Response to disturbance and plant-animal interactions of grassland swards for chicken free-range husbandry

The aim of the present study was to evaluate the suitability of fourteen grassland plant species for establishing swards for free-range chicken husbandry. Nine grasses and five forbs which are known for their good performance in intensively managed grasslands and for their high tolerance towards frequent grazing and cutting, and to trampling were used. In a field experiment, we tested the tolerance of these plants to stocking with chickens. We hypothesized that the performance of the individual species would vary strongly, because the disturbance affected by chickens is particularly heavy and differs distinctly from the conditions commonly found in other systems of grassland management. Monoculture swards of the fourteen tested species, and one mixture comprising all of these species were subjected to rotational stocking with chicken at three levels of stocking duration. The effect of plant species, stocking duration, and stocking cycle on the canopy cover, density of tillers or vegetation growing points, and growth rates of aboveground biomass were analysed. From the former two target parameters, resistance of the plant species to stocking with chicken was derived; from the latter, resilience was estimated. Our data confirmed the assumption that the tested species differ strongly with respect to their resistance and resilience to stocking with chicken, and indicated that grasses are generally more suitable than forbs for greening an outdoor chicken run. The species Festuca arundinacea and Poa supina featured the best performance, with a canopy cover of over 80 % and constant tiller densities, and with a stable and comparatively high biomass accumulation of over 4.8 g dry matter m-2 d-1 after repeated long-duration stocking. By means of a supplemental pot experiment under greenhouse conditions we aimed at elucidating whether traits related to plant growth form give an explanation for tolerance to the disturbance affected by stocking with chicken. We hypothesized that pre-disturbance biomass allocation, and the location of storage and clonal growth organs would be important determinants of damage tolerance. We expected that tolerance would be high in plants with these organs concentrated in a tussock or located close to or underneath the soil surface, and with high allocation to storage organs in root or tussock. A selection of the species used in the field experiment which represented three growth forms (cespitose, stoloniferous, and rhizomatous) were subjected to three levels of a standardized damage treatment impacting simultaneously on shoot and root. Our results showed that plant species, but not growth form groups differed significantly in damage tolerance, and that F. arundinacea and Poa pratensis showed the best performance. Shoot re-growth was strongly correlated with relative post-treatment root biomass, i.e. with the ratio of root biomass of the treated plants in relation to that of the non-treated control across all species and treatment levels, but not with pre-treatment root biomass. Root resistance to mechanical damage therefore appears to be the prevalent determinant of tolerance to the applied type of disturbance. In a field experiment we thirdly investigated the impact of sward botanical composition (sward type) and sward degradation on the foraging behaviour of chickens. Monoculture swards of nine grasses and five forbs and a mixture of these fourteen species, which had also been used for the first experiment, were stocked with chickens at three levels of stocking duration. The frequency of the animals’ plant pecking, ground pecking and scratching behaviour were recorded. Our results showed that sward type and sward degradation had a significant effect on the foraging behaviour of the chickens. Prolonged stocking significantly reduced the canopy cover of green vegetation; however, the tested sward types strongly differed with regard to the degradation caused by standardised levels of stocking duration. Our data revealed that the frequency of total sward-directed pecking (plant and ground pecking together) declined with prolonged stocking and with decreasing canopy cover in most of the sward types. This result may be relevant for considerations on animal health and welfare, because in environments which offer little incentive for object-directed pecking, there is the risk of pecking to be re-directed towards conspecifics and hence for feather and injurious pecking to occur. We derive from our results that the choice of plant species for sward establishment is a strong determinant of the animals’ behavioural interactions with the sward. Species like P. supina which feature a high resistance to the damaging effects of stocking with chickens and which at the same time provide high incentive for foraging behaviour, particularly for plant pecking, benefit the contribution of the outdoor run to animal health and welfare. In synopsis of these three experiments, our results have shown that the choice of plant species for greening outdoor chicken runs is a relevant factor for the realisation of multi-functional swards for outdoor chicken runs. Plants which feature a high canopy cover and biomass accumulation under conditions of prolonged long-duration stocking enhance the potential of the sward for soil and water protection in view of the high amounts of excrement deposition in the run area. A sward composition which additionally highly triggers sward-directed pecking behaviour in the chicken can foster animal health and welfare. Among the plants we tested within the present study P. supina altogether featured the best results with regard to these criteria; F. arundinacea showed a high resistance to stocking with chickens, but was comparatively less attractive for pecking than the other species. We conclude that the choice of plant species is a promising complementary measure of run management in addition to established practices like rotational grazing with rest periods of sufficient duration and to providing incentive for the animals to evenly use the total of the run area.