Low detection of glyphosate in rivers following application in forestry

BACKGROUND

Glyphosate is the most commonly used herbicide in the world, and is used in agriculture, forestry, and urban settings. In regions with high glyphosate use, such as agricultural, glyphosate and its' major derivative aminomethylphosphonic acid (AMPA) are frequently detected in surface waters. In Canadian forestry glyphosate-based herbicides are used to control vegetation that competes with conifer trees and are applied one to two times during a rotation, leading to infrequent application to the same area. Forestry occurs over a large spatial extent, and the cumulative application in space can lead to a large percentage of the land base receiving an application through time. To assess the frequency and concentration of glyphosate and AMPA in surface waters of a region where forestry is the dominant use sector, we conducted three monitoring programs targeting: (i) immediately after application, (ii) after rainfall, and (iii) cumulative application over a large spatial extent.

RESULTS

Across all monitoring programs we collected 296 water samples between August and October from eight river systems over two years and detected glyphosate in one sample at 17 ppb.

CONCLUSION

Glyphosate is not likely present in surface waters during baseflow conditions as a result of applications in forestry. Lack of detection is likely because soil capacity to bind glyphosate remains high due to infrequent applications to the same area, and factors that limit sediment transport to surface waters such as buffers. Additional sampling is needed during other stream conditions, ideally spring freshet, to determine peak concentrations. © 2023 National Research Council Canada. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry. Reproduced with the permission of the Minister of Innovation, Science, and Economic Development.     

Long-term impact of a precision agriculture system on grain crop production

Research is lacking on the long-term impacts of field-scale precision agriculture practices on grain production. Following more than a decade (1993–2003) of yield and soil mapping and water quality assessment, a multi-faceted, ‘precision agriculture system’ (PAS) was implemented from 2004 to 2014 on a 36-ha field in central Missouri. The PAS targeted management practices that address crop production and environmental issues. It included no-till, cover crops, growing winter wheat (Triticum aestivum L.) instead of corn (Zea mays L.) for field areas where corn was not profitable, site-specific N for wheat and corn using canopy reflectance sensing, variable-rate P, K and lime using intensively grid-sampled data, and targeting of herbicides based on weed pressure. The PAS assessment was accomplished by comparing it to the previous decade of conventional, whole-field corn-soybean (Glycine max L.) mulch-tillage management. In the northern part of the field and compared to pre-PAS corn, relative grain yield of wheat in PAS was greatly improved and temporal yield variation was reduced on shallow topsoil, but relative grain yield was reduced on deep soil in the drainage channel. In the southern part of the field where corn remained in production, PAS did not lead to increased yield, but temporal yield variation was reduced. Across the whole field, soybean yield and temporal yield variation were only marginally influenced by PAS. Spatial yield variation of all three crops was not altered by PAS. Therefore, the greatest production advantage of a decade of precision agriculture was reduced temporal yield variation, which leads to greater yield stability and resilience to changing climate.     

Response of antelope bitterbrush to repeated prescribed burning in Central Oregon ponderosa pine forests

Antelope bitterbrush is a dominant shrub in many interior ponderosa pine forests in the western United States. How it responds to prescribed fire is not well understood, yet is of considerable concern to wildlife and fire managers alike given its importance as a browse species and as a ladder fuel in these fire-prone forests. We quantified bitterbrush cover, density, and biomass in response to repeated burning in thinned ponderosa pine forests. Low- to moderate-intensity spring burning killed the majority of bitterbrush plants on replicate plots. Moderately rapid recovery of bitterbrush density and cover resulted from seedling recruitment plus limited basal sprouting. Repeated burning after 11 years impeded the recovery of the bitterbrush community. Post-fire seed germination following the repeated burns was 3–14-fold lower compared to the germination rate after the initial burns, while basal sprouting remained fairly minor. After 15 years, bitterbrush cover was 75–92% lower on repeated-burned compared to unburned plots. Only where localized tree mortality resulted in an open stand was bitterbrush recovery robust. By controlling bitterbrush abundance, repeated burning eliminated the potential for wildfire spread when simulated using a customized fire behavior model. The results suggest that repeated burning is a successful method to reduce the long-term fire risk imposed by bitterbrush as an understory ladder fuel in thinned pine stands. Balancing the need to limit fire risk yet provide adequate bitterbrush habitat for wildlife browse will likely require a mosaic pattern of burning at the landscape scale or a burning frequency well beyond 11 years to allow a bitterbrush seed crop to develop.     

Long-term effects of post-harvest stump removal and N-fertilization on understory vegetation in Western USA forests

Intensive forest management practices often disturb understory vegetation, and the recovery of these plant communities may depend on the type and severity of the disturbance. We examined the effects of stump removal and N-fertilization on understory plant communities and functional group (shrubs, graminoids, forbs, and introduced species) cover and diversity at five study areas in the Pacific Northwest of North America 24–28 years after treatment. Treatments at each study area included stumped and non-stumped controls as well as four levels of broadcast ammonium nitrate (0, 336, 672, and 1345 kg N ha⁻¹) in all combinations. Stumping had significant effects on community composition at all sites, and several plant species were associated (p < 0.05) with either controls or stumped plots. Diversity of graminoids, forbs and introduced species increased in stumped areas region wide. Stumping reduced cover and diversity of shrubs at some sites. Cover of graminoids and forbs also increased in stumped plots at some study areas. Forbs like Viola sempervirens were often indicators of stump removal while shrubs such as Acer circinatum tended to be associated with non-stumped plots. N-fertilization affected community composition at only one study area, and had no effects on cover or richness of functional groups. Stump removal has lasting impacts on plant communities and may make them more vulnerable to colonization by introduced species.