Effect of heat stress and its interaction with other management and productive variables on pregnancy rate in dairy cows in Aguascalientes, Mexico

Abstract

THE OBJECTIVES OF THE STUDY WERE TO DETERMINE THE EFFECTS OF MONTH OF SERVICE AND THE TEMPERATURE- HUMIDITY INDEX ON THE DAY OF SERVICE AND ITS INTERACTIONS WITH OTHER MANAGEMENT AND PRODUCTIVE VARIABLES ON PREGNANCY RATE (PR) IN DAIRY COWS. THE STUDY WAS CARRIED OUT IN SEVEN DAIRY FARMS IN AGUASCALIENTES. A NEGATIVE RELATIONSHIP WAS OBSERVED BETWEEN THE AVERAGE TEMPERATURE-HUMIDITY INDEX (THI) AND PREGNANCY RATE (PR) (P < 0.01), WHERE WITH EACH UNIT OF INCREASE IN THI, PR WAS REDUCED BY 1.03% (P < 0.01). PR WAS LOWER FROM MAY TO SEPTEMBER IN RELATION TO THE OBSERVED FROM OCTOBER TO APRIL (P < 0.05). COWS WITH MILK PRODUCTION ABOVE 8 500 L HAD LESSER PR (P < 0.05) IN OCTOBER, NOVEMBER, DECEMBER AND JANUARY (P < 0.05). THE TEMPERATURE-HUMIDITY INDEX ON THE DAY OF SERVICE (THIS) AFFECTED PR IN ALL PRODUCTION GROUPS. HOWEVER, IN COWS PRODUCING MORE THAN 8 500 L, THE NEGATIVE EFFECT OF THIS WAS OBSERVED ONLY WHEN IT WAS ABOVE 75.9, IN COMPARISON WITH LESS PRODUCTIVE COWS THAT WERE NEGATIVELY AFFECTED FROM THIS = 74. THIS MAY HAVE OCCURRED BECAUSE OF THE LOW PR (< 35%) OF THE HIGHER PRODUCING COWS EVEN AT COMFORTABLE THIS CONDITIONS. AT THIS = 76 OR LARGER, THERE WERE NO DIFFERENCES IN PREGNANCY RATE (PR) DUE TO MILK PRODUCTION. PRIMIPAROUS COWS HAD MORE PR THAN MULTIPAROUS COWS (P < 0.01). PR OF COWS WAS NOT AFFECTED BY TREATMENT WITH SOMATOTROPIN (P > 0.05). COWS WITH THREE MILKINGS PER DAY AND HERDS WITH MORE THAN 1 000 COWS HAD LOWER PR (P < 0.01). THE INTERACTION OF THIS WITH THE NUMBER OF CALVINGS, USE OF SOMATOTROPIN OR THE NUMBER OF MILKINGS WAS NOT SIGNIFI CANT ON PR (P > 0.05). PR OF COWS WAS AFFECTED BY THE MONTH OF SERVICE AND THE TEMPERATURE-HUMIDITY INDEX. NEVERTHELESS COWS WITH THE HIGHEST PRODUCTION, THE EFFECT WAS GREATER AND IT WAS MAINTAINED LONGER.