Modeling the Impact of Grazing on Water and Nutrient Cycling

Background

The effects of nutrient run off from Canadian pastures is a concern for producers, lawmakers, and the public, but little work has been done in a Canadian-specific context to assess this problem. Water quality issues in watersheds surrounding pastureland arise from a variety of sources including nearby pastures but also other farming practices, other livestock, households or even towns, cities, or factories. Attributing water quality issues to a single source fails to consider complex interactions among water movement, land management, and nutrient sources. Models developed for other locations and management conditions don’t account for some of the unique complexities of Canada’s pastures including beef production systems, vegetation, and cold-regions hydrological processes (e.g., blowing snow, freeze/thaw cycles, frozen ground).

Objectives

• To develop a Canadian-specific model to evaluate the relative contributions from cattle production, vegetation and soil to nutrient export from pastures taking into consideration pasture conditions, beef production management practices, climate and hydrology; and to use this model to estimate nitrogen and phosphorus export through surface runoff from pasture landscapes.

What they Did

These researchers built an integrated model to estimate N and P export in surface run off under different pasture management and weather conditions. The integrated model was comprised of animal, pasture, soil, hydrology and water quality components using western-Canada specific algorithms to predict nutrient run off. The model was based on weather data from the past 15 years for three watersheds across the prairies.

What They Learned

Analysis of the modelling simulations indicated that, as expected, pasture yield was driven by precipitation. However, in Manitoba and Saskatchewan, productivity was more influenced by previous-fall precipitation, while in the watershed at the foothills of the Rockies in Alberta, growing season precipitation had a more prominent effect. Despite fluctuations in pasture productivity due to precipitation, the manure nutrient content remained relatively constant during the simulation period between 2005 and 2019. Simulation of stream discharge was more challenging due to lack of data to fine tune the models. However, simulations were able to capture the dynamics in snow accumulation and stream discharge in all three watersheds. The fine-tuned model was then used to estimate the relative contribution of soil, vegetation and manure to nutrient export using complex statistical methods. The results of this analysis showed that soil, vegetation and manure consistently ranked as the first, second, and third most important source for total phosphorus, respectively, in all three watersheds (Fig. 1). In terms of nitrogen export, vegetation ranked as the primary source and manure was the secondary source in two (AB, SK) out of three watersheds (Fig. 1). The contribution from manure to the total nutrient export ranged between 7.9% and 20.2% for total phosphorus, and 6.9% and 11.8% for total nitrogen.

What it Means

When looking at policy and practices to reduce phosphorus and nitrogen exports into watersheds should focus on soil and vegetation, although manure does still play a role. The differences seen across the different locations suggests there is no one size fits all solution for preventing nitrogen and phosphorus export to watersheds and potential management solutions should be regionally specific.