Effects of in ovo herbicide exposure in newly hatched domestic chickens (Gallus gallus) and ducks (Anas platyrhynchos)
Stoddart, Reagen A
Agriculture is a valuable economic resource in western Canada, but for decades farmers have focused on intensive production practices while ignoring the long-term health and maintenance of the land. In recent years, the use of conservation agricultural techniques has been encouraged in an effort to conserve prairie landscape while sustaining cropland productivity. Sustainable agricultural practices that promote soil and water conservation and benefit wildlife and prairie biodiversity include conservation tillage and planting of winter cereal crops. Many species of wild birds nest in the ground cover provided by minimum tillage and fall seeded cropland in the spring. Although habitat quality in conservation areas is superior for birds, there is potential for eggs of ground nesting birds to be exposed to herbicides during spring weed control operations. Herbicides commonly used on the prairies to control weed growth in conservational systems include 2,4-D and Buctril-M®. Since the subtlethal effects of exposure to these herbicides may include DNA damage and immunomodulation, the overall goal of this study was to assess whether in ovo exposure to the herbicides 2,4-D and Buctril-M® adversely affects genetic material and/or immune system function in newly hatched domestic chickens (Gallus gallus) and ducks (Anas platyrhynchos), as surrogates for wild bird species.Study design attempted to reproduce actual field exposures by use of an agricultural field spray simulator to apply formulated herbicides (as opposed to pure active ingredients) at recommended crop application rates. In three separate experiments, fertile chicken eggs were sprayed with 2,4-D ester formulation or with Buctril-M® formulation, and fertile duck eggs were sprayed with 2,4-D ester formulation, during either an early (embryonic day 6) or late (embryonic day 15 for chickens or embryonic day 21 for ducks) stage of incubation. Genotoxicity and immune system function were evaluated in the hatchlings as the main toxicological endpoints to assess potential subtle effects from herbicide exposure, but additional measures of general health and development were also evaluated. Two endpoints were used to assess subtle changes to genetic integrity. The comet assay was used to detect structural damage (strand breaks) in avian lymphocyte DNA, as an index of acute genotoxic effects. Flow cytometry was used to examine potential clastogenic effects of the herbicides, by determining if chromosomal changes resulted in variability in the DNA content of avian erythrocytes. Several endpoints were examined to evaluate potential exposure-induced effects on the immune system. Immunopathological assessment of chicks and ducklings included differential lymphocyte counts, as well as immune organ weights and histopathology. The cell-mediated and humoral immune responses in hatchlings were assessed using the delayed-type hypersensitivity test and measurement of systemic antibody production in response to immunization, respectively. Exposure of fertile chicken and duck eggs to Buctril-M® or 2,4-D had no effects on the biomarkers of genetic integrity in this study. Differences in herbicide treatment (high and low concentrations) and times of exposure (early and late incubation stages) did not translate into noticeable factor effects in final model analyses for any of the genotoxicity assay variables evaluated in newly hatched chickens exposed in ovo to 2,4-D. Similarly, comet assay outcomes in chicks exposed to Buctril-M® were not significantly associated with either herbicide treatment or time of exposure as fixed effect factors. Results of the comet assay using peripheral lymphocytes from ducklings provided evidence of potential primary genetic damage associated with the time of spray exposure in ovo. Comet tail DNA content was significantly associated (P = 0.0269) with exposure times, suggesting that ducks may be increasingly sensitive to spray exposure conditions at an early stage of embryological development. Effects of exposure timing were not attributable to herbicide treatment. Although 2,4-D exposure time was associated with DNA strand breakage in ducklings, there was no evidence of chromosomal damage. However, an association between the HPCV values (a measure of DNA content variability) and time of spray exposure was observed in the experiment where 21-day-old chickens were treated in ovo with Buctril-M®. The mean HPCV value for the early exposure group (E6) was significantly greater (P = 0.0210) than that of the group treated later in incubation (E15). However, Buctril-M® the concentration of herbicide did not have any influence on this outcome, and the reason for the difference between exposure times is uncertain, but may be attributed to stress associated with manipulations during spraying. An increase in HPCV, reflecting greater intercellular DNA variability, is indicative of increased incidence of chromosomal damage, which may be an effect of disturbance during early periods of incubation as a result of exposure conditions.Among the panel of immunotoxicity tests conducted to evaluate the effects of in ovo exposure to 2,4-D and Buctril-M® on the developing avian immune system, only heterophil/ lymphocyte (H/L) ratios and relative immune organ weights were significantly associated with either herbicide treatment or time of spray exposure in all three experiments. In 21-day-old chicks exposed in ovo to 2,4-D, relative bursa weight was associated with the different herbicide treatments (P = 0.0006). Relative bursa weights were significantly lower in chicks in the low dose group, while the opposite effect was observed in the high dose chicks, compared with the controls. It is unlikely that the observed decrease in bursa weight in the low dose group is causally related to herbicide exposure because a consistent dose-response effect was not observed, but this outcome may be explained by a compensatory immune response. The relative spleen weights of newly hatched chickens exposed in ovo to Buctril-M® exhibited a significant association with herbicide treatment (P = 0.0137). Relative spleen weights for birds in the low dose treatment groups were significantly different than both the control (P = 0.0179) and high dose groups (P = 0.0125). However, there was no significant difference between high dose and control groups, and this outcome reduces the likelihood of a causal relationship between spleen weight and herbicide exposure. In the parallel experiment involving in ovo exposure to 2,4-D to ducklings, relative bursa weight was associated with time of spray exposure (P = 0.0434). Ducklings that hatched from eggs exposed to spray on day 6 of incubation exhibited greater mean relative bursa weights than the birds exposed to spray at a later incubation stage (E21). This result implies that spray exposure during earlier stages of development may result in conditions which affect the humoral immune response, if increased bursal weight is associated with increased B lymphocyte and antibody production. In the same experiment, mean H/L ratios in peripheral blood samples from 21-day-old ducklings were significantly different between the groups treated with the high concentration of 2,4-D and water (control) (P = 0.0395). Although ratios from the birds in the low dose groups were not significantly different from the control groups, changes in H/L ratio values demonstrate a dose dependent relationship with increasing herbicide exposure.Residue analysis of chicken and duck eggs in this study measured transfer of herbicide through the shell and into the embryo 24 hours and up to 5 days (chickens only) after spraying. Mean 2,4-D residue concentrations were higher in both chicken and duck eggs from the high dose (10X) groups than in eggs exposed to the recommended field rate of herbicide application (1X). Embryo residue concentrations in the chicken eggs increased from the day following exposure to 5 days after spraying, in both low and high dose groups. This observation indicates that the risk of contaminant-induced adverse effects may continue to increase for at least several days after exposure, thereby influencing the concentration of herbicide to which the developing embryo is exposed.On the Canadian prairies, wild bird eggs are potentially to be exposed to 2,4-D and Buctril-M® during various stages of embryonic development. The present study examined effects of herbicide exposure at two distinct times during incubation, and demonstrated the potential for subtle impacts on genetic integrity and the immune system. Results indicate that spray exposure during earlier stages of organogenesis may cause more significant adverse effects. Given the possible harmful consequences of the observed changes on the long-term health of wild birds, further research is needed in order to better characterize the risks of in ovo agrochemical exposure in prairie ecosystems.
DegreeMaster of Science (M.Sc.)
CommitteeWayland, Mark; Smits, Judit; Blakley, Barry
Copyright DateDecember 2006
herbicide exposure in birds