Factors affecting pregnancy rate in Holstein-Friesian cattle mated during summer in a tropical upland environment |
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| Authors: | WN ORR RT COWAN TM DAVISON |
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| Institution: | Queensland Department of Primary Industries, Mutdapilly Research Station, M S 825, Ipswich, Queensland 4305 |
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| Abstract: | SUMMARY 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-Frieslan milking cows and non-lactating growing helfers maintained at the Kalri 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°C to 27.5°C. At mean maximum temperatures above 27°C for cows and 27.6°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. There was a positive association between pregnancy rate and rainfall 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 intense humidity in very wet years causing heat stress in cows and associated problems such as feet infections, or a deterioration in pasture quality. We conclude that much of the variation in pregnancy rate during summer may have been related to heat stress and the level of feed supply in the form of pasture and supplements. |
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