Miller, A. J., Bork, E. W., Hall, L. M. and Summers, B. 2015. Long-term forage dynamics in pastures sprayed with residual broadleaf herbicide: A test of legume recovery. Can. J. Plant Sci. 95: 43–53. Legumes such as alfalfa (Medicago sativa L.) and white clover (Trifolium repens L.) are important components of northern temperate pastures where they increase forage productivity and quality, but are susceptible to decline when exposed to broadleaf herbicides. Little is known about the long-term sward responses following herbicide use in northern temperate pastures, including the recovery of legumes and their subsequent contribution to forage production. We established five field sites over 2 yr to assess changes in grass, legume, total forage (grass+legume) and other forb biomass, as well as the recovery of a common weed, dandelion (Taraxacum officinale Weber), for up to 3 yr following a single application of two broad leaf herbicides with residual properties (aminopyralid and aminocyclopyrachlor). The importance of defoliation and the legume seed bank in facilitating legume biomass recovery were also evaluated with mowing and legume overseeding, respectively, in a strip/split-split design. Both herbicides had similar functional impacts on sward composition based on peak annual biomass, reducing legume biomass by an average of 71–100% across the 3 yr, equivalent to 63.4–22.6 g m−2 from year 1 to year 3, respectively. Although grass biomass did not change significantly with herbicides, net reductions in total forage were limited to 6.8% (28 g m−2) over the study, suggesting at least some ability of grasses to compensate for legume removal. Legume biomass was greater following overseeding and only in non-sprayed controls, but then decreased over time. Conversely, biomass of other forbs and cover of dandelion were lower shortly following herbicide application, only to reach levels similar to non-sprayed controls by the second growing season. Defoliation also influenced sward composition, favoring dandelion recovery following herbicide application. As both weed control and legume re-establishment are important objectives for livestock producers, the result of this study provides useful insight into the long-term impact of broad-leaf weed control on forage production in mixed swards of northern temperate pastures.

Bork, E. W. and Blonski, L. J. 2012. Short-term native grassland compositional responses following liquid hog manure application. Can. J. Plant Sci. 92: 55–65. Intensive livestock operations (ILOs) are becoming more common in remote regions of the Canadian prairies in an effort to reduce conflict with other land uses. This has led to ILOs situated where the typical sink for manure application, cultivated land, is not available, leading to growing interest in using native grasslands for manure disposal. Significant opposition exists to this practice, in part due to limited information available on the impact of manure application to native grassland diversity and species composition. We examined plant species composition changes over two growing seasons following varying rates (∼9.5, 19, 38, 75 and 150 kg ha−1 available N), methods (surface broadcast vs. coulter injected) and timing (fall vs. spring) of one-time liquid hog manure (LHM) application. Our results revealed divergent responses between study sites, with metrics of plant diversity declining in mixed prairie but increasing in fescue grassland with increases in manure application rate. Both communities also demonstrated minor changes in plant species composition, primarily in response to LHM rate and manure application method. Responsive plant species included a mix of native grasses and dicots, some of which may be regarded as undesirable (e.g., Artemisia frigida): however, most vegetation responses were temporary and disappeared by the second year. Although invasion of non-native plant species was not observed, suggesting these communities are tolerant of LHM application, changes in the reproductive effort of dominant grasses (negative in Hesperostipa curtiseta; positive in Pascopyrum smithii and Festuca hallii) suggest LHM application could alter long-term grassland composition. Thus, despite exhibiting resilience to one-time LHM application, future use of native grasslands for manure disposal should be done cautiously.

Bork, E. W., Lambert, B. D., Banerjee, S. and Blonski, L. J. 2013. Soil mineral nitrogen responses following liquid hog manure application to semiarid forage lands. Can. J. Soil Sci. 93: 369–378. Expansion of intensive livestock operations into semiarid regions lacking cultivated lands requires consideration of perennial forages for the efficient and sustainable disposal of manure. Little information exists on the nutrient dynamics associated with the application of manure to these areas. We examined soil mineral nitrogen (N) responses in four sites of the mixed-grass prairie, including two native grasslands and two introduced pastures, following different seasons (fall vs. spring), methods (dribble broadcast vs. coulter injected) and rates of liquid hog manure application (9.4, 18.8, 37.5, 75 and 150 kg ha−1 available N). Soil mineral N, including NO3-N, NH4-N and total mineral N, were assessed after application but prior to plant growth in April 1999, and again one growing season later in April 2000. Initial soil N did not vary with season of application. Soil mineral N predictably increased with application rate, but only in the upper soil profile (0–20 cm). Decreases in soil mineral N after one growing season in all treatments highlighted the ability of these perennial forage lands to immobilize large amounts of soil N, a significant portion of which was related to N uptake by vegetation. Compared with broadcast application, manure injection led to 35% greater soil mineral N (both NO3 and NH4) prior to plant growth, a response that persisted 1 yr later (+12%), thus demonstrating the N conserved benefits of manure incorporation. Overall, increases in soil mineral N within these forage lands appeared to be relatively short-term in nature, largely depleting over the course of a single growing season, suggesting one-time liquid hog manure application at low to moderate rates may be sustainable in this region of the mixed-grass prairie.