Steers bred for improved net feed efficiency eat less for the same feedlot performance

Abstract

Animal Production in Australia 1998 Vol. 22 STEERS BRED FOR IMPROVED NET FEED EFFICIENCY EAT LESS FOR THE SAME FEEDLOT PERFORMANCE. E.C. RICHARDSONA, R.M. HERDB, J.A. ARCHERA, R.T. WOODGATEB and P.F. ARTHUR A B A NSW Agriculture, Agricultural Research Centre, Trangie, NSW 2823 Cattle and Beef CRC for Meat Quality, University of New England, Armidale, NSW 2531 SUMMARY Angus and Angus X Hereford and Angus X Shorthorn steer progeny of parents previously ranked for postweaning net feed efficiency (NFE) were grown out in a feedlot. Net feed efficiency is measured as net feed intake, which is the difference between feed consumed and the feed intake predicted from liveweight and growth rate. Selection for postweaning NFE produced steers that grew as fast, or faster, than low NFE steers but ate less feed per unit gain and had more favourable feed conversion ratios. The high NFE steers had slightly less subcutaneous fatness and smaller eye-muscle area than the low NFE steers which could imply an association of efficiency with maturity pattern. However, there was no evidence to suggest that a single generation of selection for and against NFE had produced steers that differed in their ability to meet market specifications, nor differed in the percentage yield of retail beef from their carcases. Keywords : cattle, feed efficiency, feedlot performance. INTRODUCTION Traditionally, beef cattle breeders have concentrated on increasing outputs to improve production efficiency. However, such selection strategies are generally accompanied by an increased demand for inputs to the system. Net feed efficiency (NFE), measured as net feed intake, is the difference between the kilograms of feed consumed and the kilograms of feed predicted to be required based on an animals liveweight and growth rate. It is thus kilograms of feed eaten net of the predicted feed intake based on liveweight and growth rate. High NFE cattle have low net feed intake and eat less than expected for their size and growth rate. Since net feed intake is calculated to be independent of liveweight and growth, selecting cattle for NFE should improve efficiency without increasing demand for inputs, particularly for feed. Postweaning tests for net feed intake on British breed cattle at the Trangie Agricultural Research Centre, NSW, have demonstrated that variation in NFE exists, and found the trait to be heritable (h2 = 0.44; Arthur et al. 1997). Preliminary results suggest selection for NFE improved the efficiency of progeny in postweaning tests for net feed intake (Herd et al. 1997). This study was designed to demonstrate the potential benefit to the feedlot industry from breeding to improve NFE. This paper reports the performance of yearling steer progeny from high or low NFE parents when the steers were finished on a feedlot ration. MATERIALS AND METHODS Animals Postweaning net feed intake tests were performed at Trangie on Angus, Shorthorn, Hereford and Poll Hereford heifers and Angus bulls. Details of these tests are given in Arthur et al. (1997). At the end of each 120 day test, heifers and bulls were ranked for net feed intake. The top 50% of heifers were then mated to the top 5% of bulls, and the bottom 50% of heifers mated to the bottom 5% of bulls to produce progeny bred for either high or low NFE. Steer progeny (n=79) of high NFE parents (high NFE steers) and low NFE parents (low NFE steers) were available for this study. Following weaning in October 1996, Angus (n=38) and Angus x Hereford and Angus x Shorthorn crossbred (n = 41) steers were transported from Trangie to the Cattle and Beef CRC feedlot Tullimba, near Armidale, NSW, to be grown on pasture and then finished on a grain-based ration to one of two market specifications. The Angus steers were targeted for the Australian supermarket grainfed trade which had a preferred range in liveweight of 300 to 390kg and P8 fat of 5 to 9mm. The crossbred steers were targeted for the Korean grainfed export trade which had a preferred range in liveweight of 420 to 630kg with an average P8 fat of 13mm (6 to 22mm being acceptable; Gaden 1993). The purebred Angus steers were divided into two groups: group 1 consisted of nine steers from high NFE parents and ten from low NFE parents; group 2 consisted of eight steers from high and eleven from low NFE parents. After a two week introductory period, the steers were fed a ration containing 70% oat grain, 15% 213 Animal Production in Australia 1998 Vol. 22 hay, 5% protein meal, plus molasses and mineral additives. This ration had a dry matter (DM) content of 89.0% and the DM was analysed in vitro to have a metabolizable energy (ME) content of 10.7 MJ/kg and 14.3% crude protein. The steers had ad libitum access to this ration: group 1 for 70 days, and group 2 for 56 days. Individual daily feed intake and weekly liveweights were recorded. The crossbred steers were split into high and low NFE groups, and each group sub-divided into two, to give two feedlot pens containing nine and 10 high NFE steers, and two pens of 11 low NFE steers. The steers were fed a ration consisting of 75% rolled barley, 10% hay ration, plus 5% of a protein pellet, 8% molasses/ water/mineral suspension, and buffer additives. The ration had a DM content of 90.1%, an energy content of 12.0 MJ ME/kg DM and crude protein content of 15.8% DM.. Within a pen the group feed intake was recorded over 84 days, together with individual steer weekly liveweights. At the start (Angus steers) and end (all steers) of the measurement period, subcutaneous rib (12/13th) and rump (P8) fat depths were measured using an Aloka 500 ultrasound scanner. The area of the eye-muscle (M. longissimus dorsi) was measured subsequently by computer analysis of stored images. Data analysis Weekly weights for each steer were regressed against time to calculate individual steer start weights, midweights, endweights and average daily gains for the measurement period. Net feed intake is the difference between an animals actual feed intake over the measurement period, and its feed intake predicted on the basis of liveweight and growth rate. Individual animal data for the Angus steers were used in the multiple regression equation: (actual) feed intake = a x midweight0.73 + b x average daily gain + residuals. From this regression the predicted feed intake of each steer was then calculated based on its size and growth rate, and the difference between the actual feed intake and predicted feed intake was the value for net feed intake. By this calculation, steers with high NFE in the feedlot, ie those eating less than predicted, will have negative net feed intakes. Feed conversion ratio was calculated as the ratio of total feed DM eaten to total weight gain for the test period. For the crossbred steers the mean feed intake by each pen was used, together with individual daily weight gain, to calculate feed conversion ratio. Differences between high and low NFE steers were analysed using the general linear models (GLM) procedure of SAS (1989). For the Angus steers, the group (1 or 2) effect and the interaction for group with efficiency class were included in the GLM model. The interactions were not significant and least-squares means (LS means) were calculated for each efficiency class. Crossbred steer data were analysed fitting efficiency class as the only fixed effect (feed pen was assumed to have no significant effect). RESULTS LS means for all Angus steer progeny (pooled group 1 and 2) of high and low NFE parents are presented in Table 1. There was no difference in start weight, end weight and average daily gain between the high and low NFE steers. High NFE steers had lower daily feed intakes and lower net feed intakes than low NFE steers. Consequently, the steers from high NFE parents had a more favourable feed conversion ratio. Steers from high NFE parents had less subcutaneous fat and smaller eye-muscle areas at the start, and these differences persisted until the end of the measurement period. The composition of the liveweight gain, as indicated by similar gain in subcutaneous fat and eye-muscle area, was the same for high and low NFE steers. Results for crossbred steer progeny of high and low NFE parents are presented in Table 2. There was no difference in start weight between high and low NFE steers. Steers from high NFE parents grew faster and as a result were heavier at the end of the measurement period, than steers of low NFE parents. The groups of high and low NFE steers had similar daily feed intakes, but the two groups of steers from high NFE parents had a more favourable feed conversion ratio. While there were no start fat or eye-muscle area scan results for the crossbred steers, the high NFE steers had less subcutaneous rib and rump fat than, and similar eye-muscle areas to, the low NFE steers at the end of the measurement period. DISCUSSION Selection for high NFE produced Angus steers that gave similar daily gains to Angus steers selected for low NFE, for reduced daily feed intakes. In the crossbred steers tested, selection for high NFE produced steers that had superior weight gains and resultant final liveweights, but no increase in daily feed intake, compared to steers selected for low NFE. In both the Angus and crossbred cattle examined, selection for high NFE produced steers that had superior feed conversion ratios and would have been more profitable to feed. 214 Animal Production in Australia 1998 Vol. 22 Table 1. Feedlot performance of Angus steers selected for high or low net feed ef ficiency (values are LS means � s.e.) Trait Net feed efficiency of parents High Number of animals Start age (days) Start weight (kg) Start rib fat (mm) Start rump fat (mm) 2 Start eye-muscle area (mm ) Average daily gain (kg/day) Gain in rib fat (mm) Gain in rump fat (mm) 2 Gain in eye-muscle area (mm ) End weight (kg) End rib fat (mm) End rump fat (mm) 2 End eye-muscle area (mm ) Average daily feed intake (kg DM) Net feed intake (kg DM/day) Feed conversion ratio (kg DM/kg) 17 264 � 3.16 283 � 6.22 3.80 � 0.30 4.28 � 0.38 43.1 + 1.0 1.35 � 0.05 4.06 � 0.58 3.19 � 0.42 5.2 + 0.9 369 � 6.78 7.06 � 0.47 8.31 � 0.62 48.5 + 1.1 9.22 � 0.18 -0.20 � 0.11 6.97 � 0.23 Low 21 271 � 2.93 293 � 5.60 4.65 � 0.27 5.88 � 0.34 46.2 + 0.9 1.30 � 0.04 4.35 � 0.52 3.80 � 0.37 5.2 + 0.8 375 � 6.10 8.35 � 0.42 10.25 � 0.56 51.4 + 0.9 9.78 � 0.16 0.17 � 0.10 7.60 � 0.20 ns ns * ** * ns ns ns ns ns * * * * * * Significance ns, non significant; * P<0.05; ** P<0.01 Table 2. Feedlot performance of crossbred steers selected for high or low net feed efficiency (values are LS means � s.e.) Trait Net feed efficiency of parents High Number of animals Start age (days) Start weight (kg) Average daily gain (kg/day) End weight (kg) End rib fat (mm) End rump fat (mm) End eye-muscle area (mm2) Group mean intake (kg DM/day) Group feed conversion ratio (kg DM/kg) a Significance Low 22 405 � 3.23 388 � 2.51 1.36 � 0.01 502 � 2.80 12.2 � 0.23 14.3 � 0.23 50.4 + 1.1 10.21 7.69 ns ns *** *** *** ** ns ns a *a 19 402 � 2.45 392 � 2.70 1.48 � 0.01 517 � 3.01 10.1 � 0.25 13.3 � 0.24 48.2 + 1.1 10.41 7.14 t-test performed using standard errors for these traits from the Angus steers above At the end of the feeding period, the high NFE Angus and crossbred steers had slightly less subcutaneous fat than the low NFE steers, but their fat depths were still more than sufficient to meet market specifications. Wolcott et al. (1997) using pre-slaughter real time ultrasound scanning, found that as subcutaneous fat depths increase, the percentage yield of retail beef decreases from steers with similar preslaughter liveweights. This would suggest that the high NFE steers, with their lower subcutaneous fat, might be expected to have had a better percentage retail yield than their low NFE counterparts, making their carcases more valuable. The smaller eye-muscle area of the high NFE Angus steers, and similar eye-muscle area and heavier pre-slaughter liveweight of the high NFE crossbred steers, might be associated with a reduction in percentage retail yield from their carcases. However, the effect on yield of reduction of eye-muscle area is much less than the effect of increase in subcutaneous fat (Wolcott et al. 1997). Using the regression equations for domestic feedlot steers in Wolcott et al. (1997) and from Wolcott pers. comm., the percentage retail beef yields from the carcases of the Angus high and low NFE steers predicted from their preslaughter measurements was 63.0 and 215 Animal Production in Australia 1998 Vol. 22 62.9%, respectively. For the crossbred steers finished for the Korean market, eye-muscle area has no effect on yield and the high NFE steers would be expected to have produced carcases with a higher percentage yield of retail beef by virtue of there lower subcutaneous fatness. There was, therefore, no evidence to suggest that a single generation of selection for and against NFE had produced steers that differed in their ability to meet market specifications nor differed in the percentage yield of retail beef from their carcases. Selection for postweaning NFE produced steers that ate less per unit gain and should be more profitable to feed in a feedlot. High NFE steers grew as fast, or faster, than low NFE steers. The composition of the liveweight gain (as indicated by gain in subcutaneous fat and eye-muscle area) was measured in the Angus group of steers and was the same for high and low NFE steers. The small differences in subcutaneous fatness and eye-muscle area between the high NFE steers and the low NFE steers could imply an association of efficiency with maturity pattern. Concurrent research is examining the physiological basis of the variation in NFE and the relationships between efficiency, body composition, meat quality and other production traits. ACKNOWLEDGMENTS The assistance of M Wolcott, C Webber, P Reynolds, C Jones and D Perry is gratefully appreciated. This work was supported by the Meat Research Corporation, NSW Agriculture and the Cattle and Beef Industry Co-operative Research Centre. REFERENCES ARTHUR, P.F., ARCHER, J.A., HERD, R.M., RICHARDSON, E.C., WRIGHT, J.H., DIBLEY, K.C.P. and BURTON, D.A. (1997). Proc. Assoc. Advan. Anim. Breed. Genetic. 12, 234-7. GADEN, B. (1993). 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