Basics of growth and development

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Chapter III Relationships among lifetime measures of growth and frame size for commercial beef females in a pasture-based production system in the Appalachian region of the United States

A paper to be submitted to the Professional Animal Scientist  C. Echols1, M. L. Wahlberg1, W. S. Swecker Jr.2, S. P. Greiner1

ABSTRACT:

The beef cattle industry has placed increased focus on mature cow size as a result of its influence on production efficiency and profitability. The objectives of this study were to evaluate relationships among lifetime measures of body weight (BW) and frame score (FS) for commercial beef females, and to assess the value of immature measures as predictors of mature cow size. Measurements of BW, hip height (HH), body condition score (BCS), and calculated FS were recorded at weaning (WN), breeding at 13 mo age (BR), and 8 subsequent periods, ceasing at approximately 5 yr of age for 232 Angus-cross females born 2004 through 2008. Correlation analysis revealed significant (P < 0.001) relationships among BW taken at WN and BR with BW measurements taken at 2.5, 3.8, and 4.8 yr of age (WN r = 0.70, 0.51, 0.61; BR r = 0.65, 0.57, 0.64 respectively). Significant relationships (P < 0.001) existed between FS collected at WN and BR and FS at 2.5 and 3.8 yr (WN= 0.70, 0.72; BR= 0.79, 0.82 respectively). Repeatability of lifetime FS measures was 0.73. BCS was a significant (P < 0.001) source of variation in mature BW, with a unit change in BCS accounting for 41 kg BW change at 4.8 yr (P001). BW and FS were moderately to strongly related (P < 0.001) at WN, BR, 2.5, 3.8, and 4.8 yr (r = 0.62, 0.49, 0.62, 0.62, and 0.47 respectively). Prediction models for BW at 4.8 yr were similar using weaning BW alone, or with inclusion of both weaning BW and HH (R2 = 0.57 1Department of Animal and Poultry Science, Virginia Polytechnic Institute and State University, Blacksburg, VA 24060.
Virginia-Maryland Regional College of Veterinary Medicine, Blacksburg, VA 24060. and 0.56). Similarly, breeding BW and HH were non-additive for prediction of 4.8 yr BW (R2 = 0.68, 0.58, and 0.68 for BW, HH, and BW and HH respectively). Performance at immature ages proves to be a satisfactory indicator of mature size, supporting continued incorporation of immature BW and HH/FS measurements into selection practices. Keywords: beef cow, body weight, frame score, mature size

Introduction

Selection pressure for weaning and post-weaning growth coupled with moderate to high correlations of these weights and associated frame score (FS) to the same measures at maturity have resulted in heavier body weight (BW) in the mature cow herd (McMurray, 2008; Dib et al., 2010). Increased cow size may not be desirable for the breeding herd as cattle with larger weights have greater maintenance costs (Fiss and Wilton, 1992).
Skeletal size has been considered one of the most important traits for beef cattle given reported high heritability of FS (0.60-0.64; Minish and Fox, 1979; Dib et al., 2010). Estimates of direct heritability of mature BW and mature height have been generally moderate to high (Berg and Butterfield, 1976; Petty and Cartwright, 1966; Dib et al., 2010), with correlations between the two traits also high. Body weight and FS have been commonly used to describe size for many years, and even though the relationship is not perfect, there is a direct relationship between FS and mature BW (Owens et al., 1993).
Acquiring or raising quality replacement heifers is an essential and major investment for the cow-calf producer, as the replacement female becomes the building block for the cow herd. The commercial cattleman is continually looking for management practices that will add consistency to their calf crop and improve profitability; however, their selection methods may be limited to phenotypic information. It is imperative that there is continued efficacy in selection practices utilizing BW and FS in selection decisions.
Objectives of this research project were: 1) characterize and evaluate lifetime BW and skeletal size changes for beef females, 2) identify the relationship among immature measurements of BW and skeletal size to the same measures at mature ages in beef females, and develop strategies for assessment and change of mature size to be utilized by the commercial cow-calf sector utilizing within-herd records.

Materials and Methods

Phenotypic observations were compiled from Angus-cross females that were born and raised at the Shenandoah Valley Agricultural Research and Extension Center (SVAREC) in Raphine, Virginia (37⁰ 56`N; 79⁰ 13` W; elevation 537.4 m). The cow herd at SVAREC is maintained exclusively on grazing and forage systems. A total of 232 females were utilized, including 51, 49, 36, 47, and 48 born in 2004 through 2008, respectively. Pedigree information (sire, dam, and maternal grandsire), along with repeated measures of BW, HH, and BCS from weaning (WN) to oldest age or approximately 4.8 yr of age were evaluated.
During initial compilation of data, all observations for BW, HH, and BCS were included when organizing data by birth year. Weaning HH was not measured on calves born in 2005. To account for variation of BCS, BW measurements not accompanied by a BCS were removed from further analysis when females exceeded 2 yr of age. Using days of age (DOA), observations were grouped for each birth year into 10 periods (Table A.1). Considerations for the number of observations on a particular date and average age for the respective period dictated what observations were included when animals had multiple observations within the same period.
Observations for period 1 were taken at weaning (mean DOA= 230) and period 10 contains information on cows just under 5 yr (mean DOA= 1745). Animals born prior to 2004 were not utilized due to limited weight and hip height collections.
Forage composition of SVAREC pastures was representative of that of cow-calf operations common to the Appalachian region. Tall fescue (Festuca arundinacea) was the dominant forage available for grazing and stockpiling. Cows were maintained on rotationally grazed paddocks until stockpiled forages diminish, typically in January or February.  After all stockpiled grass had been consumed, cows were fed free-choice fescue (Festuca arundinacea)/orchardgrass (Dactylis glomerata) hay placed in round hay feeders. A mineral and vitamin supplement was provided ad libitum, but no other supplements were provided to the cow herd. The year-round forage systems provided for the changing nutritional demands of the beef cows and allowed calves to have continuous access to high-nutritive value forages through creep grazing of adjacent paddocks within the system during the grazing season.  First calf heifers were managed similarly in separate paddocks.  In addition to orchardgrass hay, heifers received some orchardgrass baleage.   Weaning measurements were collected on 12/16/2004, 10/20/2005, 10/13/2006, 09/07/2007, and 09/10/2008. Heifers retained post-weaning were developed on stockpiled fescue followed by either orchardgrass baleage (2006-2008) or orchardgrass baleage supplemented with 75% corn gluten and 25% wheat midds at 1% of bodyweight every other day (2004-2005). Approximately April 1 each yr, heifers were rotationally grazed on fescue pastures until calving.
Cows were synchronized and artificially inseminated once prior to exposure to Hereford or Angus bulls for a 63 d breeding season. Semen (AI) from commercial companies, and cleanup bulls (leased from seedstock producers in Virginia) were used. Within 45 to 60 d after the end of the breeding, cows were rectally palpated for pregnancy diagnosis, and open cows were culled. Calves were born in January through March. Sires of females in the dataset were Angus (N=51), Hereford (N=6), and Red Angus (N=1).
The number of replacements annually was determined by the number of females needed to populate research trials in addition to replacement of open females. An average of 46 (SD= 5.6) heifers were kept as replacements. The selection process began at weaning by first eliminating any heifers with low weaning weights or heifers born late in the calving season. Disposition was a secondary consideration. Heifers were ranked on weight per day of age, providing additional information for final selection. Females kept through 1 yr of age were bred.
Body weight and HH measurements were collected periodically throughout the year when cattle were gathered for vaccinations, synchronization and breeding (BR), pregnancy diagnosis, and WN. Body weights and/or HH were not collected at every handling; therefore, the database was limited to observation dates when BW, HH, and BCS (when appropriate) were documented. An adjusted weaning BW was calculated adjusting age to 205 d and additionally adjusted for age of dam using BIF Guidelines (2002). Mean adjusted weaning weight was 224 kg (SD = 22.54). Hip heights and FS were measured and calculated according to BIF Guidelines (2002). A FS was calculated using HH adjusted to 205 d of age and further adjusted for dam age using adjustments included in the BIF Guidelines (1986). No subsequent observations were adjusted for dam age. Body condition score was based on a subjective, 9-point classification scale, from extremely thin (1= very emaciated) to extremely fat (9= very obese) (Richards et al., 1986). Cows were assigned physiological codes for pregnancy (1= open under 2 yr of age, 2= bred 13-24 mo of age, 3= open over 2 yr of age, 4= bred over 2 yr of age) and lactation (1= not lactating, 2= lactating). Technicians determining HH and assigning BCS varied, but were not documented.
Statistical analyses were conducted using SAS 9.2 (SAS Institute Inc., Cary, NC). Pearson correlation coefficients were used to evaluate the relationships between BW and FS at all 10 periods. Since the BW could be influenced by BCS, a partial correlation was conducted when BW was of interest. Repeated measurements were analyzed using GLIMMIX procedure.
Models fitted for repeated measurements of BW, HH, and FS, respectively are presented in Tables A.2-A.4. Quadratic estimates were not significant for HH (Table A.3). Differences in least squares means were evaluated using the Tukey adjustment.
Multivariate analyses were conducted to determine traits contributing to prediction of BW at specific ages. The GLM procedure of SAS was used to determine the effects of birth yr, lactation, BCS, along with combinations of WN and BR observations on BW at 2.8, 3.8, 4.5, and 4.8 yr.

Results and Discussion

Means and standard deviations for BW, HH, FS, and BCS of all cows included in this study are presented in Table 3.1. The information was further organized in the same format for each birth yr in Table A.5 – A.9 to better describe the observations collected for each birth yr.
Correlation values for lifetime measures of BW are presented in Table 3.2. Correlations were similar between BW taken at both WN and BR to those taken at mature ages. The correlations of BW at WN and BW at 2.5 yr, 3.8 yr and 4.8 yr were 0.70, 0.51, and 0.61, respectively (P < 0.001). The correlations of breeding BW to BW at the same ages were 0.65, 0.57, and 0.64 (P < 0.001) respectively. Additional correlations are included in Table 3.2.
Correlations reported in the current study are higher than those reported by Northcutt and Wilson (1993) who found correlation between 205 d BW and mature BW of 0.37 (P < 0.001) and a correlation between 365 d BW and mature BW of 0.41 (P < 0.001). Differences may be attributed to the variation in management and environment of the 28,391 head, multiple herd database studied by Northcutt and Wilson (1993). Brinks et al. (1964) reported similar correlations to Northcutt and Wilson (1993) in Hereford cattle, with r = 0.45 between weaning BW and mature BW. Mature BW in the Brinks et al. (1964) study was collected on 5-, 6-, and 7-yr-old Angus cows.
Klosterman et al. (1968) reported that mature weight is greatly influenced by body condition. Accounting for 16% of the total variation in weight (Northcutt et al., 1992), BCS was found to be a significant source of variation in weight (P < 0.001). While inclusion of BCS as a covariate had no effect on HH or FS in the present study, BCS was a significant source or variation of mature cow BW (P < 0.001); therefore, it was included as a variable when weight was included in correlations. The mean BCS was 5.81 (N= 399). There is apparent merit in adjusted weights for body condition, as measures of cow maintenance requirements are based on weight at an identified condition. Bensyhek and Marlow (1973) suggested that adjusting weight for body condition removes environmental variation. Additionally, adjusting for BCS removes some genetic variation in BW as genetic variation in BW includes genetic variation in body composition (Choy, 1996).
Strong genetic and phenotypic correlations between BW and height traits have been documented in previous studies. Bourdon and Brinks (1986) reported genetic and phenotypic correlations of 0.77 and 0.62 respectively, between yearling weight and height. Phenotypic correlation between yearling BW and FS in the current study was 0.48 (P < 0.001, Table 3.3).
Phenotypic relationships between BW and FS at 3.8, 4.2, and 4.8 yr were 0.62 (P < 0.001), 0.58 (P < 0.05), and 0.47 (P < 0.001), included in Table 3.3. Northcutt and Wilson (1993) reported a phenotypic correlation between mature weight and height of 0.58 when weight was adjusted for BCS, which is similar to correlations reported by Bourdon and Brinks (1986).
Correlation coefficients among FS at different age periods are presented in Table 3.4. Some of the coefficients lacked significance, likely attributed to the limited number of observations. As expected, some of the highest correlations existed between observations in subsequent periods. Breeding FS tended to be more highly correlated than weaning FS as compared to measurements taken at maturity. Weaning FS was significantly related to FS at 2.5, 2.8, 3.8, and 4.2 yr (r = 0.70, 0.60, 0.72, and 0.60 (P < .001)), but not at 4.5 or 4.8 yr. The limited number of observations at 4.5 and 4.8 yr (n=25 and 22), likely contributed to lack of relationship. Breeding FS had r = 0.79, 0.71, 0.82, and 0.58 (P < .001) at 2.5 yr, 2.8 yr, 3.8 yr, and 4.8 yr respectively. These relationships suggest selection on FS at an early age will be related with mature FS.

Chapter 1. Introduction
Chapter 2. Literature Review
Overview of Growth
Basics of growth and development
Factors influencing growth.
Assessing Mature Cow Size.
Weight.
Describing growth
Frame/Skeletal size
Relationships between weight and frame.
Tools for selection
Importance of Cow Size
Breed Implications
Summary
Chapter 3. Relationships among lifetime measures of growth and frame size for commercial beef females in a pasture-based production system in the Appalachian region of the United States
INTRODUCTION
MATERIALS AND METHODS
RESULTS AND DISCUSSION
IMPLICATIONS
FIGURES
TABLES
APPENDIX
REFERENCES
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Relationships among lifetime measures of growth and frame size for commercial beef females in a pasture-based production system in the Appalachian region of the United States

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