David L. Morris: Vet Column 4-18-11
For those who pay the feed bill, it comes as no surprise that up to 70 percent of the total costs of beef production is related to feed. Closer inspection reveals that the majority of feed costs can be attributed to mature females where feed is primarily for maintenance. Selection for feed conversion ratio has in the past been used to improve feed utilization efficiency. This does not account for difference in maintenance requirements, however, and although body weight gain performance increased, so did mature size and feed intake of mature females. Increased mature size has been related to delayed onset of puberty and reduced lifetimes productivity of replacement females.
More recently, feed efficiency measures have focused on residual feed intake (RFI). RFI accounts for difference in maintenance requirements and is genetically independent of body weight, body weight gain, and mature size. Selecting for decreased RFI values improves feed efficiency. In order to look at overall cow-calf production efficiency, investigators from West Virginia University evaluated the relationship between RFI and fertility as determined by age at puberty and conception rate in yearling beef heifers.
This study was conducted from February to June in 2008 and 2009 involving 137 spring-born yearling beef heifers in Wardensville, West Virginia. Heifers were Angus (42 head), Angus-cross (89 head), and Hereford (six head). They were between 11 and 14 months-of-age at initiation of the trial and average 743 pounds. Heifers were maintained on trial for 84 days in year one and 71 days in year two. Average weight off test was 934 pounds. Diets were formulated for the heifers to gain one pound per day. RFI was measured for each heifer.
Interpretation of the results from this study was based upon dividing the heifers into high, medium, and low groups based upon RFI values, with low being the most feed efficient. In this study, body weight and average frame score did not differ based upon feed efficiency measures. As might be expected, the less feed efficient heifers possessed more back fat and rump fat.
Reproductively, neither first-service conception rate nor pregnancy rate were affected by differences in feed efficiency measures. Less feed efficient heifers did reach puberty earlier than heifers with better feed efficiency measures. For every 1-unit increase in RFI (less feed efficient), age at puberty decreased by 7.5 days. A certain level of stored fat may have hastened reproductive maturity.
Based upon the average, the data revealed that selecting for improvements in feed efficient may delay reproductive maturity. Reflecting upon these data, it is important to note that a large variation in age at puberty existed within all groups tested. Within the most feed efficient group and the least feed efficient group, heifers were identified that reached puberty early in the breeding season.
This study indicates that if feed efficiency based upon RFI is important to selection within the beef production system, it will be important to also be concurrently selecting for reproductive maturity within the replacement heifer program.