While fleabane (Conyza spp.) has been a major cropping and fallow weed issue for some time, especially in zero till or no till systems, it can also be a major problem in pastures. Where it thrives in pastures it tends to use most of the soil water, many of the nutrients, and crowd out other species for sunlight, space and drymatter. Purple top is a similar pasture weed problem.
A run of wet springs and summers, as occurred in many areas over the past three years, especially favours weeds like fleabane. Fleabane germinates in autumn, winter and spring provided soil moisture is good. In wet springs and summers it can grow profusely and can dominate pastures. It mainly behaves as an annual but can grow right through summer into the following autumn and early winter.
Fleabane in pastures is best controlled by aggressive species that grow at the same time it is most active. It's a case of "fire fighting fire". Pasture most overrun by fleabane tend to be those that have low to no population of spring/summer growing perennials, or the spring summer perennial growth is compromised because of previous grazing management. Sometimes the species type is suitable to outcompete fleabane but previous grazing management, often exacerbated by prior drought, has reduced regrowth rate of perennials.
Perennial tropical grasses, with proven long term persistence for a given area, plus high growth rates, are ideal types to compete against weeds like fleabane.
Strong summer growing native grasses can also be competitive. In our central west area, on medium textured and lighter soils Premier digit and Consol love grass best fit this role. On heavier soils bambatsi panic combines well with Premier digit grass. In more western areas other species like buffel and Mitchell grass are also worth considering.
Grazing management is important to help ensure summer growing species preform at their most competitive level. For example stands heavily grazed for long periods tend to have lower root reserves and are less competitive against weeds like fleabane. A system of flexible rotational grazing, ensuring plants are not grazed longer than a few weeks and given ample opportunity to regrow before re-grazing, including allowing periodic flowering, is an example of ensuring competitive pastures.
Good soil fertility is also important for competitive pastures. Nitrogen, sulphur and phosphorus soil deficiencies are in much of NSW key deficiencies to address if soils are low in them. Legumes, which build soil nitrogen, will only do so if deficiencies like sulphur are corrected. Correcting nitrogen deficiency, via legumes or fertiliser, or a combination of both, commonly leads to tropical grasses doubling or more in productivity, and combine with being more competitive against weeds.
One key aspect of establishing competitive tropical pastures, is to firstly reduce the level of weeds like fleabane prior to sowing them. Tropical grasses tend to be poor competitors against weeds in their establishment period. Once well established they can be very weed competitive.
A common way to reduce weed level prior to perennial grass sowing is to crop for three years prior to pasture sowing. Weeds need eliminating in the pre-crop phase as well as the fallow phase. Herbicides can be effective against winter early spring fleabane germinations, in the winter crop phase, provided they are treated when young and actively growing. The main problem is during the summer fallow period.
Fleabane commonly is difficult to kill with glyphosate, likely a natural weed characteristic as well as herbicide resistance developing quickly. Research has shown a sound practise, even though a bit expensive, is to do what is termed "double knock". Apply glyphosate plus 2,4-D (note restrictions of some products in cotton areas) for multipurpose fallow weed control, and follow up seven days later with gramoxone.
Slashing can also have a role in infested pasture paddocks, especially to open up for autumn germination of winter legumes. Slashing prior to weed flowering commonly results in regrowth if conditions are moist. Further slashing may be needed.
Next week: Why some pastures good feed quality, some poor.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email firstname.lastname@example.org or contact (0428) 752 149.