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Are herbicides affecting the food supply of moose and deer?

Category: Environment and Wildlife

The woody browse that deer like to eat is reduced on sites treated with herbicides, such as glyphosate. However, deer and moose are well adapted to locate and feed on the most nutritious food sources possible. A large scale study is underway to better understand how deer are using our forests, how suitable our forests are to support deer in summer and winter months, and the effects of glyphosate on availability of food for deer.

Both deer and moose consume significant amounts of non-woody foods during the growing season, including foliage, shoots and twigs from deciduous trees and shrubs (e.g. maples, pin cherry, birch, dogwood). These food sources are commonly referred to as "woody browse". These same species are often the target of conifer release treatments and their biomass is commonly reduced 50-70% by successful conifer release treatments. However, herbaceous species contribute significantly to deer diets and are eaten by moose, and they commonly remain, or quickly “rebound,” in treated areas. Moose, which feed more on woody browse, do reduce their use of treated areas for five to seven years after treatment. However, that is only important in the short-term and then only if the landscape is dominated by young released stands. When the landscape is not dominated by young released stands these animals simply find and use better habitat found in other parts of their range.

Large mammals range over large areas of forested landscapes and hence potential effects must be considered in relation to the dynamic mosaic of forest conditions that exist on that landscape (Lautenschlager and Sullivan 2002). On an annual basis, based on a national average, less than 1/3 of clear cut harvested areas are treated with glyphosate-based herbicides. There is no question that the abundance of deciduous woody browse species will be at least temporarily reduced on these treated areas. However, as many of these plant species regenerate from the soil seed bank, reduced browse effect, even on treated sites, will be transient as evidenced in the studies noted above. Moreover, copious amounts of browse material will typically occur on the 2/3rds of the forest cutover lands which are not treated with herbicide.

A specific area of concern is the potential for reduced winter browse availability for deer whose winter foraging range area may be limited by snow depth. The potential for such effects would depend on both the proportion of the foraging area around deer yards that is treated with glyphosate-based herbicides and the actual reduction in winter browse species typically used as winter forage by deer on those areas. Considering information from New Brunswick as an example, on the broader landscape, much of the clearcut area (66% as a long term average estimate on crown lands) is left to regenerate without the aide of glyphosate treatment. Assuming this is true on cut over sites close to deer wintering areas as well, suggests that 66% of the open areas in which deer might browse in winter would be unaffected by glyphosate-based herbicide treatment. Potential effects would also be further ameliorated given that:

  • Deer habitat management in crown lands of New Brunswick is focused on identification of Deer Wintering Areas (DWAs) and maintenance of suitable winter habitat in these areas. This includes harvesting restrictions which are limited to single entry partial harvests. Under such harvest systems, herbicide treatments are not used and hence there can be no effect of herbicide treatments within the DWA per se.
  • Under increasingly severe winter conditions and snow depths, deer tend to remain closer to mature conifer or conifer dominated mixed-wood stands that intercept snowfall and thus facilitate localized movement (Morrison et al. 2003; Sabine et al 2001). While deer are known to browse on a wide variety of species in winter (Morrison et al. 2002) including in particular red and striped maple, under restricted movement scenarios cedar and balsam fir are important food sources (Morrison et al. 2002, Telfer 1972, Mautz et al. 1976, Gray and Servello 1995, Ditchkoff and Servello 1998) and neither of these species are influenced by glyphosate-based herbicide treatments (e.g. Gagne et al. 1999).

As noted by Morrison et al. (2002), by identifying browse species selected by deer and by understanding their associations with forest stand types, managers will be better able to meet the dual objectives of providing appropriate conifer dominated cover and an adequate supply of browse to support deer through winter. In this context, although it is quite unlikely that glyphosate-based herbicide use is a significant factor influencing deer winter browse availability, detailed geographic analysis of the proportion of deer foraging areas actually receiving glyphosate-based herbicide treatments would provide useful information furthering our understanding on this specific aspect. Broader assessment of deer population viability must consider a number of other potentially influencing factors such as natural predation (e.g. by coyotes, bears and wolves) facilitated predation through predator use of of roads and trails, hunting pressure and poaching, disease and deleterious effects of artificial feeding.

Lautenschlager, R.A., and Sullivan, T.P. Effects of herbicide treatments on biotic components in regenerating northern forests. For. Chron. 2002; 78: 1–37.

Gagné N, Belanger L, Hout J. Comparative response of small mammals, vegetation and food sources to natural regeneration and conifer release in boreal balsam fir stands of Quebec. Can. J. For. Res. 1999; 29:1128-1140

National Forestry Database (NFD). Pest control products use and silviculture sections; accessed summer 2015. Available electronically at: http://nfdp.ccfm.org/index_e.php

Morrison SF, Forbes GJ, Young SJ. Browse occurrence, biomass and use by white-tailed deer in a northern New Brunswick deer yard. 2002. Can. J. Forest. Res. 2002; 32:1518-1524

Morrison SF, Forbes GJ, Young SJ, Lusk, S. Within-yard habitat use by white-tailed deer at varying winter severity. Forest Ecolgy and Management. 2003; 172:173-182.

Telfer, ES. Forage yield and browse utilization on logged areas in New Brunswick. Can J. Forest Res. 1972; 2: 346-350.

Sabine DL, Ballard WB, Forbes G, Bowman J, Whitlaw H. Use of mixedwood stands by wintering white-tailed deer in southern New Brunswick. For. Chron. 2001; 77:-97-103.

Mautz WW, Silver H, Holter JB, Hayes HH, Urban WE Jr. Digestibility and related nutritional data for severn northern deer browse species. The Journal of Wildlife Management 1976; 40:630-638.

Ditchkoff SS, Servello FA. Litterfall: An overlooked food source for wintering white-tailed deer. The Journal of Wildlife Management 1998; 62: 250-255.

Gray PB, Servello FA. Energy intake relationships for white-tailed deer on winter browse diets. The Journal of Wildlife Management 1995; 50:147-152.