Influence of establishment timing on growth and fecundity of two itchgrass (Rottboellia cochinchinensis) biotypes grown in Louisiana

Authors: Spaunhorst, Douglas

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/16/2020
Publication Date: 5/18/2020
Citation: Spaunhorst, D.J. 2020. Influence of establishment timing on growth and fecundity of two itchgrass (Rottboellia cochinchinensis) biotypes grown in Louisiana. Weed Science. 68:418-425. https://doi.org/10.1017/wsc.2020.30.
DOI: https://doi.org/10.1017/wsc.2020.30

Interpretive Summary: Itchgrass is a weed commonly found in sugarcane fields in Louisiana and if not controlled can reduce sugar yield by 43%. Two distinct populations have been identified. Each population exhibits unique physical characteristics and reproduction during the growing season that distinguish them from one another. The Louisiana-1 population flowers as early as 6 weeks after emergence, but the Louisiana-2 population flowers when day length is less than 13 hours. Herbicides are generally more effective at controlling weeds smaller than 8 inches. Itchgrass growth to 8 inches tall occurred quicker for Louisiana-1 when compared to Louisiana-2, and the maximum height of Louisiana-1 was 11 inches taller than Louisiana-2. Faster early-season growth reported with Louisiana-1 when compared to Louisiana-2 may compromise herbicide performance if weather conditions prevent growers from timely applying herbicides. Seed production by October did not occur when Louisiana-2 emerged in June or later, but Louisiana-1 emerging in June produced 202 seed heads; and in some years produced up to 32 seed heads when plants emerged in July. The present study demonstrated the importance of managing the Louisiana-2 population in March and April to limit seed production, but fields infested with Louisiana-1 were at greater risk for potential crop yield loss because plants produced high quantities of seed when established over a wide period of time.

Technical Abstract: Itchgrass [Rottboellia cochinchinensis (Lour.) Clayton] is among the most troublesome weed in subtropical climates where sugarcane (Saccharum spp. interspecific hybrids) is cultivated. Two R. cochinchinensis biotypes commonly infest sugarcane in Louisiana. The Louisiana-1 biotype is day length neutral, but Louisiana-2 flowered when day length decreased to 13 h. Coupled with biotype diversity, seedling emergence has been reported to occur earlier in the growing season when sugarcane emerged from winter dormancy. Both R. cochinchinensis biotypes were established in a common garden experiment in Louisiana during periods of sugarcane development and field preparation to simulate discontinuous emergence. Weekly, plant height and raceme production were recorded for each biotype and establishment timing; aboveground biomass was harvested in autumn. Louisiana’s subtropical humid climate stimulated rapid plant growth that typically began in May and persisted through September. Without sugarcane competition, maximum R. cochinchinensis height for Louisiana-1 and Louisiana-2 was 206 and 179 cm and growing degree days to 20-cm height in 2017 ranged from 546 to 832 and 865 to 1160, respectively. Slower initial growth reported with Louisiana-2 would allow more time for growers to treat escaped plants with POST herbicides. Total raceme production, by autumn, was not reported for Louisiana-2 established in June or later, but Louisiana-1 established in June produced up to 202 racemes. The present study demonstrated the importance of managing the Louisiana-2 biotype in March and April to limit seed production, but fields infested with Louisiana-1 were at greater risk for potential crop yield loss because plants produced high quantities of seed when established over a wide period of time.