Action: Use mowing techniques to reduce mortality
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- Eight studies investigated the effects of different mowing techniques on wildlife. Seven studies (including four replicated trials of which one randomized, and one controlled and three reviews) from Germany, Ireland, Switzerland and the UK found that using specific mowing techniques can reduce mortality or injury in birds, mammals, amphibians or invertebrates. A review found the UK corncrake population increased around the same period that Corncrake Friendly Mowing schemes were introduced.
- One replicated trial found that changing the mowing pattern reduced the number of corncrake chicks killed. Sixty-eight percent of chicks escaped mowing when fields were mown from the centre outwards, compared to 45% during conventional mowing from the field edge inwards.
- Six studies looked at the effects of using different mowing machinery. Two studies (one review, one randomized, replicated trial) found bar mowers and one report found double chop mowers caused less damage or lower mortality among amphibians and/or invertebrates than other types of mowing machinery. A review found evidence that twice as many small mammals were killed by rotary disc mowers with conditioners compared to double blade mowers. Two studies found that using a mechanical processor or conditioner killed or injured more invertebrates than without a conditioner, however one replicated controlled study found mower-conditioners resulted in higher Eurasian skylark nest survival than using a tedder. A review of studies found that skylark chick survival was four times higher when wider mowing machinery was used, whilst a replicated controlled trial found skylark nest survival was highest when swather mowers and forage harvesters were used.
Mowing and harvesting operations may have a negative impact on farmland wildlife. This intervention involves using different mowing machinery or mowing patterns to reduce the impact of mowing on field-dwelling animals. Adjusting mowing techniques may benefit ground-nesting birds that frequently remain in long grass or crops for as long as possible; mowing from the centre of the field outwards, rather than from the field edge inwards, may allow birds to escape the mowing machinery before the last patch of long grass is harvested.
See also ‘Raise mowing height on grasslands’ for studies that examined the effects of raising the mowing height on grasslands, and ‘Provide refuges during harvest or mowing’ for studies investigating the effects of uncut refuges in mown grasslands.
Supporting evidence from individual studies
A replicated trial from 1992 to 1995 in Scotland and the Republic of Ireland (Tyler et al. 1998) found that a larger proportion of corncrake Crex crex chicks escaped when meadows were mown using a corncrake-friendly mowing pattern, where fields were mown from the centre outwards (inside-outwards) (68% of 76 chicks escaped) compared to the standard outside-inwards mowing pattern (45% of 31 chicks escaped). Fewer chicks were killed by inside-outwards mowing, even when the grass surrounding the plot being mown had already been cut. In plots mown from the inside-outwards, the proportion of chicks that escaped mowing declined as the distance to cover (vegetation over 20 cm tall) increased, and was higher for older chicks. Corncrakes were able to move away from the mower fast enough to escape mowing if an escape route to a refuge area was available, except for the youngest chicks (i.e. less than 2 days old). Hay and silage meadows in one area in Scotland were studied in 1994, and in two areas in Ireland from 1992 to 1995. Corncrakes were seen on a total of 59 meadows. Female corncrakes were radio-tagged to assess movement patterns. Fields were observed during both mowing patterns, and the number of chicks that escaped or were killed recorded. Meadows were searched following mowing to record the number of dead chicks and nest remains.
A 2000 literature review (Aebischer et al. 2000) found that the UK population of corncrakes Crex crex increased from 480 to 589 males between 1993 and 1998 (an average rise of 3.5%/year) (Green & Gibbons 2000) following the introduction of Corncrake Friendly Mowing schemes to increase the number of chicks that survive mowing. Management includes leaving unmown ‘corridors’ to allow chicks to escape to field edges. The reviewers acknowledge that the corncrake population increase and the introduction of these schemes may be coincidental and a longer monitoring period is required to assess the effects of these schemes on corncrake numbers.
Green R.E. & Gibbons D.W. (2000) The status of the Corncrake Crex crex in Britain in 1998. Bird Study 47, 129-137.
A replicated trial in Switzerland from 1996 to 1999 (Fluri & Frick 2002) found seven times more honey bees Apis mellifera were killed or unable to fly when white clover Trifolium repens plots were mown with a rotary mower and mechanical processor (which crushes mowings to accelerate drying) than without a processor (14,000 vs 2,000 honey bees/ha dead or unable to fly respectively). The height of flowers in relation to mower height affected bee survival. Honey bees foraging on phacelia Phacelia tanacetifolia flowers taller than the upper edge of the mower (70 cm) were shaken off and able to escape. Bee losses were higher on two white clover plots where the flower height was 25-30 cm (53% and 62% bees lost after mowing) than on one phacelia plot (flower height not given, but taller than the mower) (35% bees lost). Mowing speed did not have a significant impact on bee losses. Three plots were located on one trial farm, plots measured approximately 0.3 ha. One plot was sown with phacelia in 1996. Two plots were sown with 50% white clover in 1998 and 1999. Five to six honey bee colonies were established adjacent to the plots several days before surveying. Plots were mown with a 1.8 m-wide drum mower with integrated conditioner fixed to the side of a tractor. In 1999 the clover plot was mown with and without the processor. The number of bees before and after mowing was recorded in 1-4 m2 quadrats.
A 2003 report documenting the results of a large-scale agro-conservation project in northeast Germany (Flade et al. 2003) included a review of five studies investigating the impacts of meadow mowing techniques on animal mortality. One study found that a double chop mower killed or injured fewer amphibians than the higher performance rotary drum or disc mowers. Cutting using scythes, considered a more cautious method, still injured more amphibians than a double-chop mower, and killed a similar number of amphibians. Another study found that double-chop mowers preserved more ground beetles (Carabidae), perhaps due to their higher (and variable) cutting height. A further three studies demonstrated the importance of mowing technology for amphibians, invertebrates and nesting birds.
A 2009 literature review (Humbert et al. 2009) of thirteen studies found marked differences in the number of small mammals, reptiles, amphibians and invertebrates killed or damaged by different meadow harvesting techniques. Highest mortality was caused by flail and suction flail mowers, which killed or damaged on average 60% and 49% of the invertebrates studied, respectively. Rotary mowers with a conditioner were the next most damaging, killing/damaging on average 21% of amphibians and 35% of invertebrates studied. Bar mowers were the least damaging, with on average 11% mortality among amphibians and 18% among invertebrates. A single study showed that rotary disc mowers with conditioners caused double the number of small mammal deaths compared to double blade mowers (Oppermann et al. 2000). Three studies reported higher cutting heights were less damaging to field-dwelling animals (Löbbert et al. 1994, Classen et al. 1996, Oppermann et al. 2000). The review notes that later harvesting stages also have a considerable impact, especially the removal of baled grass. The collection of lines of mown grass for baling, had a greater impact on grasshopper (Orthoptera) populations than mowing (Oppermann et al. 2000). The review recommends leaving uncut grass strips when mowing to benefit field-dwelling organisms. It also notes that several of the studies reviewed were poorly replicated or designed.
Löbbert M., Kromer K.-H., Wieland C.C. (1994) Einfluss von Mäh- und Mulchgeräten auf die bodennahe Fauna [Influence of mowing and mulching equipment on ground fauna]. Pages 7-26 in: Forschungsberichte ‘‘Integrative Extensivierungs-und Naturschutzstrategien’’ H. 15. Universität Bonn, Institut für Landtechnik, Bonn.
Classen A., Hirler A., Oppermann R. (1996) Auswirkungen unterschiedlicher Mähgeräte auf die Wiesenfauna in Nordost-Polen [Effects of different mowing equipment on meadow fauna in northeast Poland]. Naturschutz und Landschaftsplanung, 28, 139-144.
Oppermann R., Handwerk J., Holsten M., & Krismann A. (2000) Naturverträgliche Mähtechnik für das Feuchtgrünland [Nature-friendly mowing for wet grassland, preliminary study for E & E projects]. In: Voruntersuchung für das E & E - Vorhaben, ILN Singen, Bonn.
A 2010 review of four experiments on the effects of agri-environment measures on livestock farms in the UK (Buckingham et al. 2010) found one trial from 2006 to 2008 that tested the effect of mowing techniques in reducing mortality of Eurasian skylarks Alauda arvensis nesting in silage fields. Preliminary results showed that chick survival was strongly affected by the type of machinery used. Survival was four times higher using wider machinery and reducing the number of machinery passes than without these changes. However, the number of new birds produced each year (productivity) was more sensitive to re-nesting rates than chick survival. This study formed part of a Defra-funded project BD1454 which is also summarized in (Defra 2010).
A replicated controlled study from 2006 to 2008 in Dorset, UK (Defra 2010) found that the type of silaging machinery used affected Eurasian skylark Alauda arvensis nest survival. Survival rates were highest when swather mowers and forage harvesters were used. Fewer nests were covered by grass when using a swather mower (23% nests covered) compared with bar mowers (60% covered) and fewer nests were abandoned at the egg stage. Survival rates (clutches: 43%, older nestlings: 62%) were highest with swather mowers across the silage harvest. Survival was higher using a mower-conditioner to spread cut grass rather than using a tedder separately. More nests survived forage harvesting than baling (probability of survival 0.83 vs 0.67). During silage collection, most nest failures were caused by being run over by machinery. Between 2006 and 2008, silage fields were subject to either different mowing regimes or normal (control) management. Disc mowers with one, two or three cutting bars were used on all farms, whilst two farms used swather mowers (single bar). Cut grass was normally spread to allow the grass to wilt, or placed in rows which were later collected. Machinery traffic was high during the collection process. Skylark nests were monitored to assess daily productivity, and survival rates and chicks were radio-tagged to assess their survival after fledging. Stochastic simulation modelling was used to investigate the effects on skylark productivity.
A randomized replicated trial in 2007 and 2008 in Switzerland (Humbert et al. 2010) found that harvesting meadow plots using a hand-pushed bar mower killed or injured on average 20% of caterpillars (Lepidoptera) added to plots before mowing, compared to 37% when using a tractor-pulled rotary drum mower, and 69% if a conditioner was attached to the rotary mower. Using a conditioner also increased the proportion of damaged wax invertebrate models from on average 11% (bar mower, no conditioner) to 30% (rotary mower with conditioner). Large (4 cm) invertebrate models were damaged more often than small (2 cm) models. Caterpillars and models placed on the ground or in vegetation (30 cm high) before mowing were affected differently by mowing treatments. Organisms on the ground were strongly impacted by tractor wheels, whilst those in vegetation were damaged by the mower/conditioner. Cutting height did not affect mortality in this study, but the authors note it is likely to be important for larger animals. Nine meadows were studied, with four 2.5 m-long plots each. There were four mowing treatments: hand-pushed bar mower (cutting height 6 cm, 1.7 m-wide), tractor-pulled rotary drum mower (2.5 m-wide) with two cutting heights 9 cm and 6 cm (one with, one without conditioner). Before mowing, 50 small and 50 larger wax invertebrate models were placed either on the ground or tied to vegetation 30 cm high. In 2008 on five meadows, large white butterfly caterpillars, Pieris brassicae, were placed on the ground (50 caterpillars) and in the vegetation (50 caterpillars). After mowing, both wax models and caterpillars that survived mowing were checked for injuries.
- Tyler G.A, Green R.E. & Casey C. (1998) Survival and behaviour of corncrake Crex crex chicks during the mowing of agricultural grassland. Bird Study, 45, 35-50
- Aebischer N.J., Green R.E. & Evans A.D. (2000) From science to recovery: four case studies of how research has been translated into conservation action in the UK. Pages 140-150 in: J.A. Vickery, P.V. Grice, A.D. Evans & N.J. Aebischer (eds.) The Ecology and Conservation of Lowland Farmland Birds. British Ornithologists' Union, Tring.
- Fluri P. & Frick R. (2002) Honey bee losses during mowing of flowering fields. Bee world, 83, 109-118
- Flade M., Platcher H., Schmidt R. & Werner A. (2003) Nature conservation in agricultural ecosystems: results of the Schorfheide-Chorin Research Project. Quelle & Meyer.
- Buckingham D.L, Atkinson P.W., Peel S. & Peach W. (2010) New conservation measures for birds on grassland and livestock farms. Proceedings of the Lowland Farmland Birds III: delivering solutions in an uncertain world, 60.
- Defra (2010) Modified management of agricultural grassland to promote in-field structural heterogeneity, invertebrates and bird populations in pastoral landscapes. Defra BD1454 report.
- Humbert J.Y., Ghazoul J., Sauter G.J. & Walter T. (2010) Impact of different meadow mowing techniques on field invertebrates. Journal of Applied Entomology, 134, 592-599