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 stu died using records for Holstein-Friesian milking cows and non-lactatin g growing heifers maintained at the Kairi Research Station on the Athe rton Tableland in north Queensland. The Station is situated In a tropi cal upland environment with warm, wet summers and dry, cool winters. D ata were collected over a 12-year-period (1977 to 1988) for cows grazi ng nitrogen (N)-fertilised grass pastures and receiving either no supp lement or about 3.5 kg molasses/cow/day. Cattle were mated over a 10-w eek 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 var iation 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 y ear was reduced for cows grazing pastures receiving 300 kg N/ha/yr aft er molasses feeding commenced in 1984-85. Pregnancy rates after molass es Introduction were higher for cows grazing pastures receiving 300 co mpared with 100 kg N/ha/yr. Pregnancy rate in cows and heifers was red uced from 80 to 55% as mean dally maximum temperature increased from 2 6-degrees-C to 27.5-degrees-C. At mean maximum temperatures above 27-d egrees-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 va riation in pregnancy rate was related to rainfall during the mating pe riod. There was a positive association between pregnancy rate and rain fall up to 1000 mm, and this was associated with differences In growth rate of pasture. In years of very high rainfall, above 1500 mm, there was a depression in pregnancy rate, which may be related to the Inten se humidity in very wet years causing heat stress in cows and associat ed problems such as feet infections, or a deterioration in pasture qua lity. We conclude that much of the variation in pregnancy rate during summer may have been related to heat stress and the level of feed supp ly in the form of pasture and supplements.