Hickson listed other benefits such as a shorter unproductive period, more calves over a lifetime, an increased rate of genetic gain and the opportunity to make an earlier selection of replacements.
A survey she conducted in 2007 of 331 farmers managing 16,000 in-calf two-year-old heifers revealed, however, that there were a number of negative aspects to mating yearling heifers. The main issues were:
- a poor rebreeding rate
- stunted growth
- calving problems
- the absence of a buffer mob (no empty two-year heifers)
- a low in-calf rate, and
- the requirement for different management skills.
Hickson’s research showed that one of the more important components of achieving good profitability from mating heifers early is to grow them to their maximum potential through early to mid-pregnancy. Only then should restricting their intake be considered and it should not be severe. Heifers should still be allowed to continue to grow but at a slower rate.
Her research also showed that restricting feed to heifers too much in late pregnancy did not reduce calving problems but reduced milk production and the number of heifers getting back in calf. Hickson’s general advice was to feed heifers moderately well throughout pregnancy.
Bull selection was another important ingredient. A mismatch between heifer pelvic size and calf birthweight is the main contributor to calving problems. Hickson suggested two means for overcoming issues were to grow heifers well and select mating bulls that leave small calves. She recommended buying bulls from a breeder who measures birthweight then target bulls with low birthweight estimated breeding values (EBVs) and good positive EBVs for direct calving ease. Hickson stressed not to neglect growth EBVs, particularly the ones relating to early growth when selecting easy-calving bulls.
As part of her PhD research Hickson intensively monitored calving heifers and found that 84% of non-assisted heifers calved within 150 minutes of the onset of the birth process. This has prompted her to develop assistance guidelines for the Massey herd. Heifers are assisted if there is no calf on the ground after four hours, or if there is no progress in the birth process after two hours, or if there is a malpresentation. These guidelines have resulted in most assisted calves surviving and most heifers getting back in calf.
By the numbers
A national survey carried out in 2007 by Massey senior lecturer Dr Rebecca Hickson of herds mating yearling heifers revealed:
- An 86% pregnancy rate in herds that were pregnancy diagnosed
- The percentage of calves alive at weaning for each heifer joined was 78%
- The percentage of calves alive at marking for each heifer calving was 94%, and
- The percentage of heifers that calved in 2006 and that calved again in 2007 was 84%.
