Organic Perennial Pastures Outperform Organic Annual Pastures with Lower N Losses and Equivalent Yields When Exposed to Drought, Extreme Rainfall and Simulated Grazing.

Recent organic dairy market growth drives new organic dairy establishment, which include new pastures and more grazing stipulated by the NOP’s Pasture Rule policy. In the Northeast, these market and policy changes also drive diversification from perennial C3 plants to annual C4 grasses to allow grazing through the summer. Regional hydroclimatic variability is also increasing with more droughts and extreme rainfall events. These factors inspired the question: how will interactions between pasture change and climate change affect environmental and agronomic performance? We assessed environmental and agronomic performance of perennial orchardgrass-red clover (Dactylis glomerata-Trifolium pratense, “C3”) and annual Sorghum Sudangrass-red clover (Sorghum bicolor x S. bicolor var. sudanese-Trifolium pratense, “C4”) pastures exposed to grazing and extreme precipitation in a 2012 through 2014 field trial on certified organic land in central Pennsylvania. Metrics of environmental and agronomic performance included yield quantity and quality; N uptake; soil to atmosphere NH3, N2O, CO2 emissions; soil N pools and potentially leached soil nitrate. Drought hindered C4 establishment and performance in June 2012, but did not limit C3 establishment in March 2012. Poor establishment handicapped C4 resilience to grazing and extreme precipitation simulations in 2012, limiting yields and environmental performance. Adequate moisture in 2013 supported yields in excess of county averages and soil ratings for grass-legume hay, but also drove large N2O emissions from establishing corn silage following the 2012 C4, but not in the C3. In both years, soil-atmosphere gas emissions associated with grazing (clipping) and extreme rainfall (irrigation) were higher in the C4 immediately after the experimental manipulations, but declined with decreasing temperatures in both pasture types. While market and policy shifts may promote wider planting of C4 annual pastures, this transformation may increase susceptibility to early summer droughts and N2O losses from intense rain events later in the growing season.