Research has shown considerable variation in feed efficiency between individual cattle, and heritability of feed efficiency suggests producers could reduce production costs through genetic selection. Measuring individual cattle for feed efficiency, however, particularly at multiple production stages, presents a challenge. Also, questions remain about the relationships between feed efficiency and other important traits such as those involved in reproduction. Researchers at Texas A&M University recently conducted a two-year trial to determine if residual feed intake (RFI) classification of beef heifers, measured after weaning, affected efficiency of forage utilization, body composition, feeding behavior, heart rate, and physical activity of those heifers during pregnancy.
The researchers weaned 115 Bonsmara heifers (a tropically-adapted Bos taurus breed) at average of 202 days of age. After a backgrounding period, they placed the heifers in a drylot at average 280 days of age and 642 pounds, feeding them a 70 percent roughage ration for a 70-day growing period. During the growing period, the researchers measured individual feed consumption and performance, and selected for breeding 24 heifers with the lowest RFI, meaning they were the most efficient, and 24 of the least efficient heifers with the highest RFI.
Of those, 20 low RFI and 22 high RFI heifers became pregnant and then were adapted to an all-roughage ration starting at 155 days of gestation. The researchers measured individual feed intake, weight gain, and body-condition scores during a 77-day post-breeding period. For measuring individual intake, they used individual GrowSafe bunks, which incorporate radio frequency ID readers and load cells to record how much feed each animal eats every time it visits the bunk.
They found that on average, the high efficiency heifers weighed 44 pounds more than low-efficiency heifers at weaning. During the post-weaning growing trial, high efficiency heifers also ate 4.4 pounds or 19 less feed per day, had similar weight gains, had 23 percent higher gain per feed ratios and had slightly smaller ribeye areas at end of trial compared with the less-efficient heifers.
During the post-breeding trial, the pregnant females that earlier had been identified as having lower RFI, or higher efficiency, ate 5.1 pounds or 17 percent less feed per day, spent 26 percent less time at the feeder, continued to have lower RFI, had 7 percent lower heart rates and similar initial and final body weight and body condition compared with the heifers initially determined to have lower efficiency.
There was no significant difference in calving date between the two efficiency groups. Calves from first-calf low-RFI females were lighter at birth than calves from high-RFI females, but RFI classification did not affect birth weight of calves born to second-calf females.
The researchers conclude their results indicate that heifers identified as having low postweaning RFI have greater efficiency of forage utilization as pregnant females, with minimal impacts on growth, body composition, calving date, and calf birth weight, compared to their high-RFI counterparts.
Results of the research are reported in the Journal of Animal Science.