ESTRUS SYNCHRONIZATION EFFICIENCY OF PGF(2-ALPHA) INJECTION IN SHORTHORN-HEREFORD AND CROSSBRED CHAROLAIS CATTLE NOT HAVING EXHIBITED ESTRUS AT 4 OR 7 DAYS PRIOR TO TREATMENT

The objective of this study was to compare the estrus synchronization efficiency of PGF(2 alpha) in cattle (n=470) not having shown estrus f or 4 (D4 treatment) or 7 d (D7 treatment) after onset of the breeding season. The physiological status of crossbred Charolais cows was studi ed, and their reproductive performance was compared to that of Shortho m-Hereford cows. The percentage of cows in estrus during the 7 d prior to PGF(2 alpha) treatment was superior (P<0.01) to that observed duri ng the 4 d preceding PGF(2 alpha). The daily rates of estrus were simi lar during the 2 periods. For both the Shorthom-Hereford (87.8 vs 74.7 %; P<0.03) and crossbred Charolais (87.8 vs 66.3%; P<0.005) females, t he estrus synchronization rate during the 5 d post PGF(2 alpha), was h igher in treatment D7 than in the D4 treatment. Therefore, for both Sh orthorn-Hereford (92.8 vs 81.4%) and crossbred Charolais (93.1 vs 75.0 %) the D7 program permitted insemination in 12 d (before and after PGF (2 alpha)) of a higher percentage (P<0.01) of females than the D4 prog ram did in 9 d. The intervals PGF(2 alpha) to estrus and their varianc es were similar for both treatments. The conception rate of Shorthorn- Hereford was the same whether they were inseminated after spontaneous (73.4%) or PGF(2 alpha)-induced estrus (D4 treatment: 78.5%; D7 treatm ent: 72.2%). However, crossbred Charolais cows inseminated after a PGF (2 alpha)-induced estrus in the D4 treatment had a fertility rate (50. 9%) slightly inferior (P<0.07) to that of cows inseminated after spont aneous estrus (70.1%) or after PGF(2 alpha)-induced estrus in the D7 t reatment(67.4%). The reproductive performances of multiparous Shorthom -Hereford and crossbred Charolais were similar in treatment D7; howeve r, in the D4 treatment, the conception rate of Shorthorn-Hereford was higher than that of crossbred Charolais (81.0 vs 42.9%; P<0.002). Prim iparous crossbred Charolais in the D4 treatment had a slightly lower ( P<0.10) synchronization rate (48%) than nulliparous (71%) and multipar ous crossbred Charolais (78%). In contrast, the reproductive performan ces of nulliparous, primiparous and multiparous crossbred Charolais we re similar in the D7 treatment. These results indicate that the effici ency of PGF(2 alpha) to synchronize estrus is greater when the estrus detection period increases from 4 to 7 d before PGF(2 alpha). Only in the D7 treatment was the fertility rate of crossbred Charolais similar to that of Shorthorn-Hereford.