Comparison of Spayed Heifer Performance to Intact Heifers Fed Melengestrol Acetate on Feedlot Performance and Carcass Characteristics

Authors: Tate S. Johnson, Rebecca L. McDermott, Halden J. Clark, Veterinarian, Jim C. MacDonald, Jessica L. Sperber*.

Summary with Implications

Pregnancy and estrous cycle management are critical considerations when feeding intact heifers in the feedyard. Alternatively, producers can opt to spay heifers, which eliminates hormonal fluctuations associated with the estrous cycle. A proportion of U.S. feedlots have spayed heifers on feed, although there is limited data comparing the performance of spayed heifers to intact heifers. A commercial feedlot experiment was conducted to compare performance between spayed and intact heifers fed melengestrol acetate. Intact heifers had greater ADG and tended to have greater DMI compared to spayed heifers, although no difference in feed conversion was observed. Carcass weight and final BW were heavier for intact heifers. Spayed heifers were leaner and had a lower USDA Yield Grade compared to intact heifers. These results indicate that spaying heifers upon feedlot entry and feeding similar DOF as intact heifers does not result in improved performance. 

Introduction

Heifers make up 32 to 40% of cattle on feed in the U.S., and it is estimated that 2 to 15% of intact heifers entering the feedlot are pregnant. Heifers in the feedlot pose several economic and management challenges regardless of pregnancy management method, including riding-behavior management and estrous cycle control. Feedyards have the option to purchase spayed heifers or perform the spaying procedure at the time of feedyard receiving. The spaying procedure removes the ovaries, eliminating hormonal fluctuations during the estrous cycle, which may result in more energy for gain as the heifer is no longer expending energy towards estrous-associated behaviors like mounting and riding. The exact percentage of spayed heifers on feed in the U.S. is unknown and the literature comparing performance and carcass traits between spayed heifers and intact heifers is limited. Therefore, the objective of this experiment was to evaluate the impact of spaying heifers using the Willis drop spay procedure at feedlot arrival on live performance and carcass characteristics compared to intact control heifers fed melengestrol acetate (MGA; Phibro Animal Health Corporation).

Procedure

A feedlot finishing experiment was conducted at a commercial feedyard in central Nebraska using 666 yearling heifers (allocation BW = 696; SD = 65 lb) in a generalized, randomized, block design. Eight pens were used, with four replicates per treatment (approximately 85 heifers per pen) comparing two treatments: heifers spayed upon arrival to the feedyard and intact heifers fed MGA. Heifers were blocked into two groups based on feedyard arrival date. Prior to experiment trial initiation, heifers were held off feed for 24 h prior to performing the ‘Willis Drop” spay procedure at time of initial processing for heifers enrolled on the spay heifer treatment. Heifers were gate-sorted at initial processing with three heifers sorted to be spayed, followed by three intact heifers sorted to another pen, until all heifers were randomized to treatment. Heifers that underwent the spay procedure received a caudal epidural injection of 3 cc of lidocaine (Lidocaine Injectable; Aspen Veterinary Resources) and 270 mg of meloxicam (Meloxicam tablets; Unichem Laboratories LTD). Spayed heifers were monitored daily for 5 d following the procedure, with antimicrobial and anti-inflammatory drugs administered when deemed necessary according to commercial feedyard treatment protocol. Heifers were implanted with 100 mg trenbolone acetate (TBA) and 14 mg estradiol benzoate (EB; Synovex Choice; Zoetis) at trial initiation, followed by 200 mg TBA and 28 mg EB (Synovex Plus; Zoetis). Initial and interim body weight (BW) was calculated using a 4% shrink. Deads and removals totaled 5.0 % for spayed heifers and 4.0 % for intact heifers as a percentage of total heifers enrolled on each treatment at experiment initiation. All heifers were on feed for approximately 212 d and were fed a common finishing ration (Table 1) for approximately 159 d. Heifers received lubabegron (Experior, Elanco Animal Health) for the final 42 d on feed with a 4 d voluntary withdrawal period prior to shipment. Heifers were harvested at a commercial abattoir and individual animal identification, hot carcass weight (HCW), and prevalence of liver abscess were recorded. Marbling score, ribeye area (REA), and 12th rib fat thickness were recorded following a 48-h chill, and USDA Yield Grade was calculated. Performance and carcass data were analyzed using the MIXED procedure of SAS (version 9.4, Institute SAS Inc., Cary, NC), with pen as the experimental unit and block as a fixed effect. Liver abscess prevalence was analyzed as a binomial distribution using the PROC GLIMMIX procedure of SAS. Significance was considered at α ≤ 0.05 and a tendency was considered at 0.05 < α ≤ 0.10. Initial BW was used as a covariate for carcass-adjusted final BW and HCW due to a difference in initial BW at experiment initiation.

Results 

Feedlot Performance

Since intact heifers averaged 9 lb heavier than spayed heifers at time of initial processing, initial BW was utilized as a covariate and the covariate-adjusted data are reported for performance and carcass outcomes where the covariate was significant (P ≤ 0.05; Table 2). The two variables impacted when initial BW was a covariate were HCW and carcass-adjusted final BW. When initial BW was used as a covariate, intact heifers had a 17 lb heavier final BW (P = 0.05). When the data were not adjusted for initial BW, there was a tendency for intact heifers to have increased carcass-adjusted final BW (P = 0.09). Similarly, when initial BW was used as a covariate, intact heifers had an 11 lb HCW advantage over spayed heifers (P = 0.05). When the data were not adjusted, there was a tendency for intact heifers to have greater HCW (P = 0.08). Intact heifers tended to have an increased DMI (P = 0.09) and had a 2.95% increase in ADG compared to spayed heifers (P = 0.02). Since DMI and ADG were greater for intact heifers, there was no difference in feed conversion between intact and spayed heifers (P = 0.82).

Carcass Characteristics

There was no difference in REA or marbling score between intact and spayed heifers (P > 0.16). Back fat thickness was 0.08 inches thicker in intact heifers relative to spayed heifers (P = 0.02) and USDA calculated Yield Grade was greater for intact heifers (P = 0.05), with intact heifers averaging a USDA YG of 3.15 and spayed heifers averaging 2.94. Liver abscess prevalence was not different between spayed and intact heifers (P = 0.58) and occurred in approximately 34% of the heifers. 

Conclusions

Spaying heifers upon arrival into the feedyard resulted in lower ADG and tended to suppress DMI compared to intact heifers but did not impact feed conversion. When initial BW was used as a covariate, final BW and HCW were 17 and 11 lb heavier, respectively, for intact heifers compared to spayed heifers. There was no difference in REA or marbling score between the two treatments; however, intact heifers had greater 12th rib fat thickness and USDA YG. These data would suggest that intact heifers reach a fat thickness endpoint earlier than spayed heifers and that it may be beneficial to increase DOF for spayed heifers to reach a similar fat endpoint and degree of finish. Further research is necessary to evaluate economic implications and management benefits for feeding spayed heifers. 

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