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. 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

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 2nd or 3rd 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 ( )
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 (ME)
Mid-late – born d (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
abcMeans with unlike superscripts differ (P<0.05)

8. 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)

9. 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)

10. 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
$ a
$ ab
$ ab
$ b
a bMeans within a row with unlike superscripts differ (P<0.05)
1Marbling score: 400=Small0; 500=Modest0

11. 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

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
Than their latest born counterparts