Improving Nutritive Value of Native Warm‐Season Grasses with the Plant Growth Regulator Trinexapac‐Ethyl

Abstract

Core Ideas Gibberellin inhibitors may decrease stem growth, improving late fall forage nutritive value during late summer and fall. Examining the short‐ and long‐term impacts of suppressed stem growth or altered tiller number could result in improved forage nutritive traits. Warm‐season grasses are characterized by high forage mass but low nutritive value, especially when mature. Livestock producers in the southeastern United States utilize native warm‐season grasses (NWSG), however, the best management practice to not harvest during late‐summer and fall results in large quantities of low quality forage. Gibberellin inhibitors can alter plant regrowth and forage value. This study assessed the impact of trinexapac‐ethyl [TE; ethyl 4‐(cyclopropyl‐hydroxy‐methylene)‐3,5‐dioxo‐cyclohexane‐1‐carboxylate] on fall NWSG forage. Application of TE occurred in late July at four levels (0, 0.3, 0.6, and 1.2 kg a.i. ha−1) to switchgrass (SG; Panicum virgatum L.) and a mixed sward (BBIG) of big bluestem (Andropogon gerardii Vitman) and indiangrass (Sorghastrum nutans L.) during 2016 and 2017. Forage mass (FM) and nutritive value [crude protein (CP), neutral detergent fiber (NDF), in vitro true dry matter digestibility (IVTDMD)] were evaluated monthly. Application reduced fall FM (SG: 37%; BBIG: 42%). Switchgrass nutritive value did respond to TE application. Application of TE reduced BBIG NDF overall, but strong annual variation occurred due to a drought (2016). Analyzed separately, 2017 treated BBIG samples (1.2 kg a.i. ha−1) exhibited altered FM (−57%), CP (up to 54%), NDF (−9.2%), and IVTDMD (11%). Further analysis provided tentative evidence that TE improved leaf nutritive value and increased leaf proportion, although overall leaf yield was not improved. Overall, TE improved BBIG nutritive value but at a substantial FM loss. Since poor nutritive content forage has negligible value regardless of quantity, growth regulators could improve outcomes of fall NWSG management by adding value to an unutilized resource.