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Genetic parameters for carcass traits and their live animal indicators in Simmental cattle¹

D. H. Crews, Jr.^*,2, E. J. Pollak, R. L. Weaber^,, R. L. Quaas and R. J. Lipsey

^* Agriculture and Agri-Food Canada Research Centre, Lethbridge, Alberta T1J 4B1 Canada; and Department of Animal Sciences, Cornell University, Ithaca, New York 14853; and and American Simmental Association, Bozeman, Montana 59715

² Correspondence:
5403 1st Ave. South (phone: 403-317-2288; fax: 403-382-3156; E-mail:
dcrews{at}agr.gc.ca).

	Abstract

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The objective of this study was to estimate parameters required for genetic evaluation of Simmental carcass merit using carcass and live animal data. Carcass weight, fat thickness, longissimus muscle area, and marbling score were available from 5,750 steers and 1,504 heifers sired by Simmental bulls. Additionally, yearling ultrasound measurements of fat thickness, longissimus muscle area, and estimated percentage of intramuscular fat were available on Simmental bulls (n = 3,409) and heifers (n = 1,503). An extended pedigree was used to construct the relationship matrix (n = 23,968) linking bulls and heifers with ultrasound data to steers and heifers with carcass data. All data were obtained from the American Simmental Association. No animal had both ultrasound and carcass data. Using an animal model and treating corresponding ultrasound and carcass traits separately, genetic parameters were estimated using restricted maximum likelihood. Heritability estimates for carcass traits were 0.48 ± 0.06, 0.35 ± 0.05, 0.46 ± 0.05, and 0.54 ± 0.05 for carcass weight, fat thickness, longissimus muscle area, and marbling score, respectively. Heritability estimates for bull (heifer) ultrasound traits were 0.53 ± 0.07 (0.69 ± 0.09), 0.37 ± 0.06 (0.51 ± 0.09), and 0.47 ± 0.06 (0.52 ± 0.09) for fat thickness, longissimus muscle area, and intramuscular fat percentage, respectively. Heritability of weight at scan was 0.47 ± 0.05. Using a bivariate weight model including scan weight of bulls and heifers with carcass weight of slaughter animals, a genetic correlation of 0.77 ± 0.10 was obtained. Models for fat thickness, longissimus muscle area, and marbling score were each trivariate, including ultrasound measurements on yearling bulls and heifers, and corresponding carcass traits of slaughter animals. Genetic correlations of carcass fat thickness with bull and heifer ultrasound fat were 0.79 ± 0.13 and 0.83 ± 0.12, respectively. Genetic correlations of carcass longissimus muscle area with bull and heifer ultrasound longissimus muscle area were 0.80 ± 0.11 and 0.54 ± 0.12, respectively. Genetic correlations of carcass marbling score with bull and heifer ultrasound intramuscular fat percentage were 0.74 ± 0.11 and 0.69 ± 0.13, respectively. These results provide the parameter estimates necessary for genetic evaluation of Simmental carcass merit using both data from steer and heifer carcasses, and their ultrasound indicators on yearling bulls and heifers.

Key Words: Beef Cattle • Carcasses • Genetic Parameters • Simmental • Ultrasound

	Introduction

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Recently, breed associations have begun collection of real time ultrasound (RTU) and live weight data to augment national cattle evaluations for carcass merit (Bertrand et al., 2000; Crews and Kemp, 2002). Several studies have shown that RTU traits measured in yearling bulls and heifers have positive genetic correlations with corresponding carcass traits of progeny (Reverter et al., 2000; Crews and Kemp, 2001; Bertrand, 2002). The combination of RTU and carcass data in national cattle evaluation provides for estimation of EPD for a larger and more random sample of cattle than when evaluations are based on carcass data alone (Crews and Kemp, 2002).

Genetic evaluation of carcass traits requires estimation of genetic parameters among the traits to be evaluated. To date, most reports in the literature estimate genetic parameters among carcass traits and their RTU indicators using data from organized progeny tests or relatively small experimental populations. Whereas the estimates reported have largely been similar across studies (Bertrand, 2002), there is limited information about genetic parameters estimated from field data for live animal and carcass data. Therefore, the objective of this study was to estimate parameters required for genetic evaluation of Simmental carcass merit using a combination of carcass and live animal data.

	Materials and Methods

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Live Animal Data

Live weights and RTU measures of subcutaneous fat thickness, longissimus muscle area, and intramuscular fat percentage at 369 d of age (SD = 41 d) were available on 3,409 bulls and 1,503 heifers from the American Simmental Association (ASA) performance database. Yearling bulls with live animal data were sired by 366 bulls and out of 2,858 dams, whereas yearling heifers were sired by 204 bulls and out of 1,315 dams. A total of 151 sires and 211 dams had both yearling bull and heifer progeny with live animal data.

Carcass Data

Carcass measurements of weight, subcutaneous fat thickness, longissimus muscle area, and marbling score were available on 5,750 steers and 1,504 heifers (453 d, SD = 49 d) in the ASA carcass database. Steers and heifers with carcass data represented 585 sires and 6,300 dams.

Pedigree

In order to estimate genetic correlations of carcass traits measured on steers and heifers with their live animal indicators measured on yearling bulls and heifers, a pedigree file was constructed, which included animals that provided genetic ties between the live and carcass data. A total of 140 sires and 160 dams had progeny with live animal data and progeny with carcass data. These relatively low numbers of sires and dams are due in part to the fact that no animal had both live and carcass data. The final pedigree file used for calculation of the inverse numerator relationship matrix contained 23,968 animals, including those augmented so that each animal with data had two ancestral generations.

Analysis Models

Prior to analysis, response variables were edited to remove unusable records and outliers. Outliers were defined as observations that deviated from their sample mean by more than 5 SD, whereas unusable records were those on animals for which no parentage information or slaughter date were available. Less than 1% of the original data were deleted under these criteria.

Models were written to estimate heritability and genetic correlations required to produce EPD for four carcass traits, including hot carcass weight, subcutaneous fat thickness, longissimus muscle area, and marbling score. Similar to Crews and Kemp (2001), the model for all traits except carcass weight could be represented in matrix notation as

where X and Z were design matrices relating observations (y) to their respective fixed (b) and random (u) effects, and e was a vector of random residuals. Subscripts denote observations and model terms relative to yearling bulls (B), yearling heifers (H), and steer and heifer carcasses (C), respectively. Random effects were assumed to have null means, and variances represented as

and

In these equations, A represents the additive relationship matrix and identity matrices of order appropriate to the numbers of bulls (I_B) and heifers (I_H) with live animal data, and of steers and heifers (I_C) with carcass data were used to disperse residual variances. Because no animal had both live and carcass data, residual covariances were zero.

The models for fat thickness, longissimus muscle area, and marbling score were three trait models defined such that RTU indicators measured on bulls and replacement heifers were assumed to be separate but correlated traits, which were in turn separate but correlated to the carcass traits measured on steers and cull heifers (Reverter et al., 2000; Crews and Kemp, 2001). It is common for replacement bulls and heifers to be managed differently during the period between weaning and their yearling breeding season, resulting in differences in deposition patterns of carcass yield traits (Crews et al., 1998). In the finishing period, however, steers and cull heifers would be managed more similarly; therefore, carcass characteristics of steers and heifers were assumed to be genetically equivalent. Additionally, Crews and Kemp (2001) estimated a genetic correlation of 0.95 between yearling weights of replacement bulls and heifers, indicating that yearling weights were nearly genetically equivalent between sexes. Therefore, the model for carcass weight was bivariate, with live weight at scanning (i.e., yearling) on replacement bulls and heifers treated as one trait that was separate but correlated to carcass weights of slaughter animals. Representation of the weight model by reduction of the general model previously described is straightforward.

Fixed effects for live animal traits on purebred replacement bulls and heifers included scan contemporary group (herd x scan date x sex) and the linear regression of age at scanning. Technician code was also available; however, technician was nearly completely confounded with herd x scan date and was therefore not included in the contemporary group definition. Yearling bulls and heifers with RTU data were assigned to 254 multiple sire contemporary groups. Fixed effects for carcass traits included harvest contemporary group (harvest date x sex x percentage Simmental [50, 62, 75, 94%]) and the linear regression of age at harvest. Steers and heifers with carcass data were assigned to 662 contemporary groups. Estimates of (co)variances and genetic parameters were obtained by average information REML using ASREML (A. R. Gilmour, NSW Agriculture, Orange, NSW, Australia).

	Results and Discussion

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Summary statistics for live animal traits measured in yearling bulls and heifers, and for carcass traits measured on steer and heifer carcasses are presented in Table 1. All of the RTU traits measured in yearling bulls and heifers are presented by gender except for weight, which was considered genetically equivalent in the two sexes. Mean fat thickness was equal for bulls and heifers (4.06 mm) and similar to the bulls (4.07 mm) used in Crews and Kemp (2001). Bertrand (2002) noted that in studies involving mostly British breed types, ultrasonic fat thickness of yearling bulls was typically higher than 4.00 mm. He also pointed out that more study involving the continental European breeds was warranted, given their later maturity and potential for limited fat deposition by yearling age. It is interesting to note that although fat thickness was equal in replacement bulls and heifers, the estimates of percentage of intramuscular fat for heifers (3.40%) was higher than that for bulls (2.68%). The average carcass marbling score of 5.01 corresponded to the USDA Small marbling degree (i.e., Small⁰¹) and, assuming young maturity scores, the lower one-third of the USDA Choice quality grade. Considering the average carcass fat thickness was less than 10 mm, these results indicate that genetic resources exist within the Simmental breed to produce Choice carcasses without excess subcutaneous fat deposition. Among the carcasses that had both fat thickness and marbling score recorded (n = 6,452), 43% had sufficient marbling score (i.e., 5.00) to grade USDA Choice. Of those with at least a small amount of marbling, 41% had a fat thickness less than 10 mm.

Table 3 contains parameters estimated with the model for weight. As previously discussed, live weights of replacement bulls and heifers were assumed to have a genetic correlation of one. Heritability estimates for weight at scanning (0.47) and carcass weight (0.48) were nearly identical, and estimated phenotypic SD were similar. Koots et al. (1994a) reported unweighted and weighted mean heritability for yearling weight of 0.35 and 0.33, respectively, however, heritability estimates for yearling weight were higher (0.40) in Robinson et al. (1993) and Moser et al. (1998). The heritability of steer carcass weight in Crews and Kemp (2001) was 0.38, and the mean weighted heritability reported by Koots et al. (1994a) was 0.23, both lower than in the present study. In a study comparing genetic parameters for carcass traits adjusted to different end points, Shanks et al. (2001) reported a heritability of 0.32 for carcass weight based on a subset of the present data. Estimates more similar to that of the present study for carcass weight heritability were found by Gregory et al. (1995), Splan et al. (1998), and Reverter et al. (2000). The genetic correlation between live and carcass weights was 0.77, similar to the estimate of 0.82 reported in Crews and Kemp (2001). A higher genetic correlation (0.91) between yearling and carcass weight was summarized by Koots et al. (1994b), although this weighted mean heritability was probably estimated from data with both traits measured on the same animal. Devitt and Wilton (2001) estimated a genetic correlation of 0.72 for postweaning ADG of bulls with postweaning ADG of steers; however, the genetic correlation of bull ADG with steer carcass weight (0.53) was not as high.

Parameter estimates from the model for fat thickness model are reported in Table 4. Phenotypic SD for ultrasound fat measurements from yearling bulls and heifers were 1.28 and 1.26 mm, respectively, which were similar and significantly less than the phenotypic SD for carcass fat thickness (3.44 mm). Wilson (1992) stated that phenotypic expression of fat measurements may be limited by nutritional management of growing cattle. It appears that although RTU yearling fat measurements have lower estimates of phenotypic variance, most of this can be explained in the present study by the lower mean because CV (Table 1) were similar among both RTU and carcass fat measurements. The estimated phenotypic SD for bull ultrasound fat was essentially equivalent to the estimate (1.26 mm) reported by Crews and Kemp (2001), but the estimate for replacement heifers was higher (1.68 mm) in that report. Reverter et al. (2000) showed mean additive genetic SD estimates of 0.62 and 0.28 mm for Angus bull and heifer ultrasound fat, respectively, and 0.99 and 0.47 for corresponding measures in Hereford bulls and heifers. Additive genetic SD for replacement bull and heifer RTU fat in the present study were 0.93 and 1.05 mm, respectively, which were generally higher than those reported by Reverter et al. (2000). Heritability estimates were 0.53 and 0.69 for replacement bull and heifer RTU fat, respectively. Low heritability estimates (<0.15) were reported by Johnson et al. (1993) and Moser et al. (1998) for yearling RTU fat thickness when bull and heifer data were combined. Shephard et al. (1996), however, estimated a heritability of 0.56 for yearling RTU fat depth in Angus cattle. The high heritability estimates in this study also compare more favorably to the estimates of 0.47 and 0.54 reported by Reverter et al. (2000) for Angus bulls and heifers; however, they were higher than their estimates of 0.09 and 0.27 for Hereford bulls and heifers. The heritability of 0.35 for carcass fat thickness was similar to the weighted mean (0.42) found by Koots et al. (1994a) and intermediate to other recent published estimates, which ranged from 0.28 (Reverter et al., 2000) to 0.66 (Splan et al., 1998). Fat thickness heritability estimates ranging from 0.10 to 0.14 were reported by Shanks et al. (2001). Genetic correlations of carcass fat thickness with RTU fat thickness of bulls (0.79) and heifers (0.83) were high. A high estimate (0.88) for this parameter was reported by Devitt and Wilton (2001). Similarly, three of four estimates of the correlation between RTU and carcass fat were greater than 0.79 in Reverter et al. (2000). Bertrand (2002) summarized four estimates of the genetic correlation between yearling RTU and carcass fat thickness wherein the average was 0.62.

Table 5 contains parameters estimated with the model for longissimus muscle area. Phenotypic SD estimates for replacement bull and heifer RTU longissimus muscle area were 7.55 and 6.94 cm², respectively. Devitt and Wilton (2001) reported phenotypic and additive genetic SD estimates of 7.64 and 5.43 cm² for bull RTU longissimus muscle area. Similarly, Crews and Kemp (2001) reported phenotypic and genetic SD estimates of 7.30 and 5.70 cm², respectively, for yearling bull RTU longissimus muscle area, and 6.14 and 4.29 cm², respectively, for yearling heifer RTU longissimus muscle area. Lower estimates of additive genetic SD ranging from 3.21 to 4.08 cm² were reported by Reverter et al. (2000) for RTU longissimus muscle area in Hereford and Angus bulls and heifers in Australia. Heritability estimates were 0.37, 0.51, and 0.46 for bull RTU, heifer RTU, and carcass longissimus muscle area, respectively. Moderate to high heritability estimates for RTU measures of longissimus muscle area have been reported by Reverter et al. (2000); however, the estimate in Shepard et al. (1996) was low (0.12). Koots et al. (1994a) reported a weighted mean heritability of 0.42 for carcass longissimus muscle area, which compares favorably with the present study. However, Shanks et al. (2001) reported heritability estimates ranging from 0.22 to 0.29 for carcass longissimus muscle area using Simmental field records adjusted to different end points. In the present study, the genetic correlation of carcass longissimus muscle area with bull RTU longissimus muscle area was 0.80. The estimate of this correlation was lower for the Angus (0.29) but higher for the Hereford (0.94) in the study by Reverter et al. (2000). Both Moser et al. (1998) and Devitt and Wilton (2001) reported a genetic correlation of 0.66 between yearling RTU longissimus muscle area in bulls and carcass longissimus muscle area in slaughter animals. Similarly, Crews and Kemp (2001) estimated genetic correlations of 0.71 and 0.67 for carcass longissimus muscle area with bull and heifer RTU longissimus muscle area, respectively. In the present study, the genetic correlation between yearling heifer RTU longissimus muscle area and carcass longissimus muscle area was 0.54, which appeared to be lower than that reported in Crews and Kemp (2001), but higher than the estimates of 0.16 and 0.46 reported for Angus and Hereford by Reverter et al. (2000).

Parameters among RTU measures of intramuscular fat and carcass marbling score are reported in Table 6. Heritability estimates were 0.47, 0.52, and 0.54 for bull RTU intramuscular fat, heifer RTU intramuscular fat, and carcass marbling score, respectively. Heritability estimates of 0.42 and 0.23 were reported for RTU intramuscular fat by Wilson et al. (1999) and Devitt and Wilton (2001), respectively. Further, Bertrand (2002) summarized unpublished heritability estimates for RTU intramuscular fat of 0.19 and 0.39 from analyses of Brangus and Hereford field data. The estimated heritability for carcass marbling score (0.54) was generally higher than the weighted mean heritability (0.38) in the summary of Koots et al. (1994a), and the estimate of 0.12 reported by Shanks et al. (2001). The estimate in the present study was similar to that (0.55) reported by Gregory et al. (1995); high heritability estimates for marbling score have also been reported by O’Connor et al. (1997), Pariacote et al. (1998), and Splan et al. (1998). Genetic correlations of carcass marbling score with bull and heifer RTU intramuscular fat were 0.74 and 0.69, respectively. Devitt and Wilton (2001) estimated a genetic correlation between bull RTU intramuscular fat and steer carcass marbling score of 0.80, while Wilson et al. (1999) found this correlation to be 0.77. Further, Bertrand (2002) reported a genetic correlation of 0.70 between RTU intramuscular fat percentage and carcass marbling score in Brangus field data. These results suggest that a favorable association exists between carcass marbling score and its RTU indicator, and that selection for carcass marbling score would be improved by the addition of RTU measures of intramuscular fat in yearlings.

	Implications

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	Footnotes

 
¹ AAFC-LRC contribution No. 38702066. Funding support provided by the American Simmental Association, and the AAFC Matching Investment Initiative is gratefully acknowledged.

Received for publication July 30, 2002. Accepted for publication February 26, 2003.

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