1. 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. Fike 1, M.E. King 1, L.R. Corah 1 and W.D. Busby 2 1 Certified Angus Beef LLC 2 Iowa Tri-County Steer Carcass Futurity 2010 Annual ASAS Meeting

2. 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 2 nd or 3 rd 21 d intervals; however, no differences were found in ADG

3. 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

4. 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

5. 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)

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

7. Percent Angus of the Calf by Birth Sequence Within the Spring Calving Season a abc Means with unlike superscripts differ (P<0.05)

8. Disposition Score 1 of the Calf by Birth Sequence Within the Spring Calving Season a ab Means with unlike superscripts differ (P<0.05) 1 Disposition score: 1=docile; 6=very aggressive

9. Feedlot Delivery Weight, Adjusted Final Weight, Harvest Age and HCW by Birth Sequence Within the Spring Calving Season Variable Birth Sequence EMEMLL Delivery Wt., kg328.2 a 321.7 b 310.8 c 291.4 d Adjusted Final Wt., kg554.2 a 550.7 ab 552.8 ab 538.6 b ADG, kg/d1.46 a 1.49 ab 1.53 b 1.51 ab Harvest Age, d476.5 a 463.8 b 448.3 c 435.2 d HCW, kg341.1 a 338.4 ab 342.1 ab 332.5 b abcd Means within a row with unlike superscripts differ (P<0.05)

10. Carcass Merit by Birth Sequence Within the Spring Calving Season Variable Birth Sequence EMEMLL Fat Thickness, cm1.00 a 1.02 ab 1.07 b 1.05 ab REA, sq. cm.84.1384.2684.1382.77 MS 1 459.3 a 453.7 a 436.0 b 430.0 b Calculated YG2.59 2.692.65 Carcass Value, $/hd$1046.88 a $1035.93 ab $1035.63 ab $1006.61 b a b Means within a row with unlike superscripts differ (P<0.05) 1 Marbling score: 400=Small 0; 500=Modest 0

11. Quality Grade by Birth Sequence Within the Spring Calving Season USDA Quality Grade Birth Sequence EMEMLL Chi-Square P value Prime0.53%0.78%0.00% 0.3915 CAB ® 28.60%24.37%16.28%11.24%<.0001 All Choice85.05%83.63%76.88%77.89%0.0087 Select13.88%14.81%23.12%21.05%0.0043 Standard0.53%0.78%0.00%1.05%0.9695

12. 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 o Than their latest born counterparts