ADJUSTMENT OF POSTWEANING FEED:GAIN RATIOS OF ANGUS BULLS FOR DIFFERENCES IN MAINTENANCE REQUIREMENTS 1'2'3

ADJUSTMENT OF POSTWEANING FEED:GAIN RATIOS OF ANGUS BULLS FOR DIFFERENCES IN MAINTENANCE REQUIREMENTS 1'2'3 M. E. Davis 4, G. R. Wilson 4, W. R. Harvey a and T. B. Turner 4 The Ohio State University, Columbus 43210 and Ohio Agricultural Research and Development Center, Wooster 44691 ABSTRACT Effects of adjusting postweaning feed:gain ratios for differences in maintenance requirements as recommended by the Beef Improvement Federation were examined utiliing 171 individually fed, purebred Angus bulls. The residual correlation (year and sire effects removed) between unadjusted and adjusted feed efficiency was.85 (P.01). The Spearman correlation between ranks of the bulls based on the two measures of efficiency was.79 (P.01). Although product-moment and rank correlations between unadjusted and adjusted feed:gain ratios were large, the two methods of evaluating efficiency often identified different bulls as being among the four most or four least efficient in the conversion of feed into post-weaning gain. When unadjusted feed:gain ratios were used to evaluate efficiency, bulls that were more efficient were lighter in weight at the beginning of the 140-d postweaning performance test, gained more rapidly and consumed less feed during the test, and were taller and leaner at the conclusion of the test. When differences due to maintenance requirements were removed, bulls that gained more rapidly and were heavier, taller and slightly fatter at the end of the test were superior in the conversion of feed into postweaning gain. Correlations of on-test weight and feed consumption with adjusted feed:gain ratios were near ero. On-test age was not closely related to either adjusted or unadjusted feed efficiency. (Key Words: Beef Cattle, Feed Conversion, Efficiency, Maintenance Adjustment.) Introduction Postweaning feed efficiency, generally expressed as the feed required per unit of gain, or the reciprocal, is an important component of life cycle efficiency of beef production. Variation in feed efficiency among young growing cattle of the same age and sex and treated alike is primarily a function of maintenance requirements and rate and composition of gain (Cartwright, 1983). Maintenance requirements are 1Salaries and research support provided by State and Federal Funds appropriated to the Ohio Agr. Res. and Dev. Center, The Ohio State Univ. Journal Article No. 208-84. 2The experiment was a contributing project to North Central Regional Project NC-1, "The Improvement of Beef Cattle Through Breeding Methods." athe authors wish to thank Mr. C. B. Boyles and Mr. J. D. Wells for their excellent technical assistance. 4Anim. Sci. Dept., The Ohio State Univ., Columbus 43210. Received January 11, 1985. Accepted July 18, 1985. generally considered to be proportional to metabolic weight (weight raised to the.75 power). Therefore, the Beef Improvement Federation (B/F, 1981) recommends that when feed consumption per unit gain is evaluated in time-constant intervals, the values should be adjusted for differences in maintenance requirements by multiplying by a ratio of test group average metabolic weight to the individual's own metabolic weight. The present study was undertaken to compare unadjusted and adjusted (BIF, 1981) feed efficiency ratios of yearling Angus bulls and to examine relationships of efficiency ratios with various other performance traits. Materials and Methods Source of Data. As part of a divergent selection experiment (high vs low feed efficiency) designed to obtain a realied heritability estimate for postweaning feed efficiency, 35 bull calves were selected each year from 1979 through 1983 from the purebred Angus herd located at the Eastern Ohio Resource Develop- 1395 JOURNAL OF ANIMAL SCIENCE, Vol. 61, No. 6, 1985

1396 DAVIS ET AL. ment Center, Belle Valley, to be fed in a 140-d postweaning performance test. Bulls selected for the performance test generally were those with superior weaning weight and conformation. An effort also was made to select sire groups that contained at least three bull progeny. Numbers of bulls completing the performance test in 1979 through 1983 were 35, 34, 35, 34 and 33, respectively. Numbers of sire groups represented in 1979 through 1983, respectively, were 16, 7, 10, 8 and 7. Feeding and Management System. Following weaning at approximately 7 mo of age, bulls were placed in a three-sided barn where they were group-fed for 1 wk. Bulls then were randomly assigned to individual feed bunks where they were tied for 2 h each morning and 2 h each afternoon. After allowing 1 wk for the bulls to adjust to the tieing procedure, on-test weights were taken and recording of individual feed consumption was begun. Average on-test age and weight was 223 d and 235 kg, respectively (table 1). Throughout the 2-wk adjustment period and 140-d test period, bulls were fed the concentrate diet shown in table 2. Bulls also received approximately 2.27 kg of hay. head -1 "d-1 during these periods. Weights and feed consumption were recorded by 28-d periods. Weights were taken in the morning before feeding. At the conclusion of the 140-d performance test, the bulls were weighed on two consecutive days and the average of the two weights was used as the final weight. On the day the second off-test weight was taken, hip height and backfat thickness (as estimated by sonoray) were recorded. Adjustment of Feed:Gain Ratios. From these individually-fed bulls, the three most efficient bulls (in terms of kg of feed required per kg of gain) and the three least efficient bulls were selected each year to be mated to approximately 20 cows each in a test herd of Angus cows also located at the Eastern Ohio Resource Development Center. The fourth most and fourth least efficient bulls were kept also in the event of injury to one of the three most or three least efficient bulls. Examination of the effect adjustment of feed efficiency ratios for differences in maintenance requirements using the recommended BIF procedure (BIF, 1981) would have on the identification of the four most and four least efficient bulls was desired. Adjustment was accomplished by multiplying feed:gain ratios by the ratio of test group average metabolic midweight (Wil 7s ) to the individual's own metabolic midweight (Wii7s) as follows: adjusted feed efficiency = (W i-..ts/wijts.)(feed/gain), where subscript i refers to the i th year of the test (1979, 1980, 1981, 1982 or 1983) and subscript j refers to the jth bull within the i th test. Midweights were estimated as 1/2 (initial weight on test + final weight off test). This procedure adjusts feed:gain ratios of heavier-than-average bulls downward because they would be expected to have above-average maintenance requirements and, therefore, above- TABLE 1. MEANS AND STANDARD DEVIATIONS FOR WEANING WEIGHT AND POSTWEANING PERFORMANCE TRAITS Trait Mean SD a 205-d weight, kg 229 16 On-test age, d 223 7 On-test weight, kg 235 17 Average daily gain, kg/d Final weight, kg 1.36 4-25.14 26 Feed consumption, kg 1,152 112 Backfat thickness, mm 8.24 1.84 Height, cm 118 3 Unadjusted feed efficiency, kg feed/kg gain 6.09.50 Adjusted feed efficiencyb, kg feed/kg gain 6.11.52 acomputed as the square root of the error mean square after removing variation due to the fixed effect of years and the random effect of sires nested within years. badjusted for differences in maintenance requirements as recommended by BIF (1981).

ADJUSTMENT OF POSTWEANING FEED:GAIN RATIOS 1397 TABLE 2. COMPOSITION OF EXPERIMENTAL DIETa, b Ingredient %e Corn, shelled, crimped (IFN 4-02-931) 30.00 Oats, crimped (IFN 4-03-309) 25.00 Corn cobs, ground (IFN 1-02-782) 10.00 Dehydrated alfalfa (IFN 1-00-023) 10.00 Wheat middlings (IFN 4-05-205) Soybean meal (IFN 5-04-604) 10.00 10.00 Sugarcane molasses (IFN 4-04-696) 3.00 Dicalcium phosphate (IFN 6-01-080).50 Limestone (IFN 6-02-632).55 Salt, trace mineralied d.50 Sodium bentonite.40 Selenium.05 athe diet also contained 545 to 682 IU vitamin A/kg, 45 to 68 IU vitamin D/kg and.9 to 1.4 IU vitamin E/kg. bthe diet contained 68.5% TDN and 13.7% protein. CDry matter basis. dcontained.35% Zn,.28% Mn,.175% Fe,.035% Cu,.007% Co and.007% I. average metabolic weights. Feed:gain ratios of lighter-than-average bulls will be adjusted upward because they are expected to have below-average maintenance requirements and metabolic weights. It should be noted that even though maintenance requirements are generally considered to be proportional to metabolic weight, there may be differences in maintenance requirements that are not eliminated by the metabolic midweight adjustment used in this study. Statistical Analysis. To examine effects of adjusting feed efficiency ratios for differences in maintenance requirements, product-moment correlations (SAS, 1979) were calculated among the dependent variables: 205-d weight, on-test weight, on-test age, average daily gain, final weight, feed consumption during the 140-d test, backfat thickness, hip height, unadjusted feed efficiency and adjusted feed efficiency, using residual variances and covariances obtained after removing variation due to the fixed effect of years and the random effect of sires nested within years. Correlations were also calculated separately for each year after removing variation due to sires. Spearman correlation coefficients (SAS, 1979), which are correlations between ranks of variables, were obtained between unadjusted and adjusted feed efficiency as an indication of the degree of correspondence between ranks of the bulls based upon the two measures of efficiency. In order to examine the relationship between feed efficiency adjusted for differences in maintenance requirements and feed efficiency during a weight-constant interval, weightconstant efficiency was predicted by adjusting feed:gain ratios to the mean on-test weight and mean final weight using multiple regression analysis (SAS, 1979). Data were analyed separately by years and with all years combined. Results and Discussion Means and standard deviations for 205-d weight and for postweaning performance traits considered in this analysis are presented in table 1. Relationships of Unadjusted Feed Efficiency with other Performance Traits. The residual correlation between unadjusted feed efficiency and 205-d weight was positive (r =.28, P.01; table 3), indicating that bulls that were heavier at weaning were less efficient in their postweaning gains. Bulls that were heavier at the beginning of the test had higher feed:gain ratios, as evidenced by the correlation of.50 (P.01) between unadjusted feed efficiency and on-test weight (table 3). This result was likely due to the greater maintenance requirements of the heavier bulls. The larger correlation of unadjusted feed efficiency with on-test weight than with 205-d weight presumably resulted from the fact that postweaning feed consumption was more closely correlated with on-test weight (r =.43, P.01) than with 205-d weight (r =.28, P.01). No tendency existed for bulls that were younger or older at the beginning of the 140-d test to be more efficient in the conversion of feed into gain. The lack of a significant effect of on-test age may have been due to the small amount of variation in this trait (coefficient of variation = 3.2%). In a postweaning performance test involving Hereford, Angus and Red Angus bulls, Linton (1977) observed that older and heavier bulls were less efficient because of higher maintenance requirements, fatter composition of gain and intrinsic differences in efficiency inherent with advancing age. He suggested that feed:gain ratios should be adjusted for age on test and metabolic weight before individuals are compared for genetic merit. Adjustment for on-test age would not have resulted in

1398 DAVIS ET AL. significant changes in phenotypic rankings of bulls based on feed efficiency in the present study. Bulls with larger daily gains were more efficient (r -- -.47, P.01). Therefore, bulls with superior unadjusted feed efficiencies were light in weight at the beginning of the performance test and gained rapidly during the test. The net result was a near ero relationship between final weight and unadjusted feed efficiency (table 3). As an animal increases in weight, his maintenance requirement increases proportionately. In addition, bulls with heavier final weights were fatter (r -.40, P.01). Both of these factors have a negative influence on efficiency. Bulls that had heavier final weights also gained more rapidly on test (r =.75, P.01) a factor, however, that increases efficiency by reducing maintenance requirements per kg of gain. Smith et al. (1976) and Cundiff et al. (1981) reported that evaluation of postweaning feed efficiency over time-constant intervals gives an advantage to breed groups that gain rapidly relative to the weight they maintain. This finding was true also for individuals within the breed used in the present study. A number of authors have reported favorable genetic and phenotypic relationships between gain and efficiency of gain during time-constant intervals (Pierce et al., 1954; Carter and Kincaid, 1959; Brown and Gifford, 1962; Rollins et al., 1962; Koch et al., 1963; Blum, 1976; Linton, 1977). A certain portion of this favorable relationship is automatic because unadjusted efficiency is simply the ratio of feed to gain. As an animal's gain increases in a time-constant period, his feed:gain ratio decreases. A portion of this correlation also is attributable-to the negative relationship between rate of gain and maintenance requirement per kilogram of gain. Bulls consuming more feed had higher feed:gain ratios (r =.36, P.01; table 3). similar results were reported by Brown and Gifford (1962), Bogart and England (1971), Blum (1976) and Linton (1977). To some extent, this correlation is automatic, as feed is the numerator of the feed:gain ratio. The correlation of unadjusted feed efficiency with backfat thickness at the end of the 140-d test was positive but nonsignificant in all years except 1980, in which the correlation was negative but slight. A slight tendency, therefore, existed for fatter bulls to be less efficient in most years. Blum (1976) reported a near ero genetic correlation between backfat thickness at the conclusion of a 140-d performance test and feed:gain ratios. Linton (1977) found that fat thickness did not have a significant effect on feed efficiency, although bulls that were fatter phenotypically were less efficient. Bulls with greater skeletal sie, as indicated by hip height, tended to have superior unadjusted feed efficiencies (r = -.14, P>.05). Blum (1976) found that bulls that were taller at the shoulder at a year of age were genetically more efficient in their postweaning gains. Relationships of Adjusted Feed Efficiency with other Performance Traits. When differences due to maintenance requirements were removed from the efficiency estimates, correlations with 205-d weight and on-test weight were reduced to nonsignificant levels (table 3). Therefore, the higher unadjusted feed efficiency ratios of the bulls with heavier on-test weights were due to their greater maintenance requirements. Correlations with on-test age remained nonsignificant after adjustment for differences in maintenance requirements. Blum (1976) found that adjusting feed:gain ratio for midweight m and age on test using partial regression coefficients reduced the genetic correlation between 205-d weight and feed:gain ratio adjusted for age and weight on test from.11 +.62 to.08 +.62. Carter and Kincaid (1959) reported genetic and phenotypic correlations of.66 and.24, respectively, between 182-d weight and total digestible nutrients/cwt gain when the latter was adjusted for mean live weight. Correlations with average daily gain were more strongly negative (r = -.71 vs -.47) when adjusted feed:gain ratios were used than when unadjusted feed:gain ratios were used. A similar result was noted by Blum (1976). Linton (1977) reported an essentially ero correlation between total gain on test and efficiency of feed use when efficiency was adjusted for average daily maintenance requirement of each bull. The near ero correlation with on-test weight and large negative correlation with average daily gain resulted in a negative and highly significant relationship between final weight and adjusted feed:gain ratio (r = -.53; table 3). Correlations of feed consumption with adjusted efficiency were negative and nonsignificant in the 1979, 1980, 1981 and 1982 birth year groups, and positive but nonsignificant in the 1983 birth year group. The corre-

ADJUSTMENT OF POSTWEANING FEED:GAIN RATIOS 1399 ~J Z tl U. e~ c~ Z C~ [-'~n cn~. Z 0 5e 02: e~ c~ M II I I I I I..... ~.. ; J J I I I I I I I I I I I I I I. ~ I t l l I i l l l - :: -. : : :. ~. ~. ~. ~'~'~ ~. ~. ~ ~. I I I * I..~.. ~. I 9 " " [ I......~,, = ~ u.~h ~ o >, 9 ~,.~ r162 ~ Z g o -~..~ u ~.~..~ ~.~ ~ o lation pooled over years was near ero. The genetic correlation between feed consumption and feed efficiency adjusted for midweight was.04 in a study by Koch et al. (1963). Blum (1976) reported a genetic correlation of.06 -+.43 between feed consumption and adjusted feed:gain ratio. Linton (1977), however, obtained a genetic correlation of -.65 +.47 between feed consumption and gain:feed ratios adjusted for daily maintenance requirements. Adjustment for maintenance requirements changed correlations with backfat thickness from slightly positive to slightly negative. Therefore, bulls carrying more condition were slightly more efficient as measured by adjusted feed:gain ratio. Adjustment for maintenance requirements also changed correlations of feed:gain ratio with backfat thickness from slightly positive to slightly negative in the study by Blum (1976). Fatter bulls were less efficient in terms of both unadjusted and adjusted efficiency in the study by Linton (1977). Hip height exhibited a negative correlation (r = -.34, P.01; table 3) with adjusted feed: gain ratio, indicating that taller bulls were more efficient in the conversion of feed into gain. Therefore, bulls that were lighter in weight at the beginning of the postweaning performance test, gained more rapidly and consumed less feed during the test and were taller and leaner at the end of the test were more efficient in terms of unadjusted feed:gain ratios. When differences due to maintenance requirements were removed, bulls that gained rapidly and were heavier, tauer and slightly fatter at the end of the test excelled in the conversion of feed into postweaning gain. On-test weight and feed consumption had essentially a ero correlation with adjusted feed:gain ratios. On-test age was not related to either adjusted or unadjusted feed efficiency. Relationships similar to those presented in table 3 are observed when comparing the performance characteristics of the four most efficient and four least efficient bulls (table 4). When means were averaged over the 1979, 1980, 1981, 1982 and 1983 data sets, the bulls with the lowest unadjusted feed:gain ratios were lighter in weight at 205-d of age and at the start of the postweaning performance test, gained more rapidly and consumed less feed during the 140 d on test, and were slightly heavier, leaner (as estimated by backfat thickness) and taller at the conclusion of the test than were the bulls with the highest unadjusted

1400 DAVIS ET AL. O O Z O~ m~ ~Z Z m [..-. ZZ OU~ rn [.. ea ~176 r = >, g g ~ e,l e,l e-i e,l eq e,l ~,1 eq e,i eq e,i e,i ~o....~ ~o ip..~ + + + v 7 7 + e~ N.~.~ ~ ~ '~ ~ ~ ~ ~'~ ~gg~ feed:gain ratios. When identified using adjusted feed efficiency ratios, the four most and four least efficient bulls were similar in 205-d weight and on-test weight. The most efficient bulls gained more rapidly and consumed more feed during the test, had slightly greater backfat thickness and were taller at the hips at the end of the 140 d on test than were the least efficient bulls. A tendency existed for the least efficient bulls to have greater on-test ages when unadjusted feed efficiency ratios were utilied, while the most efficient bulls were older at the beginning of the test when adjusted feed efficiency ratios were used. Relationship Between Adjusted and Unadjusted Feed Efficiency. Residual correlations between unadjusted and adjusted feed efficiency ratios obtained after removing variation due to sires ranged from.75 (P.01) in the 1981 performance test to.91 (P.01) in the 1980 test (table 3). The residual correlation between adjusted and unadjusted feed efficiency obtained after removing variation due to years and sires within years was.85 (P.01). Blum (1976) reported a genetic correlation of.93 -+.04 between feed:gain ratios adjusted for age and weight on test and feed:gain ratios adjusted for midweigbt "Ts and age. Linton (1977) observed genetic and phenotypic correlations of 1.00 -+.05 and.81, respectively, between gain:feed ratios and gain:feed ratios adjusted for average daily maintenance requirement of each bull. Spearman correlations between ranks of the bulls based upon unadjusted and adjusted feed efficiencies were highly significant in each of the five postweaning performance tests (r =.81,.92,.70,.71 and.70 in 1979, 1980, 1981, 1982 and 1983, respectively). Pooled over years, the rank correlation was.79. Therefore, bulls ranked similarly when adjusted and unadjusted feed efficiency ratios were used. However, a number of different bulls would be identified as having the highest or lowest feed:gain ratios if the criterion was unadjusted rather than adjusted feed efficiency. In 1982 and 1983, the four bulls having the highest feed:gain ratios were the same when unadjusted and adjusted feed efficiency were used as the methods of evaluation. In 1980, the four bulls having the lowest feed:gain ratios were the same regardless of the method of evaluation, while in 1981, 1982 and 1983 three of the four bulls were the same. Only one bull ranked among the four least efficient based upon both

ADJUSTMENT OF POSTWEANING FEED:GAIN RATIOS 1401 adjusted and unadjusted feed efficiency in the 1981 performance test, however. Cundiff et al. (1981, 1984) noted that breed group means for feed efficiency adjusted for differences in maintenance requirements as recommended by BIF (1981) were very similar to breed group means for efficiency during a weight-constant interval from 250 to 470 kg (r --.99). They suggested that the BIF procedure yields estimates for feed efficiency that are reasonably close approximations of feed efficiency during weight-constant intervals. In order to test this hypothesis, residual correlations were calculated between feed: gain ratios adjusted for differences in maintenance requirements as recommended by BIF (1981) and feed:gain ratios adjusted to a weightconstant interval via covariates for mean on-test weight and mean final weight. When data were analyed separately by year using the mean on-test weight and final weight in a given year as covariates, residual correlations between the two measures of efficiency obtained after removing variation due to sires were.57,.58,.71,.67 and.50 (P.01) in the 1979, 1980, 1981, 1982 and 1983 data sets, respectively. When data from all years were analyed simultaneously, the residual correlation (year and sire effects removed) between feed efficiency adjusted for maintenance and feed efficiency adjusted for mean on-test weight (235 kg) and mean final weight (425 kg) was.57 (P.01). Efficiency ratios adjusted using the BIF procedure, therefore, were more closely related to unadjusted efficiency measured in the 140-d time-constant interval (r =.85) than to efficiency adjusted to a weight-constant interval (r =.57). It does not appear that the BIF method of adjustment yielded efficiency ratios closely approximating those that would be found in a weight-constant interval within the Angus bulls used in this study. Conclusions Even though product-moment and rank correlations between unadjusted and adjusted feed:gain ratios were large, the two methods of evaluation did not necessarily identify the same bulls as being most efficient or least efficient. If the intent is to evaluate bulls for efficiency of gain, it is necessary to adjust the efficiency ratios for differences in maintenance requirements. Failure to do so results in feed:gain ratios that reflect not efficiency of gain, but feed needed for both maintenance and gain. Therefore, the recommendations of BIF (1981) should be followed in adjusting feed efficiency data. Literature Cited BIF. 1981. Guidelines for Uniform Beef Improvement Programs. Beef Improvement Federation, USDA Ext. Serv., Program Aid 1020. Blum, J. K. 1976. Selection for feed conversion: Direct and correlated responses and genetic parameters. M.S. Thesis. Kansas State Univ., Manhattan. Bogart, R. and N. C. England. 1971. Feed consumption, daily gain and feed required per unit of gain in beef calves. J. Anita. Sci. 32:420. Brown, C. J. and W. Gifford. 1962. Estimates of heritability and genetic correlations among certain traits of performance tested bulls. Arkansas Agr. Exp. Sta. Bull. 653. Carter, R. C. and C. M. Kincaid. 1959. Estimates of genetic and phenotypic parameters in beef cattle. III. Genetic and phenotpyic correlations among economic characters. J. Anim. Sci. 18:331. Cartwright, T. C. 1983. The role of dairy cattle genes in United States beef production. J. Dairy Sci. 66:1409. Cundiff, L. V., R. M. Koch and K. E. Gregory. 1984. Characteriation of biological types of cattle (cycle III). IV. Postweaning growth and feed efficiency. J. Anim. Sci. 58: 312. Cundiff, L. V., R. M. Koch, K. E. Gregory and G. M. Smith. 1981. Characteriation of biological types of cattle - cycle II. IV. Postweaning growth and feed efficiency of steers. J. Anim. Sci. 53:332. Koch, R. M., L. A. Swiger, D. Chambers and K. E. Gregory. 1963. Efficiency of feed use in beef cattle. J. Anim. Sci. 22:486. Linton, A. C. 1977. Genetic aspects of feed efficiency. Ph.D. Dissertation. Colorado State Univ., Fort Collins. Pierce, C. D., H. G. Avery, M. Burris and R. Bogart. 1954. Rate and efficiency of gains in beef cattle. II. Some factors affecting performance testing. Oregon Agr. Exp. Sta. Tech. Bull. 33. Rollins, W. C., F. D. Carroll, J.W.T. Pollack and M. N. Kudoda. 1962. Beef cattle performance and progeny tests for gain, efficiency, carcass conformation and earliness of maturity. J. Anim. Sci. 21:200. SAS. 1979. SAS User's Guide. Statistical Analysis System Institute, Inc., Cary, NC. Smith, G. M., D. B. Laster, L. V. Cundiff and K. E. Gregory. 1976. Characteriation of biological types of cattle. II. Postweaning growth and feed efficiency of steers. J. Anim. Sci. 43:37.