Ovine Observer

Controlling red legged earth mites using intensive spring grazing

Svetlana Micic and Paul Sanford DPIRD, Albany WA

Author correspondence: paul.sanford@dpird.wa.gov.au 

Background

Research in the 1990’s found that intensively grazing pastures for the whole of spring controls red legged earth mites (RLEM) as effectively as insecticides. Yet this tactic has not been adopted by producers due to the impractical number of sheep required. It is however reasonable to expect that some, if not all of the benefits could be realised by much shorter periods of intensive grazing.

Insecticide resistance has recently arisen in RLEM populations in parts of the grain belt, renewing interest in alternative control methods such as intensive spring grazing around the Timerite® period. Grazing is an attractive option as it possibly has multiple benefits such as improved yields in following pasture or crops and better weed control in spring pasture. The largest benefits are expected for farms in the medium to high rainfall zones with more pasture Feed on Offer (FOO) and bigger RLEM populations. The cost to implement this spring grazing package in most cases would be zero, however it may require some additional labour. In the absence of chemical control options, resistant RLEM’s will reduce pasture and crop yields, meaning a spring grazing package could recover a substantial proportion of these losses.

In 2019, we undertook three on-farm demonstrations at Boyup Brook, Cranbrook and Kalgan to compare the impact of intensive grazing for 2 and 4 weeks in spring on RLEM populations, around the Timerite® date, to an ungrazed control. Intensive grazing involved maintaining pasture FOO at approximately 1.4 tonne dry matter per hectare (t DM/ha). Pastures were assessed weekly and RLEM were sampled fortnightly in spring 2019 and after the break of the 2020 growing season.

Boyup Brook

In 2019, the intensively grazed plots were maintained at approximately 2t DM/ha (Figure 1).

Figure 1 Feed on offer (t DM/ha) of clover based pasture at Boyup Brook in 2019 that was ungrazed, grazed for 2 weeks or grazed for 4 weeks.

By the end of two weeks grazing (2 Oct 2019), there were 95% less RLEM compared to the control (Figure 2). Two weeks after livestock had been removed from this treatment, the numbers of RLEM had increased 5 fold (84%), whereas FOO only increased by 0.5t DM/ha (Figures 1 and 2). Four weeks of grazing led to a 98% reduction of RLEM compared to the ungrazed control and a fortnight later (6 Nov 2019) RLEM populations had crashed, indicating the population was undergoing summer diapause.

Sampling in early autumn 2020 only found RLEM in the ungrazed treatment, but by early winter, only the 4 week grazed treatment had less than 500 RLEM per square metre (Figure 2).

Figure 2 Number of RLEM in pasture at Boyup Brook that was ungrazed, grazed for 2 weeks or grazed for 4 weeks ± standard error of mean (SEM). Arrows indicate when livestock were removed, colour indicates treatment.

Kalgan

Within a week of imposing the intensive grazing treatment pasture FOO had fallen to 1.6t DM/ha. However, by the end of the 4 week grazing period FOO had risen to 3t DM/ha due to late rain lifting pasture growth rates (Figure 3).

Figure 3 Feed on offer (t DM/ha) of kikuyu based pasture at Kalgan in 2019 that was ungrazed, grazed for 2 weeks or grazed for 4 weeks.

Compared to the ungrazed control, 2 and 4 weeks of grazing resulted in a 90% reduction in RLEM numbers. When livestock were removed after 2 weeks grazing, RLEM numbers only increased by 10 % (Figure 4). By early November, RLEM numbers had decreased to less than 100 per square metre in the grazed treatments. By June 2020 the ungrazed treatment had 3 times as many mites as the grazed treatments. Both grazed treatments had an average of 1400 mites per square metre.

Figure 4 Number of RLEM in pasture located at Kalgan that was ungrazed, grazed for 2 weeks or grazed for 4 weeks ± standard error of mean (SEM). Arrows indicate when livestock were removed, colour indicates treatment.

Cranbrook

At the commencement of the demonstration, FOO was low for a spring pasture at 2.5t DM/ha. Grazing for either 2 or 4 weeks only reduced FOO to around 2t DM/ha. At the end of this 4 week period FOO in the ungrazed treatment had only increased by around 0.7t DM/ha, indicating the relatively low pasture growth rates compared to the Boyup Brook and Kalgan sites (Figures 1, 3, 5).

Figure 5 Feed on offer (t DM/ha) of clover, ryegrass based pasture at Cranbrook in 2019 that was ungrazed, grazed for 2 weeks or grazed for 4 weeks.

Unlike the other sites, grazing did not produce a significantly different reduction in RLEM numbers compared to the ungrazed control, as the population had declined in the ungrazed plot due to dry conditions. By mid-October, RLEM numbers had crashed (Figure 6). However, by the end of May 2020, the ungrazed plot had 40% more mites than grazed plots, but by mid-June RLEM numbers across all treatments had dropped below 500 mites per square metre.

Figure 6 Number of RLEM in pasture located at Cranbrook that was ungrazed, grazed for 2 weeks or grazed for 4 weeks ± standard error of mean (SEM). Arrows indicate when livestock were removed, colour indicates treatment.

Conclusion

Intensive grazing in spring appears promising as a tactic for suppressing RLEM for the following season. However, it should only be used when the population of mites is above 5000 per square metre and FOO is more than 3t DM/ha. The aim is to have less than 1000 mites per square metre in the following season. To reduce the number of RLEM’s the pasture needs to be grazed to a FOO of 2t DM/ha or less for 4 weeks around the Timerite® period to achieve a significant reduction in mites the following season.

Acknowledgments

This research has been made possible by funding from the Small Farms, Small grants, Department of Agriculture and Water Resources. The authors wish to thank Fitzgerald Biosphere Group, Southern Dirt, The Gillamii Centre and the farmers on whose properties these demonstrations were conducted.