Field Trial Assessing the Use of Sex-Sorted Semen in Beef Cattle Field Trial Assessing the Use of Sex-Sorted Semen in Beef Cattle

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Introduction
Sex-sorted semen utilization holds the potential to create a high percentage of either bull or heifer calves. Most of the research to date has investigated success rates utilizing "heifer" semen. The objective of this study was to evaluate the success of "bull" and "heifer" sex-sorted semen in a commercial cattle operation.

Experimental Procedures
This trial was conducted on the Odde Ranch in North Central South Dakota from the summer of 2019 through fall 2020. There was a total of 320 Angus and SimAngus cows and heifers from four separate groups utilized in this trial (Table 1). The groups were: yearling heifers housed in a dry lot system (Group 1, n = 101); young cows (nursing calves) on pasture (Group 2, n = 51); cows (nursing calves) on pasture (Group 3, n = 80); and cows (nursing calves) on pasture (Group 4, n = 88).
The yearling heifers and young cows were inseminated to a commercially available Angus sire (Sire A) sorted to contain >90% X-chromosome bearing sperm cells. The mature cows were inseminated to a commercially available Charolais sire (Sire B) sorted to contain >90% Y-chromosome bearing sperm cells. All semen was packaged at a concentration of 4 × 10 6 sperm cells per 0.25 mL dose.
The Group 1 yearling heifers were estrus synchronized using the melengestrol acetate plus prostaglandin F 2α (MGA-PGF 2α ) protocol. Heifers were fed MGA for 14 days at a concentration of 0.5 mg/head/day. After feeding MGA for 14 days, MGA was removed from the ration for 19 days. On day 19 after the final day of MGA feeding, heifers received a 5 mL injection of PGF 2α (Lutalyse; Zoetis, Madison, NJ). Estrus was synchronized in Groups 2, 3, and 4 using the 7-day CO-Synch plus CIDR protocol. Cows received a progesterone insert (CIDR; Zoetis, Madison, NJ) along with a 2 mL injection of a gonadotropin-releasing hormone (GnRH) analog (Gonadorelin; Cystorelin; Merial, Athens, GA) at the start of the protocol. After seven days, CIDRs were removed and cows received a 5 mL injection of prostaglandin F 2α (PGF 2α ; Lutalyse; Zoetis, Madison, NJ). Estrus detection aids (Estrotect, Rockway Inc., Spring Valley, WI) were applied at time of PGF 2α injection at all locations. Estrus was defined as >50% of the patch coating removed. Group 1 heifers were also visually observed for signs of estrus every four hours for five days following injection of PGF 2α .
Group 1 heifers were inseminated at 15 to 21 hours after the onset of estrus. The Group 2 young cow group was inseminated using a split-time artificial insemination (AI) system with those showing estrus by 70 hours post PGF 2α inseminated at 70 hours. Those with inactive patches were injected with GnRH and were inseminated at 94 hours post PGF 2α . Group 3 cows with active patches were inseminated at 70 hours post PGF 2α . Those with inactive patches were administered GnRH and inseminated at 82 hours post PGF 2α . Group 4 cows were inseminated at 70 hours post PGF 2α with cows with inactivated patches getting an injection of GnRH. Females were exposed to bulls approximately 5-7 days after AI for the remainder of the breeding season.
Pregnancy diagnosis was conducted approximately 65-90 days post insemination via transrectal ultrasonography (ReproScan XTC equipped with a 4.0 MHz 60 mm convex rectal probe; ReproScan, Winterset, IA). Fetal size was used to differentiate AI pregnancies from natural service pregnancies. Gender was determined at birth. Gender accuracy to sex-sorted semen for each sire was calculated at the end of the calving season for all AI pregnancies. Gender skew, defined as the number of the desired gender divided by the total in the group, was calculated for each group. All calves were weighed prior to weaning in the fall. An adjusted 200-day calf weight was calculated using the fall weight and an average birth weight of 80 lb using the following equations: Average daily gain (ADG) = [fall weight -standard birth weight (80 lb)]/days of age. Adjusted 200-day weight = (ADG × 200 days) + standard birth weight (80 lb).

Results and Discussions
The results of this trial are shown in Table 2. Group 1 yearling heifers had an observed estrus response of 95.1% 5-days post PGF 2α injection and AI pregnancy rate of 63.4%. The gender accuracy of the AI calves was 94.3% heifers with an overall gender skew of 77.7% for heifer calves. The pregnancy rate observed was acceptable for sex-sorted semen and higher than what is typically found in the literature (Thomas et al., 2017). This is likely attributed to the intensive heat detection conducted on the heifers and the increased fertility seen with heifers in general.
Group 2 young cows had an estrus response of 88.2% overall with an AI pregnancy rate of 47.1%. The gender accuracy of AI calves was 89.5% heifers with an overall gender skew being 76.1% heifer calves. Group 3 mature cows had an estrus response of 75% and an AI pregnancy rate of 46.3%. The gender accuracy of AI calves was 91.0% bulls resulting in a gender skew of 68.8% bull calves. Group 4 mature cows had an estrus response of 69.3% and an AI pregnancy rate of 40.2%. The gender accuracy of AI calves was 84.8% bulls resulting in a gender skew of 58.7% bull calves.
Results for Groups 2 and 3 are similar to what has been reported in the literature for sexed semen (Andersen et al., 2020;Thomas et al., 2019). Results for Group 4 are lower than some other reported studies, possibly due to the use of fixed-time AI instead of split-time AI.
Adjusted 200-day weights (Table 2) were approximately 100 lb heavier for Groups 3 and 4 compared to Groups 1 and 2. This is likely due to cows in Groups 3 and 4 being older and implanted, that calves in these groups were sired by high growth Charolais bulls, and that these calves have more heterosis (cows were Angus-Simmental).

Implications
These results show that sex-sorted semen has potential on beef cows and heifers. Steer calves are worth more than heifers, and this difference is increasing due to increased carcass weights. "Bull" sex-sorted semen in terminal sire programs appears to have significant commercial potential. Brand names appearing in this publication are for product identification purposes only. No endorsement is intended, nor is criticism implied of similar products not mentioned. Persons using such products assume responsibility for their use in accordance with current label directions of the manufacturer. Split-Time AI -cows that did not display estrus at 70 hours after PGF 2α received an injection of GnRH and were inseminated 12 hours later.

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Fixed-Time AI -all cows were inseminated by 70 hours after PGF 2α , and cows that did not show signs of estrus by 70 hours received an injection of GnRH. 6 Angus sire semen was sorted to contain >90% X-chromosome bearing sperm cells at a concentration of 4 × 10 6 per 0.25 mL straw. 7 Charolais sire semen was sorted to contain >90% Y-chromosome bearing sperm cells at a concentration of 4 × 10 6 per 0.25 mL straw.