Fifteenth Australian Weeds Conference - Suppression of Native Wild Rice Germination by Exotic Para Grass

Sep 15, 2026

Uploaded by Jiven

This study examines how introduced para grass (Brachiaria mutica) suppresses germination of the native annual wild rice (Oryza meridionalis) on monsoonal floodplains. Wild rice relies on seed dormancy that prevents germination after dispersal into moist soil at the end of the wet season. Field and laboratory results show para grass modifies seedbed conditions, lowers surface temperatures, and disrupts dormancy breaking cues, leading to markedly reduced germination under para grass cover compared with wild rice cover.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages4

Reading Time10 min

AccessFree to download

Summary

Wild rice and para grass interaction

Dormancy breaking cues and seedbed conditions

Field germination results

Implications for weed management

Fifteenth Australian Weeds Conference Suppression of native wild rice germination by exotic para grass Penny A. S. Wurm1, Sean M. Bellairs2 and Beckie Kernich2 1 Tropical Savannas CRC, Charles Darwin University, Darwin, Northern Territory 0909, Australia 2 School of Science and Primary Industries, Charles Darwin University, Darwin, Northern Territory 0909, Australia Summary Wild rice (Oryza meridionalis Ng) underpins vertebrate food chains on the monsoonal floodplains. This native annual grass is displaced by the introduced pasture, para grass (Brachiaria mutica (Forssk.) Nguyen). This study investigates whether the mechanism for this displacement is through effects on the germination of wild rice seed. Wild rice has a seed dormancy mechanism that prevents seeds germinating when they are first dispersed into the moist floodplain soil at the end of the annual wet season. The presence of para grass may prevent or change environmental cues that would otherwise break dormancy and stimulate germination of wild rice. Initially we found that, under wild rice cover, some 76% of seeds had germinated, 17% had died and 5% remained ungerminated after two wet seasons. When wild rice seeds were sown into a well established para grass cover, almost no seed germinated, a large proportion remained alive but ungerminated, and a greater proportion died than under wild rice cover. We then investigated how para grass may prevent germination of wild rice, by (a) determining dormancy breaking cues for wild rice seeds and (b) comparing floodplain seed bed conditions under para grass and wild rice cover. In the laboratory, breaking of dormancy was dependent on the seeds being exposed to high temperatures. In the field, para grass produced much higher above ground biomass than wild rice, and modified the seedbed. Temperatures were lower at the soil surface under para-grass cover than under wild rice cover. This study showed wild rice has a dormancy mechanism, which prevents germination under significant canopy and litter cover. Thus, a weed species (para grass) can modify habitat conditions so that dormancy breaking and germination in a native species (wild rice) is prevented. Managers may need to deal with habitat modification by weeds, as well as direct competition with natives, when dealing with environmental weeds. Keywords Oryza meridionalis, Brachiaria mutica, seed dormancy, seedbed conditions, monsoonal floodplains. INTRODUCTION This study addresses interactions between wild rice and para grass. Wild rice seeds underpin the vertebrate food chain on the monsoonal floodplains in northern Australia, being an important food resource for both magpie geese and dusky plains rats (Redhead 1979, Frith and Davies 1961) . Para grass was introduced from tropical Africa and South America, initially to Queensland in the 1880s and to the NT by 1910 (Cameron and Lemke 2002) . Its productivity and stoloniferous habit meant it was valued as a ponded pasture species and for bankstabilisation. In Queensland it is now considered an environmental weed in wetlands, a weed of sugarcane plantations and has been shown to alter stream hydrology (Bunn et al. 1998). In the Northern Territory (NT), it is still valued as a pasture species in grazed landscapes, but is an environmental weed in conservation reserves, including Kakadu National Park. It isnota declared weed in either jurisdiction. Wild rice is being displaced by para grass on NT floodplains (Knerr 1996, Wilson et al. 1990, Ferdinands et al. 2001) . On the Magela Creek floodplain, Knerr (1996) found that the distribution of para grass increased by some 300 ha in five years, while that of rice decreased concomitantly. Ferdinands et al.(2001) predict that, without proactive management, further displacement of wild rice by para grass will occur on the Mary River floodplain. The mechanism for this displacement is unknown, but may be due to direct competition or via habitat modification, an

FAQs

What native and introduced species are studied, and why are they important?

The native annual wild rice (Oryza meridionalis) supports vertebrate food chains on monsoonal floodplains, while introduced para grass (Brachiaria mutica) displaces it and is studied to understand the displacement mechanism.

How does para grass affect wild rice germination?

Para grass cover almost eliminates wild rice seed germination, leaving many seeds alive but ungerminated and increasing seed death compared with wild rice cover.

What mechanism prevents germination under para grass cover?

Dormancy breaking in the laboratory depends on exposure to high temperatures, while in the field para grass modifies the seedbed and produces lower soil-surface temperatures under its canopy and litter, preventing dormancy breaking.

Sep 15, 2026

Uploaded by Jiven