EVIDENCE continues piling up that shows heifers can be effectively bred at lighter weights.
Significant amounts of data to this effect have come from Nebraska, in particular, and now, another project at the U.S. Meat Animal Research Center in Clay Center, Neb., adds more fuel to this fire of change.
In recent history, animal scientists recommended feeding replacement heifers a diet to achieve 60% to 65% of mature body weight, or BW, by breeding at 14 months of age. This was based on research conducted during the late 1960s through the early 1980s.
However, research conducted over the past 10 years has found that feeding beef heifers to 50% to 55% of mature BW reduced body size and development costs without compromising pregnancy rate.
Recently published research from the U.S. MARC examined whether developing pre-pubertal heifers on less dietary energy and to a BW of 55% rather than 65% at 14 months of age would compromise ovarian development and reduce fertility.
These researchers used 60 head of 8-month-old Angus and MARC II heifers each year in a study replicated in 2009, 2010 and 2011. MARC II cattle are a stable composite of a quarter Angus, a quarter Hereford, a quarter Simmental and a quarter Gelbvieh.
In each year, heifers received either a low- or high-gain diet, fed to achieve an average daily gain of either 1 or 1.75 pounds per day from 8 to 15 months of age, including the first 21 days of the breeding period.
The low-gain diet consisted of 30% corn silage and 70% alfalfa haylage, which was 13% crude protein and 61.6% total digestible nutrient. The high-gain diet was 69% corn silage and 31% high-moisture corn, which was 11.8% crude protein and 74.4% TDN.
Then they were transferred to pasture. At 14 months of age, the heifers were exposed to bulls for 47 days.
Average daily gain during the treatment period was 1.74 pounds per day for the high-gain heifers, compared with 1.04 pounds per day for the low-gain heifers. The high-gain heifers were 16% heavier than the low-gain heifers at the onset of the breeding period (926 vs. 798 pounds). They were 12% heavier at the end of the 47-day breeding period (921 vs. 824 pounds).
In 2010 and 2011, 97.2% of all the heifers were cycling by 21 days of breeding. Postweaning ovarian development was not compromised by treatment.
Researchers said a greater proportion of the high-gain heifers, compared with the low-gain heifers, conceived within the first 21 days of the breeding period. The actual numbers were 64.4% vs. 49.2%. Yet overall pregnancy rate was not affected by treatment. Those actual numbers were 83.0% vs. 77.7%, respectively. This was not statistically different.
The researchers concluded that the results from this study agree with previous similar studies. They said development of replacement beef heifers on less energy and at a smaller average daily gain from 8 months of age to achieve 55% of their mature BW at breeding may enable beef producers to reduce associated feed costs without compromising ovarian development or the proportion of heifers pregnant during a 45-day breeding period.
However, they noted that the smaller proportion of low-gain heifers becoming pregnant within the first 21 days of the breeding period may compromise their stayability in the herd as a result of calving and breeding later in subsequent years. This would be especially so if a greater restriction in body weight gain was imposed during the postweaning period.