A study at the Western Beef Development Centre (WBDC) near Lanigan, Sask., shows that pregnancy rates for heifers bred at 55 per cent of their mature body weight were on par with heifers bred at a more traditional 60 to 65 per cent of their mature body weight. This pattern held true in all three years of the study whether the heifers were raised in a conventional drylot or a bale-grazed pasture.
The 60 to 65 per cent target is largely based on U.S. research in the 1960s however most of the studies done since 2004 have concluded heifers can be raised with more modest gains and still breed on time.
All of that work was done on heifers wintered on cornstalks and winter grass through their first calving season. The WBDC was the first to explore whether heifers could successfully be grown out with moderate gains on winter pastures in Western Canada. The study ran for three years to be sure the lighter weights did not jeopardize the heifers’ future fertility.
“A heifer development program is all about trying to grow a replacement heifer to breed between 14 and 15 months of age and deliver her first calf by two years of age. The real question is how they perform going forward as two- and three-year olds and retention in the herd,” says WBDC research scientist Dr. Bart Lardner.
The WBDC study involved 174 Angus-type heifers weighing 554 to 565 pounds at weaning. They were assigned to one of four groups and managed to reach 55 per cent of mature body weight (moderate gain) at breeding or 62 per cent of mature weight (high gain) in either a drylot pen or a bale-grazing system.
All four groups received good alfalfa-grass hay bales containing 60 per cent total digestible nutrients (TDN) and 10 per cent crude protein (CP) fed free choice along with a processed barley grain (85% TDN;12.5% CP). The heifers were weighed frequently to monitor performance and adjust the diet through the winter and right up to June 2 when they were moved to the breeding pasture.
From then until they were preg-checked in October the heifers were managed as a single group on mixed alfalfa-smooth brome grass-crested-wheat grass pasture. The pregnant heifers were then grazed on swathed barley (69.3% TDN;10.8% CP) from November to March. From calving through May, they were fed in pens on free-choice grass-legume hay and five pounds of range pellets per head per day.
All winter and calving diets were designed to meet NRC recommended protein and energy requirements for pregnant beef heifers of this type.
At turnout the first year, the moderate-gain group averaged 787 pounds with an average body condition score of 2.6 on a scale of one to five, where one is emaciated and five grossly fat. The high-gain group averaged 875 pounds with a 2.8 body condition score.
More from the Canadian Cattlemen website: Undernourished bull calves can’t make up lost ground
“What we saw during the summer on good-quality pasture was that the high-gain heifers gained 1.5 pounds a day, which was similar to their gain during winter. The moderate-gain heifers gained 1.1 pounds a day during development and put on more than two pounds a day during summer, which can be attributed to compensatory gain on pasture,” says Lardner.
A word of caution: the winter diet of a moderate-gain program must provide adequate nutrition so the heifers are in good body condition by breeding season.
The heifers were exposed to bulls for 63 days. Pregnancy diagnosis showed no statistical difference with 84 to 88 per cent settled for the moderate-gain groups and 85 to 90 per cent for the high-gain group.
Calculations using the projected calf birth date and a 279-day gestation put the percentage of heifers pregnant after 45 days at 98 and 95 per cent for the moderate- and high-gain heifers, respectively.
Pregnancy rates were similar: 95 per cent in year two and 94 per cent in year three for the moderate gainers: 95 to 96 per cent in year two and 93 per cent in year three for high gainers.
Finally, the proportion of heifers exposed to bulls as yearlings that were still in the herd as pregnant three-year-olds was similar for both.
Following the heifers and their calves through the first and second calving revealed no differences in calf birth weights and weaning weights.
Feed costs for post-weaning development were approximately $60 per head less for the moderate-gain groups because they ate less than the high-gain group.
The big question for these modest-weight heifers was could they reach puberty by breeding season since the age at which a heifer reaches puberty is largely determined by her weight.
In this study there was no difference in reproductive performance. In short, moderate-weight heifers do get pregnant.
Go to www.wbdc.sk.ca to find his presentation and the heifer development study fact sheet when it becomes available.