Associations between oil- and gas-well sites, processing facilities, flaring, and beef cattle reproduction and calf mortality in western Canada

From the fall of 1992 through calving 1996, detailed cow breeding outcome r ecords were maintained actively for seven large cow-calf herds in western C anada. The numbers of mature females in the study for the breeding seasons be.-inning in 1992, 1993, 1994, and 1995 were 1122, 1177, 1251, and 1236, r espectively. Outcomes included pregnancy status, calving interval, and the occurrence of twins, abortions, stillbirths, and neonatal mortality. Inform ation 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 age ncy. Each flaring site then was classified as sour or sweet based on the pr esence or absence of hydrogen sulfide in the flared gas. A detailed invento ry was prepared itemizing the type and number of facilities within 1.6 kin (I 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 determ ined for each month of the study. Appropriate risk periods where specified for each outcome and a cumulative exposure calculated for each breeding fem ale (using detailed individual-animal records of cow movements between past ures and herd-management groups). Generalized estimating equations were use d to evaluate the association between exposure and outcome and to adjust fo r potential confounders and clustering of binomial outcomes within herd. Increased risk of non-pregnancy was sometimes associated with exposure to o ne or more of the following facility types: sour-gas flaring battery facili ties, all battery-flaring sites, active gas wells, and larger field facilit ies. The associations were not, however, consistent among years or even amo ng risk periods for the same year. Facility proximity and flaring were not associated with increased abortion risk. Volume of flared sour gas from bat tery 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 incre ased productivity also were found (most of which involved either oil wells or all well sites). (C) 2001 Elsevier Science B.V. All rights reserved.