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3c. Weed management research - long version
Pastures for grazing animals such as sheep and cattle consist of both the grasses you want and the weeds you don't want. The following research by Annabel investigated whether cutting the pasture at critical times, a bit like mowing the lawn at home, could remove the weeds. Defoliating pastures at critical times in the development of a species can have a large impact on composition in future years. Using non-selective defoliation techniques have the potential to provide a useful, non-herbicide method of pasture weed management. However, timing the cut is critical to prevent regrowth of the target weed (and subsequent seed production) and minimise damage to desirable pasture species. The critical time will vary for each plant species.
Experimental design and treatments
Weeds research at WWAI has examined the effect of spring pasture defoliation (mechanical and grazing) on pasture composition and the management of targeted weed species. A degraded, mixed temperate perennial (phalaris/cocksfoot) and annual (subclover/annual grass and broadleaf) pasture was either grazed or mechanically defoliated during spring. There were four mechanical defoliation treatments and one treatment continuously grazed by sheep:
Tr 1 Continuously grazed by sheep (10 DSE)
Tr 2 Grazed by sheep 1 Jan – 1 Aug; sheep excluded and mechanically defoliated early Oct
Tr 3 Grazed by sheep 1 Jan – 1 Aug; sheep excluded mechanically defoliated late Oct
Tr 4 Grazed by sheep 1 Jan – 1 Aug; sheep excluded mechanically defoliated early Nov
Tr 5 Grazed by sheep 1 Jan – 1 Aug; sheep excluded mechanically defoliated late Nov
The mechanical spring defoliation treatments were repeated over three years (1997-1999). The experimental area contained four replicates that were separated by 10 m wide buffers. Each replicate consisted of five 10 m x 20 m plots. Defoliation treatments were allocated to the plots randomly. The uniformity of grazing in the grazed plots was improved by positioning them within the experimental design to allow maximum accessibility by the sheep from the surrounding paddock.
Mechanical defoliation treatments
The mechanical defoliation was achieved using a small plot forage harvester. The pasture was cut to 5 cm and the harvested material removed from the experimental area. The cutting dates were at approximately 17 day intervals each year. These harvest dates covered the range of phenological development stages of the pasture species during spring and a range of forage qualities.
Management of livestock
The experimental area was grazed by sheep (wethers) at 10 dry sheep equivalents (DSE) from July 1997. To prevent preferential grazing, plots that were mechanically defoliated were not reopened to grazing until the herbage in the plots was visually assessed as similar in pasture height to that in the surrounding paddock.
Botanical frequency was assessed at various times during the spring using a non-destructive technique. A modified rod-point quadrat method (Little and Frensham 1993) was used to assess botanical composition of the pastures. The rod consisted of two 50 cm pieces of steel welded in the middle to form a cross with the ends sharpened. Metal legs (4 cm high) were attached to each arm of the cross. The point-quadrat was then placed on the pasture and the vegetation closest to the four elevated points was recorded. The height of the quadrat was chosen to reduce bias from taller growing species (Kemp and Dowling 1991).
The quadrat was thrown 80 times/plot while walking back and forth across the 200 m 2 area. Species frequency was calculated and percentage contribution of each species to botanical composition was estimated. This method was used to assess the botanical composition of the grazed and previously defoliated plots at the beginning of spring and on the day prior to cutting the next designated defoliation treatments. Botanical frequency was also recorded on all plots approximately three weeks after the completion of the defoliation treatments (late January).
Large changes in pasture composition occurred as a result of the various defoliation methods and their timing, after two years (Table 1). Although in this trial the pasture regrowth was not grazed until mid-summer, a strategic grazing and/or use of herbicide on the spring regrowth may have further minimised seed production for some species (e.g. Paterson 's curse).
Continuously grazing the pastures during spring had a large impact on Paterson 's curse seed production (Table 2) and subsequent autumn seedling germination but favoured annual grass species such as vulpia (silver grass), bromes and barley grass.
Compared to grazing, defoliating in early October (equivalent to an early silage cut in this region), significantly improved subclover and annual ryegrass content, and reduced vulpia. Cutting at this time may favour an increase in the content of Paterson 's curse if the regrowth is not managed as the regrowth can successfully produce viable seed.
A traditional hay cut for this region would equate to cutting in early November. Defoliating at this time reduced annual ryegrass and subclover but significantly increased vulpia content as vulpia produced and shed vast quantities of viable seed before the defoliation occurred.
The stage of growth of a species at the time of defoliation determines its response to the cutting e.g. subsequent growth and seed production. For weed management in this trial, the optimum stage for cutting annual grasses is when the majority of the most advanced seed heads is between post-flowering and very early seed fill. For Paterson 's curse, the optimum cutting time appears to be when the earliest (lowest) flowers are starting to form green seeds on the most advanced flowering head.
Further research is required to provide clear guidelines on the critical growth stages for other pasture species, so that farmers can identify the optimum cutting time. This approach will be much more reliable with respect to weed management than setting defoliation dates by a calendar as species development will vary with region and year.
If mechanical defoliation is to be used as a weed management tool in pastures, the optimum time to manage the target weed may not correspond with the optimum time for obtaining maximum forage quality. Farmers wishing to incorporated strategic spring defoliation into their pasture weed management plans need to determine a balance between forage quality and desirable changes to pasture composition.
Table 1 Effect of grazing by wethers (10 DSE stocking rate) and cutting times on species composition of a mixed annual grass/subclover/perennial grass pasture the third spring after cutting or grazing in each of the two previous springs.
Table 2 Impact of one or two years of grazing or cutting on the seed production of annual ryegrass, vulpia, and Paterson 's curse.
Kemp, D.R. and Dowling, P.M. (1991). Species distribution within improved pastures over Central N.S.W. in relation to rainfall and altitude. Australian Journal of Agricultural Research 42 , 647-59.
Little, D.L. and Frensham, A.B. (1993). A rod-point technique for estimating botanical composition of pastures. Australian Journal of Experimental Agriculture 33 , 871-5.
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