The role of farmyard manure in the maintenance of botanical diversity in the traditionally managed hay meadows of the Pennines

Abstract

Traditionally managed hay meadows of the Pennines are highly prized for their botanical diversity. In recent decades there has been a drastic reduction in these meadows because of the negative effects on botanical diversity of widespread management changes, such as the application of inorganic fertiliser and the switch from hay to silage production. The consequences of changes in grazing regime, cut date and inorganic fertiliser application have been well documented. However, although farmyard manure application is an integral part of long term meadow management it has as yet received less scientific attention. The application of farmyard manure provides a possible mechanism by which seed contained within hay cut from the meadows can subsequently be returned to the meadow. The dispersal of seed and establishment of plants within traditionally managed hay meadows is an important area of study because of the problems associated with attempts to re-create meadows in the Pennines and throughout the UK, an important goal of agri-environment schemes. Field, glasshouse and laboratory experiments are described which attempt to establish the viable seed content in samples of hay from botanically diverse meadows and how subsequent feeding to livestock affects the viable seed content of the farmyard manure produced. The experiments also explored the extent to which seed remained viable during the time that manure was stored prior to spreading, thus affecting its capacity to germinate once spread or to become incorporated into the soil seed bank. The spreading of farmyard manure has often been observed to create bare patches within the meadow vegetation. It was therefore hypothesised that these patches may be suitable niches for the germination of seeds from within the manure itself or from the soil seed bank, and this was investigated using field experimentation. Comparison of the original vegetation with the viable seed content of hay, manure and dung from cattle fed exclusively on meadow hay from two traditionally managed sources produced some interesting results. The meadow vegetation contained a range of prominent perennial herbs such as Geranium sylvaticum, Sanguisorba officinalis and Filipendula ulmaria which are considered to be important from a conservation management viewpoint. However, germination of the seed within the hay cut from these meadows failed to provide any evidence for the occurrence of such species within the hay. Either the typical mid July hay cut comes before these species produce ripe seed or the seed was lost from the hay during the turning and baling operations. Viable seeds of the early flowering annual herb Rhinanthus minor, which is also considered by ecologists to be desirable, were also absent from the hay. The most abundant species in the hay were generally grasses, with Poa trivialis especially prominent, and less important, often annual, herb species. The feeding of this hay to cattle and subsequent germination of seed within the dung showed that the digestive system of the cattle was extremely damaging to seed viability. Proportionally P. trivialis became even more dominant within the dung samples with many of the less common species in the hay failing to have any viable seed within the dung. In contrast to the dung, the fresh manure collected from the two farms contained a larger quantity of seed, and more species than the dung. These samples were in fact more comparable to the hay samples in terms of species composition although not quantity. This suggests that the majority of the viable seed which is incorporated into manure does not pass through the digestive tract of the livestock, but rather falls directly from feed racks onto the barn floor. Confirmation of the negative effect of the digestive processes of cattle on seed viability was achieved in a laboratory experiment. A three stage process was used to mimic the effects of chewing, rumen digestion and post ruminal digestion. Five species were tested; two grasses P. trivlalis and Anthoxanthum odoratum which occurred within the hay samples, the annual herb Myosotis arvensis which was found within hay samples and two perennial herbs F. ulmaria and S. officinalis which were absent from the hay samples. Whilst the effects of chewing and ruminal digestion varied between the five species tested the post ruminal digestion was extremely damaging in all five species. These results suggested that the prominence of P. trivialis in dung samples was due more to the high quantity of seed within the hay rather than to any increased ability to survive digestion. The perennial herbs, F. ulmaria and S. officinalis were able to survive digestion at least as well as P. trivialis and so given appropriate management of hay cut timing could become incorporated into manure for subsequent dispersal via this route. By comparing the viable seed content of fresh" manure with that in samples stored for 3 months, 6 months and 12 months it was clear that the seed content of the manure did not reduce until it had been stored for a period longer than 6 months. Once again those species with the greatest quantity of seed in fresh manure were the ones which were able to survive within the older manure samples. The burial in manure of known quantities of the same seeds as used in the laboratory digestion experiment confirmed this result, and also showed that herb species absent from the hay and manure are at least as capable of survival within manure heaps as species such as P. trivialis which dominated the manure samples collected from farms. In order to assess the role that manure application to meadows may have in supplying seed to the soil seed bank, soil cores were extracted from the meadows at the two farms used in the study and the seed content estimated by seed germination. The soil seed bank contained all of the species found within the manure samples as well as a range of earlier flowering species including the annual herb R. minor. Again the longer lived perennial herb species valued from a bio-diversity perspective were largely absent from the soil seed bank or only present in very low quantities. The species make up of the seed bank within these diverse hay meadow communities was found to be more akin to the species make up of species poor pasture communities. This suggests that seed set by many of the ecologically desirable species is not a regular occurrence within Pennine meadows. The large quantity of seed of certain species found within manure applied to the meadows means that it could play a significant role in the build up of seed within the soil seed bank, although these species may be expected to be recruited to the soil seed bank via other routes such as seed rain during the period of crop growth and the hay making operations. Field experimentation with observation of the colonisation of gaps created by farmyard manure application showed that vegetation colonising these gaps did not compare to the seed content of the manure or the soil seed bank, but rather the surrounding vegetation. Colonisation of these gaps did not give rise to vegetation containing a higher quantity of species indicative of the manure or soil seed content. Perennial herb species absent from both the manure and the soil seed bank were amongst the colonisers of the these gaps suggesting that vegetative re-growth is an important process in the maintenance of long term botanical diversity within such meadows. Whilst it remains possible that during years when the hay cut is later than usual, seed production and subsequent incorporation into the hay of more desirable later flowering herbs could occur, the overwhelming weight of evidence suggests that the farmyard manure from cattle fed meadow hay is not a significant contributor to the dispersal of ecologically valuable components of meadow vegetation. Indeed, the results suggest that seed production may be less important than the vegetative propagation of many species.