Selection for Improved Feed Efficiency Affects Reproductive Performance

March, 2014
Phillip Lancaster, UF/IFAS, Range Cattle REC, Ona

Introduction
A significant challenge facing agriculture is the expected 34% increase in the human population by 2050, which will require 70% more food from existing natural and land resources.  Thus, the efficiency of natural resource use must be improved to meet the food security goals for a growing population while protecting the environment.  Due to their ability to utilize cellulose in plant cell walls for production of meat, cattle can be used to produce human food from land that cannot be used to grow food crops.  However, cattle have a low conversion rate of feed to meat. Additionally, feed represents about 70% of the production costs of beef production.  Therefore, considerable research has focused on improving nutrient use efficiency or feed efficiency in cattle.

Feed conversion ratio (FCR), also referred to as feed to gain, has typically been used as the measure of feed efficiency in beef cattle, particularly in growing cattle.  However, animals with low FCR can either have faster gain and the same feed intake or lower feed intake and the same gain indicating that selection is not consistently based on either feed intake or rate of gain making genetic improvement difficult.  Feed conversion ratio is strongly negatively correlated with growth rate suggesting that selection for faster growth rate will improve FCR.  However, results of selection studies have been mixed with some studies reporting improved FCR in cattle selected for fast rates of gain, but other studies reported no improvement in FCR between genetic lines selected for fast or slow growth rate.  Thus, selection for faster growth rates may not improve feed efficiency in growing-finishing cattle, but will significantly increase mature weight, thereby increasing feed required to maintain the cow herd.  Given that the cow herd accounts for 65% of the feed required to produce a pound of beef, selection for improved feed efficiency based on FCR or growth rate may be detrimental to feed efficiency of beef production.

Recently, considerable research has focused on evaluating residual feed intake (RFI), also referred to as net feed intake or net feed efficiency, as a trait for use in selection programs.  Unlike FCR, RFI is independent of growth rate and mature weight, and selection for RFI will not result in greater maintenance requirements of the cow herd.  Residual feed intake is calculated as actual feed intake minus expected feed intake based on growth rate and body weight; animals that consume less than expected have low RFI and are more efficient.  The use of RFI to measure feed efficiency identifies animals that consume less feed for the same growth rate and body weight, putting selection pressure directly on feed intake.  Therefore, consistent genetic improvement in feed efficiency can be made using RFI.

Relationships of RFI with reproductive performance

Body condition score or body fat is known to impact reproductive performance and pregnancy rates in beef cows; cows of low body condition with less body fat have lower pregnancy rates and longer postpartum intervals.  Thus, selection for cattle with low RFI, which have slightly less body fat, could negatively impact reproductive performance in heifers and mature beef cows.  In previous studies where RFI was calculated based on ADG and BW, authors reported no difference in age or BW at puberty between low and high RFI heifers, even though differences in body fatness were evident.  Conflicting results have been reported for calving rates of heifers differing in RFI.  In one study, no difference in calving rate was found, but in another study low RFI heifers had lower calving rate than high RFI heifers (72.6 vs. 84.2%).  Interestingly, when RFI was calculated based on ADG, BW, and rib fat thickness, the difference in calving rate between low and high RFI heifers was eliminated (75.5 vs. 81.5%;Table 1). But, when RFI was calculated based on ADG, BW, and rib fat thickness, low RFI heifers were older and heavier at puberty than high RFI heifers. These studies indicate that selection for improved feed efficiency based on RFI (low RFI) may negatively impact attainment of puberty and calving rate in heifers. Since cattle with low RFI have slightly less body fat, it has been discussed that RFI should be calculated based on ADG, BW, and rib fat thickness to reduce any negative effects on carcass quality. However, it appears that using rib fat thickness in the calculation of RFI may negatively impact attainment of puberty in heifers. Producers should be aware of this when selecting bulls based on RFI to produce replacement heifers.

Table 1. Reproductive performance of beef heifers with low and high residual feed intake in Study 2.
  RFI-11 RFI-2
Trait Low RFI High RFI Low RFI High RFI
Age at puberty, d 353 347 356a 343b
BW at puberty, lb 809 802 819a 793b
Calving rate, % 73a 84b 75 82
1RFI-1 = RFI calculated based on ADG and BW; RFI-2 = RFI calculated based on ADG, BW, and 12th rib fat thickness.
abMeans within a row and RFI calculation with different superscripts differ (P < 0.10).


Two studies have evaluated reproductive performance in mature cows (Table 2).  The first study is from an Australian project evaluating divergent selection for RFI. No difference in pregnancy, calving, or weaning rate was found between cows from the low and high RFI selection lines. These authors did report a tendency for cows from the low RFI selection line to calve 5 days later in the year indicating that these cows were bred later in the breeding season compared with cows from the high RFI selection line.  There was no difference in milk yield or pounds of calf weaned per cow exposed between cows from the low and high RFI selection lines. In the second study, the effect of RFI on cow productivity was evaluated retrospectively by classifying dams as either low or high RFI based on the RFI of their offspring; RFI was not measured on the cow.  This study also found no difference in pregnancy, calving, or weaning rate between dams based on RFI of their offspring.  Similar to the first study, the second study found that dams of offspring with low RFI calved 4 days later in the year than dams of offspring with high RFI.  But there was no difference in calving interval suggesting that the dams of offspring with low RFI were bred later in the breeding season as heifers and continually bred later in the breeding season in subsequent years.  There was no difference in pounds of calf weaned per pound of cow body weight between dams of offspring with low and high RFI.  These results suggest that selection for more efficient cattle using RFI will most likely have little impact on reproductive performance and productivity of mature cows.

Table 2. Reproductive performance and productivity of mature beef cows with low and high residual feed intake.
  Study 1 Study 2
Trait Low RFI High RFI Low RFI High RFI
Pregnancy rate, % 90.5 90.2 95.6 96.0
Calving rate, % 89.2 88.3 84.9 86.3
Weaning rate, % 81.5 80.2 81.5 82.3
Calving date, Julian day 215a 210b 92a 88b
abMeans within a row and study with different superscripts differ (P < 0.10).

 

In conclusion, selection for improved feed efficiency in growing bulls based on FCR will likely result in increased mature cow size and feed required to maintain the cow herd.  In contrast, RFI is not related to growth and mature size, and thus, is a preferable trait to use for selection of cattle with superior feed efficiency.  But producers should be aware that some antagonistic relationships may exist with attainment of puberty and reproductive performance in heifers, while productivity of mature cows is less affected.

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