Effect of Time of Birth Within the Spring Calving Season on Performance and Carcass Traits of Beef Calves Fed in the Iowa Tri-County Steer Carcass Futurity G.D. Fike1, M.E. King1, L.R. Corah1 and W.D. Busby2 1Certified Angus Beef LLC 2Iowa Tri-County Steer Carcass Futurity 2010 Annual ASAS Meeting

Introduction and Background Larson, et al., (2010) found that calves born during the first 21 d calving period had greater HCW, marbling scores and greater % grading premium Choice and higher carcass values than calves born in the 2nd or 3rd 21 d intervals; however, no differences were found in ADG

Objectives Does age of calf and time of birth within the calving season affect: Feedlot performance Marbling score and, ultimately, USDA quality grade Certified Angus Beef ® (CAB®) acceptance rate

Materials and Methods Data were analyzed on 1,369 calves from a single, central Missouri ranch for a six-year period (2002-2007) Genetics were Angus/Simmental-based, rotating sires among different years All cattle were fed a common dietary energy level and administered similar health and implant treatments in a southwest Iowa feedlot All cattle were weighed, sorted, vaccinated, implanted, disposition scored and body conditioned scored within 4 days of arrival

Materials and Methods Calves were sorted and harvested when visually determined to have one cm of fat cover Only cattle with full, detailed carcass and performance information were included in the study Calves were divided into four sequence groups: Early – born d 1-21 of calving season (E) Mid-early – born d 22-42 (ME) Mid-late – born d 43-63 (ML) and Late – born d>63 (L)

Materials and Methods Statistical analyses: Continuous outcomes were quantified using one-way analysis of variance (ANOVA) For rate analyses, Chi-square was employed

Percent Angus of the Calf by Birth Sequence Within the Spring Calving Season abcMeans with unlike superscripts differ (P<0.05)

Disposition Score1 of the Calf by Birth Sequence Within the Spring Calving Season 1Disposition score: 1=docile; 6=very aggressive abMeans with unlike superscripts differ (P<0.05)

Feedlot Delivery Weight, Adjusted Final Weight, Harvest Age and HCW by Birth Sequence Within the Spring Calving Season Variable Birth Sequence E ME ML L Delivery Wt., kg 328.2a 321.7b 310.8c 291.4d Adjusted Final Wt., kg 554.2a 550.7ab 552.8ab 538.6b ADG, kg/d 1.46a 1.49ab 1.53b 1.51ab Harvest Age, d 476.5a 463.8b 448.3c 435.2d HCW, kg 341.1a 338.4ab 342.1ab 332.5b abcdMeans within a row with unlike superscripts differ (P<0.05)

Carcass Merit by Birth Sequence Within the Spring Calving Season Variable Birth Sequence E ME ML L Fat Thickness, cm 1.00a 1.02ab 1.07b 1.05ab REA, sq. cm. 84.13 84.26 82.77 MS1 459.3a 453.7a 436.0b 430.0b Calculated YG 2.59 2.69 2.65 Carcass Value, $/hd $1046.88a $1035.93ab $1035.63ab $1006.61b a bMeans within a row with unlike superscripts differ (P<0.05) 1Marbling score: 400=Small0; 500=Modest0

Quality Grade by Birth Sequence Within the Spring Calving Season USDA Quality Grade Birth Sequence E ME ML L Chi-Square P value Prime 0.53% 0.78% 0.00% 0.3915 CAB® 28.60% 24.37% 16.28% 11.24% <.0001 All Choice 85.05% 83.63% 76.88% 77.89% 0.0087 Select 13.88% 14.81% 23.12% 21.05% 0.0043 Standard 1.05% 0.9695

Conclusions The oldest calves were/had: Heavier at feedlot delivery Heavier at harvest and greater HCW Greater marbling scores Higher percentage of calves grading USDA Choice and CAB® Greater carcass value per head Than their latest born counterparts