Abstract
Leaching of nitrate (NO3-N) from manure-applied cropping systems can represent a substantial N-loss to the environment for dairy farms, particularly in fields with artificial subsurface drainage. In this on-farm study, we used a Before/After analysis to assess the effectiveness of summer fertigation with reduced manure rates (years 2010 – 2015) versus fall injection (2007 – 2009) of dairy slurry in terms of subsequent corn silage yield, corn N removal, soil NO3-N distribution, and NO3-N losses in subsurface tile drainage from a 65-ha field in Minnesota, USA. Yield was similar between periods (average of 18.8 Mg ha-1), but crop %N, N removal, and manurial N-use efficiency were 15, 12, and 126% greater during the fertigation than injection period. Fertigation reduced spring soil NO3-N concentrations to 60-cm depth by an average of 53% relative to injection, except in the 15 to 30 cm increment, where no difference was found. Similarly, fall soil NO3-N concentrations from 30 to 90 cm were 48% lower, on average, under fertigation than injection. Weekly flow-weighted mean NO3-N concentration in tile drainage was lower during fertigation (47.7 mg L-1) than injection (56.8 mg L-1), although mean weekly drainage depth was greater during fertigation (2.3 versus 1.1 mm). This resulted in similar weekly loads between periods (mean of 0.96 kg NO3-N ha-1). For non-snowmelt flow, relationships between drainage and NO3-N load showed log–log slopes of near 1.0 for injection and 0.97 for fertigation, indicating dilution of concentrations with increased flow during fertigation, but not during injection. Differing intercepts indicated a treatment effect of fertigation independent of flow effects, and corresponded to loads of 5.9 kg NO3-N ha-1 for injection and 4.7 kg NO3-N ha-1 for fertigation, a reduction of 20% at a 10 mm weekly flow depth. The magnitude of the reduction in load increased to 22% at a 25 mm weekly flow depth. Results suggest that summer fertigation with attendant reduction in application rate is a viable method for reducing drainage NO3-N losses without impacting yield of irrigated silage corn in the U.S. Midwest.
Highlights
The US dairy industry has changed dramatically in recent decades, with a trend toward geographic consolidation into fewer, large-scale confinement operations
We have demonstrated here that summer fertigation reduced NO3-N losses from an irrigated summer-annual cropping system in the US Midwest
This greater utilization, in turn, enables lower manure application rates and results in reduced N losses in drainage
Summary
The US dairy industry has changed dramatically in recent decades, with a trend toward geographic consolidation into fewer, large-scale confinement operations. By 2012, nearly 50% of all dairy cows in the US were on farms of 1,000 cows or more, up from a mere 10% in 1992 This has resulted in a geographic consolidation of dairy manure production, and has led to an increased reliance on corn (Zea mays L.) silage for animal feed in place of perennial forages like alfalfa (Medicago sativa L.). Much of the cropland in this region is tile-drained, facilitating rapid transport of water and nutrients from the point of application to nearby waterways (David et al, 2010). These factors, coupled with the aforementioned manure consolidation and loss of perennial cover, raise questions about the water quality implications of the trend toward larger operations in the dairy industry
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