Factors affecting pregnancy rate in Holstein-Friesian cattle mated during summer in a tropical upland environment.

The relationships between some environmental factors and reproductive rates of dairy cattle during summer in a tropical upland area were studied using records for Holstein-Friesian milking cows and non-lactating growing heifers maintained at the Kairi Research Station on the Atherton Tableland in north Queensland. The Station is situated in a tropical upland environment with warm, wet summers and dry, cool winters. Data were collected over a 12-year-period (1977 to 1988) for cows grazing nitrogen (N)-fertilised grass pastures and receiving either no supplement or about 3.5 kg molasses/cow/day. Cattle were mated over a 10-week period starting from mid-January to early February, with 32 to 36 cows and 5 to 42 heifers used in each year. There was considerable variation among years in the pregnancy rate after 3 inseminations, with a low of 39% and a high of 94% pregnancy. This variation from year to year was reduced for cows grazing pastures receiving 300 kg N/ha/yr after molasses feeding commenced in 1984-85. Pregnancy rates after molasses introduction were higher for cows grazing pastures receiving 300 compared with 100 kg N/ha/yr. Pregnancy rate in cows and heifers was reduced from 80 to 55% as mean daily maximum temperature increased from 26 degrees C to 27.5 degrees C. At mean maximum temperatures above 27 degrees C for cows and 27.6 degrees C for heifers pregnancy rates to 3 inseminations were consistently below 60%. Much of the year to year variation in pregnancy rate was related to rainfall during the mating period.(ABSTRACT TRUNCATED AT 250 WORDS)