Abstract

Three experiments were designed to evaluate the effect of the duration of treatment with progesterone (P4) vaginal devices (1 g of P4, Triu-B; Biogénesis, Buenos Aires, Argentina) on pregnancy rates in crossbred zebu cattle artificially inseminated at a fixed time (FTAI). In Exp. 1, 371 non-lactating multiparous crossbred zebu cows with a body condition score (BCS) between 2.0 and 3.0 out of 5 were randomly allocated to one of nine treatment groups in a 3 × 3 factorial design. On Day 0, all cows received 2 mg of estradiol benzoate (EB, Bioestrogen; Biogénesis) i.m. and were divided into groups to receive a new Triu-B device, a once-previously-used Triu-B (2nd-use), or a Triu-B that was previously used twice (3rd-use) and in which three silicone rings (2 cm in length) impregnated with 100 mg P4 each were threaded through the wings of the device. Triu-B devices are currently approved for re-use up to three times. Cows were further subdivided to have their Triu-B devices removed on Days 7, 8, or 9. PGF (150 ¼g D(+)cloprostenol, Enzaprost-DC; Biogénesis) was administered i.m. at the time of Triu-B removal, followed by 1 mg EB 24 h later; all cows were FTAI 52 to 56 h after Triu-B removal. In Exp. 2, 417 crossbred zebu heifers, 20 to 26 months of age and BCS between 2.5 and 3.5, were treated similarly to the cows in Exp. 1, except that only new or 3rd-use Triu-B were used. In Exp. 3, 836 crossbred zebu heifers, similar to those used in Exp. 2, were also treated similarly to the cows in Exp. 1, but in this case, Triu-B devices were inserted for 8, 9, or 10 days. Pregnancy rates were determined by ultrasonography 50 to 60 days after FTAI and were analyzed by logistic regression. In Exp. 1, there were no significant effects of duration of treatment [7 days: 69/137 (50.4%), 8 days: 68/139 (48.9%), or 9 days: 41/95 (43.2%); P = 0.28], or type of Triu-B used (P = 0.91; new Triu-B: 47.7%, 2nd-use: 48.9%, or 3rd-use: 47.2%). In Exp. 2, there were no significant effects of duration of treatment [7 days: 76/139 (54.7%), 8 days: 63/143 (44.1%), or 9 days: 73/135 (54.1%); P = 0.98] or type of Triu-B used (new Triu-B: 48.6% and 3rd-use Triu-B: 53.2%; P = 0.33). In Exp. 3, there was an interaction between duration of treatment and pregnancy rates (P < 0.05). Lower pregnancy rates were obtained in 9- and 10-day treatments than in 8-day treatments, but only when previously used Triu-B devices were used. Pregnancy rates were: new Triu-B for 8 days: 42/89 (47.2%), 9 days: 37/90 (41.1%), and 10 days: 41/91 (45.1%); 2nd-use Triu-B for 8 days: 51/103 (49.5%), 9 days: 39/104 (37.5%), and 10 days: 39/109 (35.8%); 3rd-use Triu-B for 8 days: 52/90 (57.4%), 9 days: 31/90 (34.4%), and 10 days: 25/70 (35.7%); P < 0.01. Results demonstrate that treatment of crossbred zebu cattle with P4 devices for 7 to 10 days results in comparable pregnancy rates when new Triu-B devices are used. However, pregnancy rates decreased when previously used Triu-B devices were inserted for more than 8 days.