only when the BV has a reliability of more than 60%.”
He suggested farmers not use bulls with short gestation lengths in the first week of mating, but as PSMs differ throughout the country, it was impossible for LIC to take out short gestation length bulls for the first week of AI in its fresh semen teams. But if the fresh semen bull that day is a high BW bull, Gatley said farmers should think again. Howies Checkpoint had a short gestation length of -5.6, so on average cows would calve nearly three days early.
“But who wants to pass up the highest BW bull in the world to select for a minor gestation length difference?”
Using Datamate (a handheld computer used to record matings before inseminating) to warn AI technicians of short gestation bulls in the first week of mating on a farm was not justified.
“With the data we have available we have limited opportunity to manage the gestation length issue as half the genes come from the female side where there is low reliability.”
CRV Ambreed made sure clients were aware of which of their bulls had short gestation traits so they could plan their mating accordingly and if necessary avoid using them in the first week, marketing manager Peter Berney said.
“Typically what we do is suggest to farmers to use the short gestation bulls tactically – keep them until the end of the mating to bring calving dates forward, but not at the start. Farmers should be thinking about all of the traits of a bull.”
From next year gestation lengths of all CRV Ambreed bulls will be recorded in sales catalogues.