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The effects of different crop stubbles and straw disposal methods on wintering birds and arable plants. BD1610
Sponsor: DEFRA Countryside Management Division.
Collaborators: British Trust for Ornithology

Overall objective:

To quantify: (1) Seed availability and weed populations on different stubble types under different herbicide regimes. (2) Broad (national) utilisation of different stubble types by birds. (3) Detailed (local) utilisation of different stubble types by birds, in relation to disposal methods and seed abundance. (4) Patterns of seed availability and vegetation composition on sub-samples of fields on a local scale. (5) Agronomic implications of the options trialled in Objective 1. To provide (6) guidelines for policy decisions related to agri-environment scheme

Executive summary

1. There is now a wealth of evidence to link the declines of farmland bird species with agricultural change. A major change has been the switch from spring to autumn cereals, resulting in the loss of winter stubbles (a key foraging habitat for birds) and spring germinating cornfield flowers. The retention of stubbles is likely to be an important conservation measure, particularly for birds, but little is known about the relative cost-effectiveness of different stubble types or their management. In this study we quantify: (i) seed availability and weed populations on different stubble types and herbicide regimes; (ii) national availability and use of different stubble types by granivorous birds and local use in relation to disposal methods, seed abundance; and vegetation composition; and (iii) agronomic implications of stubble retention and management options. We use these data to provide guidelines for policy related to agri-environment schemes and farm management practices.

2. In a split-plot design experiment with different crop types, herbicide regimes and stubble cultivation, weed cover was generally higher in uncultivated stubbles that followed a reduced herbicide crop. The latter resulted in 10-50% higher weed cover in all crops except spring barley where weed cover in the conventional crop was already relatively high. Weed cover was lowest in maize stubble, and consistently lower in winter wheat than spring barley. Weed seeds were more abundant at harvest under reduced herbicide treatments (though there was no effect in oilseed rape where weed seed densities were highest). Spilt grain had largely disappeared by mid-winter. Weed seeds numbers also declined over winter except in spring barley, where they remained constant on both herbicide treatments, and in conventional wheat, where they increased. Weed seed densities at harvest were highest in linseed and rape stubbles. However, reduced herbicide regimes of all crops provided seed densities of about 5000m-2 through the winter and, of the conventional crops, spring barley provided the best seed resources in mid-winter.

3. A national survey of farmland birds revealed that, even in the most 'stubble rich' arable areas of eastern England, cereal, maize, linseed, sugar beet and rape stubbles together comprised only 12% of farmland in early winter (7% in late winter); equating to just two to three fields in a 1-km square (100 ha). Approximately 70% of all stubble is cereal stubble and a high proportion of farmland birds were found on this habitat. No single stubble type supported the most birds, although rape supported high densities in early winter and, within cereal stubbles, barley supported consistently higher densities of birds than wheat, which is harvested later.

4. Intensive studies of stubbles in East Anglia also revealed high densities of birds on rape in early winter. In mid/late winter barley supported the highest, and sugar beet the lowest, densities of buntings and Skylarks, whereas finches, sparrows, thrushes and Starlings were present in highest densities on sugar beet (in one winter) and similar but low densities on barley, wheat and linseed. As at the national scale, no single crop emerged as consistently supporting the highest densities of birds (across winters and species/functional groups) but, within the widespread and long-lasting cereals, barley (spring and winter crops combined) consistently supported higher densities of granivorous birds than wheat.

5. The quality of a stubble field for birds depends on the abundance and accessibility of food within it: crop and weed seeds on the soil surface at harvest and weed seed rain in the stubble phase. In fields studied in East Anglia, weed seeds on the soil surface were most abundant in two broad-leaved crops - sugar beet and oilseed rape. Crop seeds were highest on rape and lowest on winter wheat. Crop seeds, but not weed seeds, showed a large decline in number over winter. Weed seed replenishment through germination and seed set of plants was considerable (up to 30 000 seeds/m2), and a feature of several species that are important in the diet of farmland birds in late winter. There were some consistent differences in seed rain between crop types. It was higher on barley than wheat or linseed and, although it was low in oilseed rape, seed rain in this crop comprised almost entirely broad leaved species important in the diet of farmland birds e.g. Polygonum spp..

6. At both the national and local scale, most stubble fields supported no birds at all. Only a small number held high densities of granivores and the chemical management of the preceding crop was extremely important in explaining this between-field variation. Less frequent spraying, the use of a smaller number of herbicides and/or not using glyphosate prior to harvest were overriding factors explaining differences in the abundance of weed seed in the soil (at harvest and in seed rain). Stubbles preceded by crops subject to less intensive herbicide regimes tended to have higher cover of arable weeds important in the diet of farmland birds, higher seed rain in winter and a higher density and diversity of weed seeds on the soil surface at harvest. Almost 80% of the variation in the number of granivorous birds using a field in mid-winter (expressed in terms of their energy demand) was explained by three factors: the density Chenopodiaceae and Polygonaceae seeds and the number of chemicals used on the preceding crop.

7. Food abundance is modified by accessibility but there were few marked, consistent differences between crops in sward structure (stubble plus regenerating vegetation) that would influence the accessibility of seeds on the soil surface. Thus, it seems likely that food abundance is more important than accessibility in determining use of stubbles by birds. The scope for managing stubbles to improve accessibility of seed is likely to be limited.

8. The field experiment illustrated that spring crops, reduced herbicide programmes and lack of stubble cultivation encouraged more weeds and seed in the stubble phase. Maize is not likely to be a useful stubble crop, whereas oilseed rape, linseed and barley stubbles, especially with reduced herbicide programmes, may provide good resources for birds in winter. Field surveys also showed higher weed seed abundance (seed at harvest and seed rain) in stubbles following crops with reduced herbicide load. Across all crops, broad-leaved weed seed density was highest oilseed rape and, within cereals, it was higher in barley (winter and spring combined) than wheat.

9. The value of stubbles for plants and birds could be maximised by changes in (i) crop type and extent (ii) crop management. Further research is required to investigate the viability of retaining oilseed stubble over winter but this may be a valuable option. More spring cropping should be encouraged, especially spring barley which, under conventional management, provided relatively high levels of food for birds in late winter. However, most stubble types could potentially provide higher food resources for birds in winter if they follow a crop managed under reduced herbicide use. Agronomic information indicates that winter stubbles, whilst resulting in lower gross margins from following spring crops, have few other disadvantages. Adjusting area payments or other incentives could promote spring cropping. Currently, most stubble fields support no birds, almost certainly because they provide little or no food as a result of intensive herbicide regimes in the crop (and stubble) phase. Thus simply retaining stubbles (under agri-environment schemes or in wider farm management practices) will not necessarily provide resources for farmland birds - the preceding crop and stubble must be managed sympathetically. Given the extent of cereals (and the short-lived nature of rape and beet stubbles), the management of this stubble type, rather than promotion of different crop stubble, may be the most cost effective way of enhancing winter food resources for birds at a national scale unless novel schemes were developed for rape or beet. Few agri-environment scheme stubble prescriptions specifically recommend a crop type (beyond a cereal or linseed) or reduced herbicide on the preceding crop (only within the stubble itself). We recommend four possible changes to increase the value of these stubbles for birds: (i) reduced herbicide programmes on preceding crops, (ii) restrictions on the use of pre-harvest glyphosate, (iii) promotion of barley (especially spring barley) over wheat and linseed, (iv) no stubble cultivation.

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