Fallow Replacement in Dryland Wheat Cropping Systems to Increase Economic Returns and Improve Water Quality in Semi-Arid Central Montana.

Fallow replacement in dryland wheat cropping systems to increase economic returns and improve water quality in semi-arid central Montana

Groundwater nitrate (NO₃^-) concentrations above the drinking water standard of 10 mg nitrogen (N) L^-1 are common in central Montana's semi-arid Judith River Watershed (JRW), partly due to shallow soils (~30 to 100 cm to alluvial gravel layer). Dryland agriculture is prevalent in this part of the northern Great Plains (NGP) and may drive nitrate loss to groundwater, but little research has evaluated the effectiveness of alternative management practices (AMPs) intended to reduce nitrate leaching in the region. Additionally, likelihood of widespread adoption of AMPs is uncertain because their economic returns are largely unknown. We conducted a 2-yr study on JRW commercial farms, comparing nitrate leaching and net revenues for three AMPs—pea, controlled release urea, and split application—with grower standard practices (GSPs)—fallow, conventional urea, single broadcast urea. The AMPs were selected by farmers and other stakeholders based on their perceptions of likely adoption. Eight management interfaces were established on three landforms and each management practice was analyzed annually in duplicate. We used an N mass balance approach to calculate the amount of NO₃^- leached during each treatment, and standard economic techniques to estimate net revenue for each field. In 2013, wheat after pea leached less NO₃^- (20 +/- 2.5 kg N ha^-1) than wheat after fallow (56 +/- 3.6 kg N ha^-1), while alternative fertilizer treatments did not decrease nitrate leaching. Unlike the fertilizer treatments, pea reduced root zone water storage, likely decreasing deep percolation and soil nitrate pools, thereby limiting nitrate leaching. Net revenue was not different between any AMP and its corresponding GSP in 2013. In 2014, leaching was not different between each AMP and GSP; however, net revenue was higher for pea - winter wheat ($243 ha^-1 +/- $39 ha^-1) than fallow - winter wheat ($160 ha^-1 +/- $33 ha^-1). Alternative fertilizer treatments did not affect 2014 net revenues compared to standard practices. We conclude that replacing fallow with pea has more potential to reduce NO₃^- leaching and increase net revenue over multiple years than the two fertilizer practices.