Droughts provide opportunities to help improve control of some weeds, as well as present risks of new invasions via purchased fodder, wind/water erosion, vehicles and weakened pastures.
Depending on drought severity, and timing of rain events, droughts have sometimes allowed for germination of some weeds but they may not have been able to set seed because of extremely dry conditions at their flowering and seed set time.
An example in our area was good germination of weeds like saffron thistle, Paterson's curse and capeweed after March 2019 rain.
Saffron set some seed on stunted plants in parts of paddocks like gullies and depressions where more water gravitated via a bit of runoff. However, in the majority of pasture paddocks, saffron failed to set seed, especially in strong perennial pastures.
While one year's prevention of weed seeding via the drought is not enough to anywhere near eliminate many annual weeds, it can be part of a successful "integrated" control program. One would need at least three consecutive years of saffron seed prevention to go anywhere near eliminating it.
Part of long-term control can also include a strong perennial grass pasture to add to moisture competition. Herbicides like gramoxone as an at-flowering treatment can also be an effective part of integrated control.
Paterson's curse did cope better with the drought, however, even with a steer fattening business, while Paterson's curse is commonly ignored by them (not so much by sheep), because of the scarcity of feed they commonly grazed them to a degree, also contributing to low seed set. Dry weather also adversely impacted on introduced Paterson's curse predators, but one hopes these will return in numbers next season and continue with their useful control role.
St Barnaby's thistle was accidentally introduced to our property a few years ago via a pasture seed contaminant and we have worked hard at eliminating it. Drought probably meant any odd plants may have been grazed during the relatively palatable vegetative phase, with the dry spring leading to almost no plants surviving to set seed. However, we have found the odd plant and removed them and still feel we are on track to eliminate this weed.
Perennial weeds are more difficult to control and some of them are quite drought hardy. For example, over the last few years, we have experienced a couple of sporadic infestations of blue heliotrope and monitor it similar to non-drought years for plants germinating from a soil seed bank, or from shoots off existing deep-rooted parent plants. Appropriate herbicides are used to treat these areas and again we hope to be on target to eliminate this pest.
We have a long-term strategy to eliminate African lovegrass, a constant invader from an adjoining public road, via three years cropping followed by sowing down to a competitive long-lasting tropical grass. Feed scarcity, and because winter legumes normally coexist with the weed, together with a fertiliser program to correct phosphorus and sulphur deficiency, African lovegrass has been a valuable feed this drought. Steers have grown well on it, and like their cousins, introduced tropical grasses, have provided good feed from "out of season" rain.
Another useful "weed" that in spring provided good feed when little else was available was Peppercress. For some reason, it has suited a part of this difficult drought. Skeleton weed, a normal "pain in the neck" deep-rooted perennial weed has also proved good feed, even when stemmy.
As the drought breaks, we have experienced a weed problem where perennial pastures are not strong (some of our native pastures especially). Germinations of catheads, melon's and burrs occurred on November and then January and February rains where perennial pastures don't exist or were thin.
As is constantly discussed, careful surveillance of new weeds is important when drought-breaking rains occur. Introduced fodder, blown-in seed, flooding rains or arrival on transport are all possible sources of new weeds. If uncertain seek help with identification (LLS and other agronomists). Early weed identification is a major part of a successful control program.
Next week: Research shows why ground cover is so vital.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email firstname.lastname@example.org or contact (0428) 752 149.