CASE STUDY: Assessment of Relationships Between Carcass Traits and Body Measures at Conclusion of Pasture Backgrounding1

Introduction

Sorting cattle for body size and condition as they enter the feedlot is a common practice used to improve uniformity in feedlot pens. Reduction in variability of body size and condition within feedlot pens groups cattle with similar feed efficiencies and endpoint carcass traits, which is economically advantageous. Visual appraisal is the most common method of sorting cattle because it is rapid and useful if done by experienced cattlemen. As with most visual estimation techniques, subjectivity is a problem in accurately assessing body size and condition. Improvements may be possible with precise, objective measures of body size and condition that closely relate to eventual carcass traits and values. Body weight is the most common measure of body size; however, hip height (HIPH) is a better indicator of maturity than BW (Hammack and Gill, 1997). Hip height and BW are direct measures of body size and combining these two measurements to calculate BW per unit of HIPH may provide a better predictor of carcass parameters. Ultrasound predictions of carcass composition for cattle entering the feedlot have been unreliable (Brethour, 2000; Rouse et al., 2000, Aiken et al., 2004), but ultrasound technology may have potential as a tool for sorting cattle entering the feedlot. May et al. (2000) reported that ultrasound technology can accurately assess carcass traits of finished cattle, but it also could be beneficial to implement ultrasound evaluation at key points along the marketing chain. An experiment was conducted to determine whether direct measures of body size, body condition, ultrasound fat thickness (UFT), ultrasound longissimus area (LA), and HIPH can be used to objectively sort cattle entering the feedlot.

Materials and Methods

This experiment was conducted at USDA-ARS Dale Bumpers Small Farms Research Center. Experimental protocols were reviewed and accepted by the research center's Animal Welfare Committee. Two groups of yearling steers were grazed separately during the cool and warm seasons using the same three pastures (two were 16.2-ha; the other was 20.2-ha). Steers in both groups were visually observed to have variable Angus, Hereford, Simmental, Charolais, and Limousine breeding. Three feeding regimens were randomly assigned to pastures to vary BW gains and range of body size and condition. Cool-season pastures were composed primarily of volunteer ryegrass (Lolium multiflorum Lam.), cheatgrass (Bromus tectorum L.), and little barley (Hordeum pusillum Nutt.). Supplements for the cool-season pastures were 0.45, 0.91, and 1.36 kg of a soybean meakground corn (1:4)/d per steer plus free-choice bermudagrass [Cynodon dactyon (L.) Pers.] hay fed from the initiation of grazing on Dec. 11, 2000 until Mar. 2, 2001. On March 3, supplements were decreased from 0.45 to 0, 0.91 to 0.45, and 1.36 to 0.91 kg/d per steer, respectively, until termination of grazing on May 24. Warmseason pastures were predominately bermudagrass, dallisgrass (Paspalum dilitatum Poir.), and crabgrass (Digitaria sanguinalis (L.) Scop. Supplements during warm-season grazing were 0, 0.45, and 1.36 kg of the concentrate mixture/d per steer with no hay available from initiation (Jun. 6) to termination (Sep. 25) of grazing. Steers were blocked by BW into three groups, and groups were randomly assigned to pastures. Sixty steers were grazed in the cool season, and 54 were grazed during the warm season. Mean initial BW and HIPH were 245

At initiation and conclusion of grazing in both seasons, unshrunk BW and HIPH were measured. Hip height was measured at the highest point over the hip with a sliding caliper. Ultrasonic scans were taken between the 12th and 13th ribs over the longissimus using an Aloka Model 500 (Aloka, Wallington, CT) equipped with a 17-cm linear array transducer. Fat thickness was measured at 0.75 the distance between the medial and lateral ends of the cross-sectional area of longissimus. Body condition was scored on a 9-point scale (1 = emaciated to 9 = obese) and averaged between two trained observers.

Cattle grazed on cool-season pasture were transported to a feedlot in Garden City, KS on Jun. 17, and those on the warm-season pasture were transported to a feedlot in Dimmitt, TX on Oct. 1. Cool-season cattle were harvested on Nov. 5, 2001 (141 d on feed), and warmseason steers were harvested on Mar. 14, 2002 (165 d on feed).

The following carcass traits were measured after a 48-h chill: marbling scores (MSCORE); carcass backfat thickness (CFT); LA; and kidney, pelvic, and heart fat (KPH). Yield grade (YG) and percentage of retail product (%RP) yield were estimated using USDA (1997) equations. Marbling scores were assigned by a trained observer according to the following scale: 100 to 190 = practically devoid^sup 00^ to practically devoid^sup 90^, 200 to 290 = trace^sup 00^ to trace, 300 to 390 = slight^sup 00^ to slight^sup 90^, 400 to 490 = small^sup 00^ to small^sup 90^, 500 to 590 = modest^sup 00^ to modest^sup 90^, 600 to 690 = moderate^sup 00^ to moderate^sup 90^, and 700 to 790 = slightly abundant^sup 00^ to slightly abundant^sup 90^ (USDA, 1997). Carcass values were set using USDA Beef Carcass Price Equivalent Values (USDA Market News, 2005) for Jun. 20, 2005. Carcass values were expressed on a per 45.5-kg carcass weight basis [$/100 lb; value per carcass weight (VALCWT)] and on a total carcass weight basis (VALCAR). Base value for a Choice quality grade between 273 to 409 kg (600 to 900 lb) hot carcass weight (HCW) was $126.96/45.5 kg of HCW; for a Select quality grade with 273 to 409 kg of HCW, the value was $121.89/45.5 kg of HCW (USDA Market News, 2005).

Spearman correlations (Steel and Torrie, 1980) were used to estimate associations between measurements of body size, body condition, carcass characteristics, and carcass value. Multiple linear regressions (Littell et al., 1996) were performed on the carcass traits using two sets of independent variables. One set included HIPH, BW, and UFT; the other set included HIPH/BW (kilograms of BW per centimeter of HIPH), body condition score (BCS), and UFT/BW (UFT/100 kg of BW). Season was analyzed in the model as a discrete variable to partition it from experimental error. Body size and condition measures were analyzed as continuous variables. Backward stepwise regression was used to reduce models to variables significant at the 0.10.

A simulated sorting of cattle was performed by sorting data separately for cool- and warm-season cattle in ascending order by either HIPH, BW, UFT, BW/HIPH, BCS, or UFT/BW. Mean YG, HCW, VALCWT, and percentage grading Choice or Prime were calculated for the lower and upper 50% of each pasture group. Lack of replication limited statistical comparisons; therefore, numerical differences between the lower and upper percentile groups and consistency between the between the two pasture groups were used to assess each sorting method.

Results and Discussion

Steers grazed on cool-season pastures tended to gave greater BW gain and body condition as compared with steers grazed on warm-season pastures (Table 1). Average daily gains on cool-season pastures were 0.72, 0.86, and 1.0 kg/d, and on warm-season pastures were 0.88, 0.91, and 0.70 kg/d, for low, intermediate, and high supplement levels, respectively. Forage mass was not measured, but forage production was visibly low in the warm-season pasture receiving the high-concentrate treatment. This pasture never received as much fertilizer and herbicide inputs in the immediate past as the other two pastures. Lesser forage growth with warm-season pastures likely limited steer performance to a level that was not sufficiently compensated by greater consumption of supplement. Furthermore, cool-season annual grasses are generally higher in quality than warm-season perennial and annual grasses (Ball et al., 2003). Differences in carcass traits and values were not as distinct between the two groups; however, this might have been due to steers grazed on warm-season pasture residing in the feedlot 24 d longer than those grazed on cool-season pasture.

Body condition score was correlated (P<0.10) with HCW, CFT, and KPH (Table 2). Marbling scores were not correlated (P>0.10) with any body measures. Reverter et al. (2000) reported low genetic and environmental correlations for Angus cattle, and 0.39 and 0.25, respectively, for Hereford cattle. Similarly, using Hereford steers, Arnold et al. (1991) calculated a genetic correlation of 0.19 and an environmental correlation of 0.14 between CFT and MSCORE. Poor relationships between external fat and intramuscular fat suggest that body condition is a poor indicator of MSCORE.

Correlations were moderately positive (r = 0.44) between UFT (collected at yearling age) and CFT for both grazing seasons (P<0.01; Table 2). Aiken et al. (2004) reported slightly less correlation between UFT and CFT (r = 0.36) measured at the conclusion of stocker grazing and concluded that a close association between CFT and UFT measurement prior to entrance in the feedlot is unlikely because of the change to a high-energy diet that increases the rate of external fat deposition. Greater positive correlations (r = 0.68) between UFT and CFT have been reported when ultrasound measurements were taken within a week of harvest dates (Brethour, 2000; May et al., 2000; Greiner et al., 2003).

Yield grade and %RP also were correlated (P<0.01) with UFT for both seasons and with UFT/BW (P<0.01) for the cool season (Table 2). Fat thickness has a major influence as an input in the calculations of YG and %RP (USDA, 1997). Therefore, UFT at termination of grazing may provide information on future rankings for YG and %RP of carcasses.

Hot carcass weight, kilograms of retail product (KGRP), and VALCAR were positively related (P<0.001) to BW and BW/HIPH in both seasons. Body weight at the conclusion of grazing had a direct, positive influence on KGRP and VALCAR. Obviously, cattle ranking higher in BW and BW/HIPH at entrance to the feedlot should produce heavier HCW, which, in turn, should have a direct positive influence on KGRP and VALCAR.

A season effect in the multiple linear regressions was not detected (P>0.05) for most variables (Table 3). Differences between cattle on cool- and warm-season pastures were observed in LA for Sets 1 and 2, and in KGRP (P<0.10) for Set 2 (Table 3). Although steers grazed on cool-season pasture generally were heavier and in better condition than those grazed on warm-season pasture, the additional 24 d in the feedyard for warm-season steers provided needed growth and conditioning to compensate for these differences.

Significant linear coefficients were determined for relationships between independent variables in Set 1 and each carcass trait (Table 3). Ultrasound fat thickness was the sole significant variable in reduced models for CFT (P<0.001), KPH (P<0.05), YG (P<0.01), and retail product (P<0.001); however, coefficients of determination for relationships between these dependent variables and UFT were <0.20. Although a rank correlation was not detected between HIPH and MSCORE (Table 2), multiple regression indicated a slight relationship between the two variables. The strongest relationships were found between independent variables in Set 1 and HCW, KGRP, and VALCAR. Hot carcass weight was positively affected (P<0.001) by BW, both of which reflect frame size. For KGRP, a positive regression coefficient (P<0.001) was determined for BW, and a negative coefficient (P<0.01) was calculated for UFT. Body weight at the conclusion of stocker grazing provided information on future rankings in KGRP, and UFT likely would give indication of rankings in CFT. Similar to MSCORE and VALCWT, HIPH had a negative effect (P<0.05) on VALCAR, which was likely related to the negative impact HIPH had on carcass quality grade. This association could be related to a breed-type effect on MSCORE (i.e., confounding of HIPH with breed type) within these groups of cross-bred steers rather than a direct influence of body frame on MSCORE. Ultrasound fat thickness also had a negative effect of little practical significance (P<0.10) on VALCAR, which was probably due to the inverse relationship between CFT and %RP. Greater BW at the conclusion of grazing resulted in greater VALCAR and was likely linked to the previously discussed positive relationship between BW and HCW.

The reduced model contained BW/HIPH and UFT/BW for all traits except KPH, MSCORE, and VALCWT (Table 3). Kidney, pelvic, and heart fat and MSCORE had positive increases of little significance (P<0.10) with increases in BW/HIPH and UFT/BW, respectively. The strongest relationships were detected (R^sup 2^ = 0.47) between the two independent variables and HCW, KGRP, and VALCAR. Linear coefficients were positive for BF and YG regressed over UFT, which is indicative of the influence that UFT had on eventual CFT and the positive relationship between CFT and YG. Percentage of retail product decreased linearly (P<0.01) with increases in BW/HIPH and UFT/BW, which was in agreement with the inverse relationships between %RP and YG. Hot carcass weight, KGRP, and VALCAR increased with increases in BW/HIPH, but decreased with increases in UFT/BW. The three dependent variables are quantity related; cattle at the conclusion of stocker grazing with greater BW/HIPH probably have a greater proportion of BW in dense lean tissue, and those with greater UFT/BW have a greater proportion of BW in fat tissue.

The simulated sorting into upper and lower 50% groups resulted in distinct groupings for each of the sorting measures (Table 4). Body weight ranked highest in HCW and VALCAR for the upper percentiles for cool- and warm-season groups. Therefore, the measure of body density, BW/HIPH, did not offer an advantage over BW measures. As a result of ranking highest for the upper percentiles, BW ranked lowest for the two carcass traits. It is critical that feeder cattle groups requiring longer days on feed are harvested with a minimal number of carcasses discounted for high HCW (>409 kg). The highest percentage of Choice/Prime quality grades was expected with the upper percentile group, but sorting by BW resulted in the lower percentile group for the cool-season steers having a higher percentage of Choice/Prime quality grades. This was likely due to the lower percentile group being composed of steers with primarily Angus and Hereford breeding; the upper percentile groups were composed of the later-maturing continental breeds. Sorting the cool-season steers by HIPH also showed the lower percentile group to have a higher percentage of Choice/Prime quality grades. This is likely common with commercial herds containing mixtures of early- and late-maturing breed influences, but higher quality grades are not critical if HCW can be increased to add carcass value.

The other sorting measures did not provide ranges in HCW or VALCAR required for successful sorting. Lack of sensitivity with the UFT and UFT/BW methods were probably because external fat thickness is typically very low for feeder cattle entering the feedyard. These methods are likely better suited for sorting cattle later in finishing when cattle have accreted thicker layers of backfat and feeder groups. Hip height could be used for sorting at entrance to the feedlot when groups lack uniformity in HIPH. Cattle groups used in the present experiment did not offer the variability needed to accurately assess HIPH as a sorting tool.

Implications

Results of the experiment indicated that direct measures of HIPH, BW, UFT, and BCS vs visually assessing BCS can improve sorting of cattle at the conclusion of backgrounding. Correlations were detected between carcass traits and body size and condition measures, but associations were generally greater with BW/HIPH and UFT/BW measures, which are criteria related to body thickness/muscling and external fat relative to BW, respectively. In combination, these measures provide information on rankings of HCW, KGRP, and VALCAR. Visual estimation of body size and condition will remain the standard method of sorting cattle entering the feedlot, but sorting by BW provides a simple and useful tool to discern small differences in body size and improve detection and grouping of cattle for reaching an optimum finish with minimum number of days on feed.

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