Feed is the largest variable cost in beef production.  The cow herd consumes approximately 85 % and 50 % (or greater) of the annual feed budget in suckler calf-to-weanling and calf-to-beef systems, respectively.  Consequently, the suckler beef research programme at Grange is investigating the area of feed efficiency using the concept of residual feed intake (RFI).  This is an alternative measure of identifying feed efficient cows and progeny, and unlike feed conversion ratio (FCR), RFI permits selection for feed-efficient cattle without increasing their mature size and thus, maintenance requirements and costs.

Residual feed intake is defined as the difference between an animals actual intake and the intake predicted based on bodyweight maintenance and live weight gain i.e. efficient animals eat less than expected and have a negative or low RFI value, whereas inefficient animals eat more than expected and have a positive or high RFI value.

In other words, for the growing animal, RFI is the variation in intake that remains after accounting for the requirements for maintenance and growth.  What makes up the variation in the remaining or residual portion is not fully understood. In order to calculate RFI, individual intake and growth of animals needs to be recorded over at least 70 days.

The research project at Grange is examining the divergent selection for RFI using the Simmental breed as a model.  Recent results from an associated study, using data on pedigree bulls from the national performance test station in Tully, showed that the principal applies to all the main breeds, there is significant genetic variance and the heritability is ~0.46. Thus, with sufficient data, we can effectively select for it.  There is very little information published on RFI in the suckler cow or on RFI in cattle offered forage-based diets.

The objectives of the Grange project are to demonstrate the magnitude of the benefit, determine some of the biological basis underlying the trait and identify any associated changes in selecting for it. 
As part of the study, weanling heifers (purebred Simmental and Simmental × Friesian) were individually offered grass silage ad libitum and 2 kg of supplementary concentrate daily during the indoor winter period, and residual feed intake was calculated for each animal.

They were then divided in three RFI groups - low (efficient), medium and high (inefficient) RFI.  The three RFI groups did not differ in live weight, live weight gain, body and ultrasound measurements, muscularity score, blood variables or lying and standing time BUT the high RFI group consumed 19% more feed than the low RFI group.  These animals were bred to Simmental sires of known RFI.  During the following winter they were offered grass silage only ad libitum (plus a mineral vitamin supplement) and RFI was calculated for each animal during pregnancy (first calvers).  Similarly they were divided into low, medium and high RFI groups.

Results are presented in Table 1 for the low and high RFI groups.  Intake of grass silage for the high RFI group was 21% greater than the low RFI group and YET, the cow groups did not differ in live weight or live weight gain, body condition score, ultrasonic fat and muscle depth, withers height, chest depth or circumference, back length, pelvis width, calving difficulty or calf birth weight.  Time spent lying, standing or active did not differ either (not presented).  There were differences in muscularity score in favour of the low RFI animals. The repeatability of RFI through the various stages in the lifecycle of the cow (weanling to pregnant heifer to mature cow) is been examined.

Other measurements that were carried out on the cow include, grass intake, blood and rumen variables, feed digestibility, colostrum yield and quality and milk yield.  This data is being analysed.  Similarly, detailed measurements on the progeny are ongoing.

The results to date demonstrate the large variation in feed efficiency within the suckler cow herd and the potential cost savings to farmers if feed efficient animals can be identified and selected for in a breeding programme. 

Table 1. Productivity traits in pregnant beef heifers differing in phenotypic residual feed intake (RFI)