Technical Abstract: Approximately 10% of beef heifers are pregnant upon entering feedlots annually. This represents a significant problem to the beef industry because pregnant heifers have decreased feedlot performance, increased mortality, lower carcass quality and dressing percentage, and as a result are worth less to the beef industry. The increased mortality and poorer overall performance among heifers comprise the primary reasons why producers are paid less for heifer than steer calves. Surgically spaying heifers increases costs morbidity and mortality and is generally not feasible. Many feedlots examine heifers for pregnancy and abort pregnant heifers upon entering the feedlot, but this practice also increases costs, morbidity, and mortality, and the effectiveness of most abortive compounds is less than 100%. Feedlot heifers today are generally fed melengestrol acetate (MGA; a synthetic progestin) to prevent estrous activity while in the feedlot, but feeding MGA is not practical for stocker programs. We conclude that immunization against the tGnRH, oGnRH, and tGnRH/oGnRH fusion proteins produced anti-GnRH antibodies that suppressed estrous cycles in 83, 100 and 92% of heifers, respectively, without affecting feedlot or carcass performance. Implanting heifers with Synovex H improved ADG, REA, and yield grade. Protocols that yield greater than 95% estrous suppression may someday replace surgical spaying of heifers.