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Growth, carcass characteristics and meat quality of the steer and heifer progeny of autumn (AC: March-April) and winter (WC: June-July) calving cows following weaning in January in each of 3 years (2003-05) were measured. The cows were mated to sires with a high estimated breeding value for either retail beef yield (RBY), intramuscular fat (IMF) or both RBY and IMF. After weaning, the progeny entered one of three growth paths until slaughter at an average steer liveweight of 500 kg: (i) fast - fast growth from weaning on a high concentrate feedlot diet; (ii) slow - slow growth from weaning(~0.6kg/day)to400kgliveweightfollowedbygrowthatover1kg/dayonhighqualitypasture;or(iii)comp.-10% weaningweightloss,immediatelyafterweaningfollowedbycompensatoryorrapidgrowthofover1kg/dayonhighquality pasture. Steers on thefast growth path had higher (P <0.001) P8 fat thickness than those on the slow or comp. growth paths whereas heifers on the fast growth path only had higher (P < 0.001) P8 fat thickness than those on the slow growth path. Animals on the fast growth treatment had higher (P < 0.001) levels of IMF% than the slow animals which were higher (P <0.001)than the comp. growth treatment. AUS-MEAT and US marbling scores were not different amonggrowth paths. Animals finishedonthefastgrowthpathhadalower(P <0.001)RBY%thanthoseoneitherthesloworcomp.growthpaths. The RBY-sired progeny had higher (P < 0.001) finishing liveweight and hot standard carcass weight than either RBY and IMForIMF-siredanimals.IMF-siredprogenyhadhigher(P <0.01)ribfatthicknessthaneitherRBYorRBY-andIMF-sired animals. There was also a similar trend for P8 fat thickness but the effects were not significant. The RBY-sired animals had lowerAUS-MEATmarblingscores(P <0.01),USmarblingscores(P <0.001)andlevelsofIMF%(P <0.01)thaneitherof theothertwosiretreatments.RBY-siredanimalsalsohadhigher(P <0.001)estimatedRBY%thanthosefromtheIMFsires while those by RBY and IMF sires were intermediate and not significantly different from either. Calving time had little influenceonmostcarcasscharacteristics.However,WCanimalstendedtobefatterandhavehighermarblingscoresthanAC animals. The IMF% was higher (P < 0.01) in WC animals from RBY and IMF sires than in the corresponding AC animals. Heifershadlighterslaughterliveweight,carcassweight,werefatterandhadhighermarblingscoresthansteers.Heifersalso had lower (P < 0.001) RBY% than the steers. Ossification scores for heifers were higher (P < 0.001) than for steers by ~30 units inAC calvesand by20 unitsinWCcalves. Theresults of this experiment confirmtheeffectiveness of using sireswith high estimated breeding value for the required characteristics in producing the desired improvements in the progeny. The absence of any interactions of sire type with growth path indicates that differences between sire types will be similar regardlessofenvironmentalconditions.Animalsraisedonafastergrowthpathafterweaningproducecarcasseswithmorefat and more IMF% than those grown on slower growth paths.

animal production science

Effects of growth path, sire type, calving time and sex on growth and carcass characteristics of beef cattle in the agricultural area of Western Australia