Associations between air emissions from sour gas processing plants and indices of cow retainment and survival in dairy herds in Alberta

Abstract

This paper describes the results of an investigation into the effects of air emissions from sour gas processing plants on indices of retainment or survival of adult female dairy cattle on farms in Alberta; namely, the productive lifespan of individual animals, and annual herd-level risks for culling and mortality. Using a geographical information system, 2 dispersion models--1 simple and 1 complex--were used to assess historical exposures to sour gas emissions at 1382 dairy farm sites from 1985 through to 1994. Multivariable survival models, adjusting for the dependence of survival responses within a herd over time, as well as potential confounding variables, were utilized to determine associations between sour gas exposure estimates and the time from the first calving date to either death or culling of 150210 dairy cows. Generalized linear models were used to model the relationship between herd-level risks for culling and mortality and levels of sour gas exposure. No significant (P < 0.05) associations were found with the time to culling (n = 70052). However, both dispersion model exposure estimates were significantly associated (P < 0.05) with a decreased hazard for mortality; that is, in cases where cattle had died on-farm (n = 8743). There were no significant associations (P > 0.05) between herd culling risks and the 2 dispersion model exposure estimates. There was no measurable impact of plant emissions on the annual herd risk of mortality.

Figure 1. Frequency distribution of cows culled in lactation lifespan (months from first calving date until culling date) in Alberta dairy herds (n = 70 052 cows culled). The clustered vertical bars represent stratification at the median herd PD exposure level (0.98 × 10⁻¹ μg/m³ of SO₂) for the at-risk period.

Figure 2. Frequency distribution of cow fatality in lactation lifespan (months from first calving date until death) in Alberta dairy herds (n = 8743 cows died). The clustered vertical bars represent stratification at the median herd PD exposure level (0.91 × 10⁻¹ μg/m³ of SO₂) for the at-risk period.

Figure 3. Frequency distribution of annual culling risk in Alberta dairy herds (n = 7810 annual herd reports). The clustered vertical bars represent stratification at the median herd PD exposure level (1.0 × 10⁻¹ μg/m³ of SO₂) for the at-risk period.

Figure 4. Frequency distribution of annual mortality risk in Alberta dairy herds (n = 7810 annual herd reports). The clustered vertical bars represent stratification at the median herd PD exposure level (1.0 × 10⁻¹ μg/m³ of SO₂) for the at-risk period.

Figure 5. Survival curve of time to culling (months from first calving date until culling date) in Alberta dairy herds (n = 70 052 cows culled). The clustered vertical bars represent stratification at the median herd PD exposure level (0.98 × 10⁻¹ μg/m³ of SO₂) for the at-risk period.

Figure 6. Survival curve of time to death (months from first calving date until death) in Alberta dairy herds (n = 8743 cows died). The clustered vertical bars represent stratification at the median herd PD exposure level (0.91 × 10⁻¹ μg/m³ of SO₂) for the at-risk period.