154 EFFECTS OF THE BREED AND SEASON ON THE FERTILITY OF TROPICAL DUAL PURPOSE COWS

Abstract

First service fertility is an important factor affecting the calving-conception interval and the number of services per conception. In the present study, first service fertility of 618 multiparous dual purpose cows was analyzed according to breed predominance (Bos indicus and Bos taurus) and service season: dry (December-March); sub-humid (April-August); and humid season (September-November). All cows were located in a sub-humid tropical forest area, characterized by 1950 mm/year of rainfall and a mean daily temperature of 28.3°C. Cows were milked and suckled twice a day. Estrus was detected twice a day with a minimum observation period of 30 min. All cows were serviced according to the international rule AM-PM with semen of one of two Brahman bulls (A and B). All cows received water and mineral salt ad libitum during the entire year. Additionally, during the dry season cows received an energy supplementation. Data were analyzed using the chi-square procedure of SAS (SAS/STAT User's Guide, 8.2 ed. Cary, NC: SAS Institute, Inc., 2001). No bull effects were observed on first service fertility (bull A: 37.70%, 105/305; and bull B: 40.26%, 126/313; P > 0.05). Breed predominance significantly affected the first service fertility, being higher in Bos indicus cows (47.48%) than in Bos taurus cows (32.78%; P < 0.05). The higher first service fertility of Bos indicus cows seen during the year was probably because Bos indicus cattle are more thermotolerant than Bos taurus cattle (Hansen 2004 Anim. Reprod. Sci. 82-83, 349-360). Moreover, season of service did not affect the first service fertility of Bos indicus cows (dry: 48.95%, 70/143; sub-humid: 50%, 33/66; and humid: 41.30%, 19/46; P > 0.05). While in Bos taurus cows, first service fertility was higher in cows serviced during the dry season (43.24%) in comparison with those serviced during the sub-humid (28.26%; P < 0.05) or humid season (23.58%; P < 0.05). No differences were observed between Bos taurus cows serviced in the sub-humid and the humid season (P > 0.05). First service fertility did not differ between Bos indicus and Bos taurus cows serviced during the dry season (P > 0.05), which is likely due to the lower relative humidity and extra energy supplementation. This may improve oocyte quality. In conclusion, fertility of Bos taurus cattle is more sensitive than that of Bos indicus cattle under tropical conditions; therefore, the implementation of a reproductive seasonality scheme is recommended to increase the reproductive efficiency of Bos taurus cattle.