Growth, feed intake and maternal performance of Angus heifers from high and low feed efficiency selection lines S. T. Morris A,B , F. Y. Chan A , N. Lopez-Villalobos A , P. R. Kenyon A , D. J. Garrick A and H. T. Blair A A Institute of Veterinary, Animal and Biomedical Sciences, College of Sciences, Massey University, Private Bag 11222, Palmerston North 4442, New Zealand. B Coresponding author. Email: S.T.Morris@massey.ac.nz Abstract. Feed conversion efficiency is an important factor affecting profitability for cow-calf production systems. One method of characterising feed conversion efficiency is by calculating residual feed intake (RFI), the difference between measured and expected feed intakes. A high RFI value indicates low feed efficiency while a low RFI value indicates high feed efficiency. The present experiment used 49 Angus heifers sired by Angus bulls selected for high or low RFI. The progeny were bred on commercial farms and then transferred to Massey University at weaning at ~200 days of age. The heifers were weighed at ~30-day intervals and were mated at 15 months, calved at 2 years old and reared their calf to ~160 days of age. Each heifer had recorded its own liveweight gain (from its weaning to weaning of its first calf), herbage intake at 350 and 450 days of age using the n-alkane technique and maternal performance (calf production). These records were analysed to estimate the RFI of each heifer, and to determine differences in RFI between selection lines. There were no significant differences (P > 0.05) in liveweight or dry matter requirements between selection lines at Day 350, but differences were significant (P < 0.05) at Day 450 such that the high feed efficiency line were 24.6 kg heavier and required an extra 0.32 kg/head.day of herbage. There were no significant differences (P > 0.05) between the two lines in herbage intake or RFI measured at either Days 350 or 450. There were no differences (P > 0.05) in pregnancy rates, calf birth dates or birthweights, estimated 24-h milk production or calf weaning weights between the two lines. This experiment shows that beef cattle selected for low RFI have higher growth rates and heavier liveweights than cattle selected for high RFI both lines had similar calf production at first breeding. Additional keywords: cow-calf, residual feed intake. Received 12 March 2014, accepted 4 June 2014, published online 17 July 2014 Introduction Feed conversion efficiency (FCE) is one of the most important factors determining farm profitability in animal production systems. In beef cattle, FCE is defined as feed intake per unit gain of liveweight (Arthur et al. 2004). In a typical beef breeding and finishing system, providing feed for cattle incurs the highest input cost. A large proportion (65–75%) of the total feed intake goes into maintenance of the beef cow instead of production (Arthur et al. 2001, 2004). By improving FCE it is therefore possible to lower the cost of production. There are many alternative definitions of feed efficiency, each with its advantages and disadvantages (Berry and Crowley 2013). Residual feed intake (RFI) has increased in popularity as a measure of animal-level feed efficiency (Berry 2008). It is defined as the difference between actual feed intake and that predicted, based on estimates of the requirements for maintenance and production, such as changes in liveweight. Estimated requirements can be obtained from regression analyses or published nutritional requirements. Highly efficient animals will have lower RFI values than less efficient ones (Arthur et al. 2004, 2005). By reducing RFI, FCE can be improved, which will lead to an increase in profitability if productivity is unaffected (Arthur et al. 2004; Richardson and Herd 2004; Baker et al. 2006). The objective of this experiment was to measure growth, feed intake and maternal performance of Angus heifers sired by bulls selected for low or high estimated breeding values (EBV) for RFI. Materials and methods Two experimental groups were generated from 250 industry- bred 2-year-old and 3-year-old Angus dams that had been bred via AI using semen from Angus bulls (four per line). The bulls were chosen to represent the top 10% (Hi-Efficiency = low RFI, ranges –0.82 to –1.16 kg) or bottom 10% (Lo-Efficiency = high RFI, ranges 1.0 to 1.14 kg) EBV for RFI and were each artificially bred to at least 25 cows. All the female offspring (49 calves comprising 25 high RFI and 24 low RFI calves, respectively), were transferred to Massey University at weaning (~6 months of age) and managed together until weaning of their first calf. These heifers themselves were naturally bred at ~15 months of age for two oestrous cycles to breed-average merit yearling Angus bulls CSIRO PUBLISHING Animal Production Science, 2014, 54, 1428–1431 http://dx.doi.org/10.1071/AN14220 Journal compilation Ó CSIRO 2014 www.publish.csiro.au/journals/an