Negative effects of moderate drought on forage quality and quantity can be long-term and exacerbated by heavy grazing

Authors: Porensky, Lauren; KOERNER, SALLY - University Of North Carolina Greensboro; Williams, Amanda; VAN EMON, MEGAN - Montana State University; KOMATSU, KIMBERLY - University Of North Carolina Greensboro; WILCOX, KEVIN - University Of North Carolina Greensboro; Dietrich, John; Reinhart, Kurt

Submitted to: International Rangeland Congress
Publication Type: Proceedings
Publication Acceptance Date: 3/12/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Growing season droughts can reduce forage availability and quality. These droughts are predicted to become increasingly frequent in temperate rangelands due to climate change. To sustain livestock production systems, we need to understand what grazing management strategies will sustain both ecosystems and livelihoods in the face of increasing drought. Little is known about how droughts interact with grazing management to affect forage quality and quantity. In two North American rangelands, we assessed the separate and combined effects of experimental droughts and grazing management strategies on forage quality and quantity over five years. During a 2-yr experimental drought period, rainfall reductions decreased both forage quality and quantity at one site. At a second site, heavy grazing during the first year of experimental drought reduced forage biomass and digestibility during the second year. In the first year after experimental rainfall reduction treatments ended, we saw lasting effects of our former treatments. Plots that formerly received large experimental droughts had 26% to 57% less digestible forage biomass but greater forage quality than controls. Effects on forage biomass disappeared in the second recovery year, but reductions in forage quality persisted up to three years after droughts ended. These findings show that although North American Great Plains grasslands are able to tolerate both drought and grazing, these disturbances can have additive and long-term effects on forage. Moderate droughts can impact ruminant nutrition for 1-3 years after the drought ends, and heavy grazing during drought may strengthen the effects of drought on livestock nutrition.

Technical Abstract: Growing season droughts can have major impacts on grassland vegetation and are predicted to become increasingly frequent in temperate rangelands due to climate change. To sustain livestock production systems, we need to understand what grazing management strategies will best support long-term ecosystem and livelihood sustainability in the face of increasing drought. Little is known about how droughts interact with grazing management to affect forage quality and quantity. In two North American grasslands where grazing by domestic livestock is the primary land-use, we assessed the separate and combined effects of experimental rainfall reductions and grazing management strategies on ruminant forage quality and quantity over five years. During a 2-yr experimental rainfall reduction period, rainfall reductions decreased both forage quality and quantity at one site. At a second site, heavy grazing during the first year of experimental drought reduced forage biomass and digestibility during the second year. In the first year after experimental rainfall reduction treatments ended, plots that formerly received large rainfall reductions displayed strong legacy effects. These plots had 26% to 57% less digestible forage biomass but greater forage quality than controls. Experimental treatments did not induce long-term changes in forage quantity at either site, but reductions in forage quality persisted up to three years after droughts ended. Our results highlight the resilience of North American Great Plains grasslands to both drought and grazing, but also suggest that these disturbances can have additive and long-term effects on forage nutritive value. Legacy effects of droughts on forage quality and quantity may impact ruminant nutrition for 1-3 years following a drought, and heavy grazing during drought may strengthen the effects of drought on livestock nutrition.