PubMed2001
From the fall of 1992 through calving 1996, detailed cow breeding outcome records were maintained actively for seven large cow-calf herds in western Canada. The numbers of mature females in the study for the breeding seasons beginning in 1992, 1993, 1994, and 1995 were 1122, 1177, 1251, and 1236, respectively. Outcomes included pregnancy status, calving interval, and the occurrence of twins, abortions, stillbirths, and neonatal mortality. Information also was collected on other risk factors known to influence beef-herd health and productivity. Detailed maps of active and inactive oil and natural-gas sites, batteries, compressor stations and processing plants were verified. Records of flaring activity at each facility were obtained from the government regulatory agency. Each flaring site then was classified as sour or sweet based on the presence or absence of hydrogen sulfide in the flared gas. A detailed inventory was prepared itemizing the type and number of facilities within 1.6 km (1 mile) of the center of each quarter section used for pasture. The total volume of gas flared within 1.6 km of the center of each pasture was determined for each month of the study. Appropriate risk periods where specified for each outcome and a cumulative exposure calculated for each breeding female (using detailed individual-animal records of cow movements between pastures and herd-management groups). Generalized estimating equations were used to evaluate the association between exposure and outcome and to adjust for potential confounders and clustering of binomial outcomes within herd. Increased risk of non-pregnancy was sometimes associated with exposure to one or more of the following facility types: sour-gas flaring battery facilities, all battery-flaring sites, active gas wells, and larger field facilities. The associations were not, however, consistent among years or even among risk periods for the same year. Facility proximity and flaring were not associated with increased abortion risk. Volume of flared sour gas from battery sites was associated with increased risk of stillbirth. Finally, sour-gas flaring was associated with increased calf-mortality risk for the 1992-1993 calf crop. Several examples of associations between exposure and increased productivity also were found (most of which involved either oil wells or all well sites).