Long Term Cost-Benefit Analysis of Including Forages in Cropping Rotation

Background

Integrating forages into crop rotations is known to have many benefits, including weed suppression, increased productivity and various soil quality impacts, but the short-term loss of cropland often makes this seem cost inhibitive. There has been little research that looks at the long term agricultural and economic benefits of forage inclusion in different rotations.

The Breton Research Plots in west-central Alberta have been maintained in 9 different long-term crop rotations for almost 100 years. The data collected from these plots allows researchers to compare different long-term crop rotations and amendment treatments that differ from 1 to 8 years in duration and include variable amounts of perennial forage. Some of these plots date back as early as 1929 and were established on Gray Luvisolic (low organic matter) soils to address issues of chronic low fertility and productivity. Unlike many other long-term studies where forages are examined as a single treatment, forages are a significant part of at least 5 of the long-term rotations at Breton. These rotational treatments, with comprehensive agronomic and soil data dating back 40 years, provide a unique opportunity to evaluate the effects of forage inclusion on soil properties, including carbon storage and stability, as well as net present values based on historical inputs and outputs.

Objectives

• Assess carbon stock, accrual and stability, across the long-term cropping system treatments of the Breton plots.
• Evaluate soil microbial biomass and necromass, together with soil carbon respiration and mineralization, in relation to the long-term cropping treatments at Breton.
• Quantify root biomass and annual carbon enrichment during the growing season in relation to the long-term crop rotation treatments at Breton.
• Assess soil biomarkers associated with the long-term cropping treatments at Breton.
• Quantify the net present value (PV) of agronomic activities and carbon accrual among long-term treatments of the Breton plots containing variable forages.

What they DID

This research team resampled 9 different treatments across 36 replicate plots at Breton in 2021 to evaluate differences in soil carbon stock, stability, and associated biological soil indicators. In addition, this team combined the most recent data with long-term soil and agronomic data tracked from individual plots for the period 1980 through 2021 to conduct a cost-benefit analysis of these different cropping systems. The core treatments included varying complexities of agricultural systems (e.g., wheat-fallow, continuous grain, 5 and 8 year-long rotational systems, and continuous forage), which included contrasting soil amendments (none, conventional fertilization, or manure) and perennial forage inclusion (none, 2 in 5 yr, 3 in 8 yr, and continuous forage).

What they learned

Results of this study revealed a number of important findings:

First, it provided clear evidence for the size of the soil carbon stock within Gray Luvisolic soils at the Breton plots and the positive effects of specific agricultural management practices. Soil carbon stock was maximized and extended to a greater depth in the soil by an increasing use of perennial forages, relying on manure (livestock and/or green manure) as a soil amendment, and avoiding the use of fallow.

Second, soil carbon stability was generally greater in soils receiving these same practices, reflected by an increase in carbon (increasing perennials and manure) within the more protected mineral-associated organic matter pool, together with carbon in the fungal necromass pool.

Third, several mechanisms likely contribute to these increases in soil carbon stock and stability. These included an increased root standing biomass at peak growth within plots having continuous forage, and favorable annualized carbon increases within the soil (7-9% from July to October) in systems with continuous soil cover, either as continuous forage, continuous grain, or an 8-year agro-ecological rotation that included manure and forage.

Fourth, analysis of long-term soil carbon data from the Breton plots showed distinct increases in soil organic carbon over most, or all, of the last 40 yr, with the largest gains occurring in the continuous forage and 8-yr rotation systems.

Fifth, economic valuation revealed two important outcomes. In 2021, the value of standing soil carbon stocks was markedly greater, more than $5,000/ha higher, in the continuous forage and 8-yr rotation systems compared with the continuous grain (business-as-usual) system. Net present value analysis of annual historical input (cost) and output (return) data from the Breton plots indicated that the greatest annualized returns were consistently generated by the 8-year agro-ecological rotation, while the continuous forage system generally had positive returns that were more sensitive to carbon and commodity price fluctuations.

What it means

The current net present valuation of the different cropping systems examined varied widely, depending on the value of commodities produced (e.g., of forage) and whether carbon was included in the valuation, as well as the projected carbon pricing. The results provide compelling evidence for the importance of maintaining agricultural practices that support soil organic matter inputs to increase soil carbon stock and stability. Equally important, the study highlights the significant economic value of soil carbon in agricultural soils; a value expected to grow as the federally mandated carbon pricing increases. In turn, this work lays a foundation supporting the need for novel future land-use policy changes that reward producers for adopting practices that increase soil carbon stocks and stability, including continuous cropping, use of manure as a rotational amendment, and the partial or prolonged use of perennial forages.