Synovex-H®, a combination of testosterone propionate and oestradiol benzoate, was implanted on three occasions at about 90-day intervals into Hereford × Shorthorn (HS), Brahman (B) and BX(B × HS) heifers to try to increase live weights and hence pregnancy rates at first breeding. Half of both the implanted and nonimplanted heifers of each breed was treated every 3 weeks to control cattle ticks and gastro-intestinal helminths. Treatment to control these parasites increased live weights, the increase depending on the breed. Implanting with Synovex-H also increased growth, the magnitude of the increase depending on both breed and treatment to control parasites. When parasites were present in significant numbers in the environment, the implanted groups had higher tick and helminth burdens and, in consequence, their live weight gains in response to the implant were lower than those of the nonimplanted group. The reason for the increase in susceptibility to parasites of the implanted group was not sought. It is suggested that it is associated with the presence, either alone or in combination, of the androgen and oestrogen components of Synovex-H.
By the start of the 10-week breeding season, 290 days after the last dose of Synovex-H was implanted, previous advantages of the implanted groups in live weight had been eroded to the point where the differences between groups were no longer statistically significant. Subsequent fertility both as maidens and as first-calf heifers was generally significantly lower in the previously implanted groups of all breeds. Calves born to implanted heifers were not as heavy as those born to nonimplanted heifers both at weaning and 120 days after weaning. The particular regimen of use of Synovex-H followed in this study cannot be recommended if heifers are to be used for breeding.