Effects of grazing management on creeping thistle and other injurous weeds and integration of grazing with weed control

Summary

Creeping thistle Cirsium arvense is a persistent weed commonly affecting grassland conservation areas and grazing land in the UK. High levels of Cirsium infestation reduce agricultural productivity and in conservation areas if dense stands form, may result in localised reduction of biodiversity through less aggresive but desirable plants being out-competed. Current methods of thistle control involve repetitive topping or intensive herbicide use which may in themselves have conservation impacts. Previous work in New Zealand had separately examined the use of herbicide/grazing and of topping/grazing regimes in the control of C.arvense and shown a high level of reduction in thistle density (Hartley et al. 1984; Mitchell & Abernethy 1993, 1995).

A project was therefore undertaken to test an integrated approach to thistle management combining grazing, mechanical techniques and herbicide application to obtain a reduction in Cirsium density and long-term maintenance of low population levels, whilst minimising the impact upon non-target species. The study developed and tested combinations of these control techniques. The key objectives being to a) examine the effect of the timing and severity of grazing on thistle populations and b) examine the impact of cutting and herbicide application and their interactions with grazing regimes.

Study sites: The research was carried out on two semi-improved grassland sites (MG6 - UK National Vegetation Classification) with severe thistle infestation:

i) Bronydd Mawr (Wales) - a marginal, semi-upland, sheep-grazed grassland
ii) Marsh Gibbon (England) - a lowland, mixed sheep and cattle-grazed grassland

In June 2000, the mean thistle shoot density was 11.1/m² at Bronydd Mawr and 14.3/m² at Marsh Gibbon.

Experimental design: The study was undertaken from April 2000 to March 2003. Each site was subjected to a set of grazing treatments and weed control sub-treatments. Grazing regimes were implemented in 2000. Restrictions in livestock movement due to an outbreak of foot and mouth disease, prevented this implementation at Marsh Gibbon in spring 2001 but the required treatments were re-instated in the autumn. Weed control sub-treatments were applied in 2000 and 2001.

Bronydd Mawr - Six grazing treatments were applied in a randomised block with three replicates. Five weed sub-treatments were applied to 10 x 10 m plots within each grazing block of 0.25 ha. Each plot was separated by a buffer strip of at least 5 m to reduce the risk of interference effects. Grazed blocks with no sub-treatments and high densities of thistles controlled for the impact of grazing only on thistle density. Those with low initial densities were used to examine thistle propagation mechanisms.

Marsh Gibbon - The same design was applied but with five grazing treatments in 0.5 ha blocks and six weed control sub-treatments.

Thistles were topped to a height of about 8-10 cm (using a flail mower at Bronydd Mawr and a drum mower at Marsh Gibbon, both types of machinery resulting in a similar cut). Cut vegetation was not removed. Herbicide (Dow Sheild: active ingredient 18% w/w clopyralid, 3, 6-dichloro-2-pyridinecarboxylic acid) mixed 1 part to 4 parts water, was applied using a towed 'weed wiper'. The concentration used was based upon previous research on weed wipe herbicide application to thistles.

Impacts on non-target plants: Impacts of treatments on non-target plants were assessed by root frequency counts of forb species in three randomly placed 1 m² quadrats in each weed control plot.

Impacts on invertebrates: Impacts of treatments on the invertebrate communities associated with C. arvense were assessed by (i) timed counts of herbivorous and pollinator invertebrates utilising each treatment block (ii) counts of thistle stem invertebrate populations and (iii) identification of invertebrate populations within thistle seed heads.

Herbicide dosage: An experiment in June 2001 at Marsh Gibbon assessed the optimum dose rate of Dow Shield herbicide applied by weed wipe for the control of C.arvense. Six concentrations (0%, 0.002%, 0.45%, 0.90%, 1.80%, 3.60% w/w clopyralid) diluted in water were applied by weed wipe to 2.35 x 5 m blocks (fully randomised with four replicates) adjacent to the main experimental plot. Thistle densities were assessed pre-herbicide application and again in June 2002. These plots were extensively sheep grazed after herbicide application.

Herbicides & grazing: Herbicide impact on non-target plants was minimised by grazing prior to 'weed wipe' herbicide application. Grazing reduced the non-target vegetation height to below that of the less palatable thistle stems. Application of clopyralid herbicide in June, followed by light grazing in the autumn was the most effective Cirsium control method. Creeping thistle densities were reduced from 10.8 shoots/m² to 0.1/m² at Bronydd Mawr and from 11.7 shoots/m² to 0.2 at Marsh Gibbon.

The optimum dilution rate for the herbicide was found to be 1 part Dow Shield to 19 parts water, giving a concentration of clopyralid of 0.90% w/w. The effectiveness of the herbicide did not increase significantly at concentrations in excess of this, and below 0.45% clopyralid (equivalent to 1 part Dow Shield to 39 parts water) there was no significant effect on the thistle population.

Evidence from the USA points to clopyralid being highly persistent in dead plant matter, with plant growth inhibited in compost produced from vegetation where clopyralid had been applied. Therefore, raking off of dead material after spraying may be desirable.

Herbicides & cutting: Herbicide application in September following cutting in June was less effective than herbicide application and grazing, as thistles recruited that year remained as low rosettes, below the level of non-target species, and thus did not receive herbicide.

Grazing & cutting: Lenient autumn grazing with cutting gave good control at Marsh Gibbon, reducing thistle shoot density from 10.3 to 2.2 shoots/m².

Grazing only: Grazing alone was not found to be an effective means of control. Thistle populations in the low thistle density control plots increased over the course of the experiment under the grazing-only regime. Tight autumn grazing and winter grazing both increased thistle populations. The grazing effect was greater with sheep than with cattle. This was thought to have been due to lessened competition from grass and other plants in the spring rather than recruitment from seed germination. The principle mechanism for the increase in thistle numbers was found to be vegetative shoot production rather than seedling growth.

Cutting & removal: As with grazing only, hay cutting and removal on a three year rotation also led to increased thistle infestation.

Effect of treatments on non-target species: None of the treatments reduced total non-target plant species populations, though there were some changes in individual species densities e.g. at Bronydd Mawr tight autumn grazing increased the frequency of yarrow Achillea millefolium and white clover Trifolium repens, whilst tight spring grazing significantly increased the frequency of autumnal hawkbit Leontodon autumnalis. Cattle grazing benefited meadow buttercup Ranunculus acris compared with sheep grazing. Herbicide application caused a decrease in A.millefolium in comparison with the control and cutting only plots.

At both sites, the highest pollinator numbers and diversity were found on the high thistle density control plots (sub-treatment plots B). Creeping thistle is a key attractor for a wide range of pollinating insects e.g. bumblebees Bombus, and the control of thistle populations may to lead to a reduction in pollinating insect numbers.


References

Hartley M. J., Lyttle L. A. & Popay A. I. (1984) Control of Californian thistle by grazing management, Proceedings of the New Zealand weed and pest control conference, New Zealand Weed and Pest Control Society, Hastings, New Zealand.
Mitchell R. B. & Abernethy R. J. (1993) Integrated management of Californian thistle in pasture, Proceedings of the Forty Sixth New Zealand Plant Protection Conference, Christchurch, New Zealand, 10-12 August 1993. New Zealand Plant Protection Society, Rotorua, New Zealand.
Mitchell R. B. & Abernethy R. J. (1995) The effect of topping and repeat grazings on Californian thistle and pasture production. In: Popay, A. J. (Ed.), Proceedings of the forty eighth New Zealand plant protection conference, Angus Inn, Hastings, New Zealand, August 8-10, 1995. New Zealand Plant Protection Society, Rotorua, New Zealand: 1995.


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