Echinochloa in mid-southern US and California Rice: What is Known and What are the Knowledge Gaps?

Several Echinochloa P. Beauv. species, introduced at multiple events, have established themselves as a persistent concern for US rice production. In this review, we highlight the key biological characteristics of economically relevant Echinochloa in US rice, revisit their historical trajectory, and put forward research directions for their management with special reference to barnyardgrass. Ecologically-differentiated Echinochloa species have a distinct association with rice culture methods that have been practiced in a region, barnyardgrass being historically predominant in drill-seeded rice in the mid-South, and early watergrass and late watergrass in water-seeded California rice for the last few decades. However, the emerging evidence challenges the dogma that other Echinochloa species for the specific regions are of less importance. Primarily managed by the water-seeding method of rice culture in the early years, Echinochloa species have persisted in the sophisticated US rice culture through the evolution of resistance to herbicides in the later course. Accumulating knowledge, including those of recent genomic insights, suggests the rapid adaptability of Echinochloa. The last decade has seen a (re)emergence of non-chemical methods as a key component of a sustainable management approach, among which use of harvest weed seed control (HWSC) methods and cover crops in the mid-South and stale-drill seeding in California are being considered as potential tools for management of Echinochloa. In recent years, furrow-irrigated rice has rapidly supplanted a significant proportion of conventionally flooded rice in the mid-South, whereas the propensity for compromised continuous submergence is increasing in California rice. On the cusp of this shift, the question at the forefront is how this will affect Echinochloa interference in rice and how this change will dictate the management efforts. Future research will develop a clear understanding of the impact of the changing agroecosystems on Echinochloa species as well as their response to the prospective integrated control interventions.