A genetic analysis of biological and economic efficiency of post-weaning feedlot performance in beef cattle

[摘要] ENGLISH ABSTRACT:It is generally accepted that feed intake and growth (gain) are the most important economiccomponents when calculating profitability in a growth test or feedlot. Feeding cost of animals is amajor determinant of profitability in livestock production enterprises. Genetic selection to improvefeed efficiency aims to reduce the cost of feeding in beef cattle production and thereby improveprofitability.The objective of this study was to define a clear selection objective to enable South African beefbreeders and especially the feedlot industry to select for post-weaning growth or feedlotperformance and to identify factors influencing profitability in a feedlot environment.Because of the recording of individual feed intake and weight gain values in the South AfricanAgricultural Research Councils' centralized growth tests, it was also possible to calculate aphenotypic value for feedlot profitability (R-value) for each bull tested in a centralized growth test.(Co)variances, using multitrait as well as random regression models, for and between feedlotprofitability, weaning weight and other production, reproduction and efficiency traits wereestimated. Residual feed intake (RFI) and feed conversion ratio (FCR) as efficiency traits were alsocompared to growth (average daily gain (ADG), weaning weight (WW) and shoulder height(SHD)), reproductive (scrotum circumference (SCR)) and profitability (feedlot profitability) traitsmeasured in growth tests of young Bonsmara bulls.Consequently, a single post-weaning growth selection index value based on the economic andbreeding values of different selection criteria related to feedlot profitability was composed.(Co)variance components, heritabilities and genetic correlations for and between initial weight(lW), final weight (FW), total feed intake (FI) and shoulder height (SHD) were estimated throughthe use of multitrait restricted maximum likelihood (REML) procedures. These breeding values(EBV s) were then used in a selection index to calculate a single economical value for each animal. This economical value is an indication of the gross profitability value or gross test value (GTV) ofthe animal in a post-weaning growth test.The heritability estimate of 0.36 for R-value, obtained from the multitrait analysis, shows that thistrait is genetically inherited and that it can be selected for. The heritability for R-value obtainedfrom the single trait random regression model varied between 0.57 and 0.62. The geneticcorrelations between the R-value and the other traits, obtained from the multitrait analysis, variedfrom negligible to high. The heritability estimated for FCR was 0.34 and for RFl 0.31 with agenetic correlation estimate of 0.75 between the traits. The estimated genetic correlation betweenprofitability (R-value) and FCR and RFl were -0.92 and -0.59, respectively. The geneticcorrelation estimate of -0.92 between FCR and R-value is largely due to the part-whole relationshipbetween these two traits. This is also shown in their genetic trends. The genetic correlations andexpected correlated responses between RFl and FCR with R-value suggest that indirect selection forR-value through the direct selection for FCR and/or RFl will result in slower genetic progress in Rvaluethan direct selection for R-value. However, where the R-value cannot be calculated and/orwhere direct selection for R-value is not possible, it would be better to select indirectly for R-valuethrough the use of FCR rather than RF!. Consequently, a regression equation was developed (withan R2 of 0.82) to estimate a feed intake value for all performance-tested Bonsmara bulls which weregroup fed and whose feed intakes were unknown. These predicted feed intake values made itpossible to calculate a feedlot or post-weaning growth profitability value (R-value) for all testedbulls even where individual feed intakes were unknown. Subsequently, an R-value for each bull wascalculated in a favourable economic environment (FEE), an average economic environment (AEE)and in an unfavourable economic environment (VEE). The high Pearson and Spearman correlationsbetween the EBV s based on AEE and the other two environments suggested that the averageeconomic environment could be used to calculate EBVs for R-value or feedlot growth profitability.It is therefore not necessary to change the carcass, weaner or feed price on a regular basis to accountfor possible re-rankings based on R-value EBVs.Heritabilities for lW, FW, Fl and SHD were 0.41, 0.40, 0.33 and 0.51, respectively. The highestgenetic correlations between these traits were the 0.78 (between lW and FW) and 0.70 (between Fland FW). GTV values varied between -R192.l7 and R231.38, with an average of R9.31. ThePearson correlations between EBVs (for production and efficiency traits) and GTV range from-0.51 to 0.68. The lowest correlation (closest to zero) was 0.26 between the Kleiber ratio (KLB) andGTV. Correlations of 0.68 and -0.51 were estimated between average daily gain (ADG) and GTVand feed conversion ratio (FCR) and GTV, respectively. The heritabilities of the different traitsincluded in the selection index suggest that it is possible to select for a GTV. The selection index can benefit feedlotting In selecting offspring of bulls with high GTV values to maximizeprofitability.The Pearson and Spearman correlations between the R-value EBVs and the index values (GTV)were very high (0.97). This high correlation of 97% indicates that it is not important which methodis used to calculate a genetic post-weaning growth of feedlot profitability value. The selection indexvalue is, however, more simplified than the feedlot profitability with less assumption. Therefore, itis recommended that the post-weaning selection index value be used as a selection objective inbreeding programmes to improve post-weaning growth profitability rather than the more complexfeedlot profitability value.

[发布机构] Stellenbosch University