Herbage selection and landscape utilisation by suckler cows on a sub-alpine pasture
Meisser M.1, Chassot A.2 and Freléchoux F3.
1Research Station Agroscope Changins-Wädenswil ACW, CH-1260 Nyon 1, Switzerland
2Research Station Agroscope Liebefeld-Posieux ALP, CH-1728 Posieux, Switzerland
3Swiss Federal Research Institute WSL, Case postale 96, CH-1015 Lausanne, Switzerland Corresponding author: marco.meisser@acw.admin.ch
Abstract
In many Alpine countries, the use of mountain pastures for cattle is in decline, thus favouring the development of shrubs in these areas. The objectives of this study were to assess the herbage selection and landscape utilisation by suckler cows on a pasture located at 1,800 m a.s.l., and covered by 31% of shrubland and forest. Vegetation surveys were performed during the first rotation to determine the browsing frequency of each herbaceous plant species. In addition, four cows were monitored by Differential Global Positioning System (DGPS) for a period of 3 to 5 days during each rotation (three in total).
The botanical observations showed that the largest consumption of herbaceous plants was not necessarily among those known to be the most palatable. Among woody species, cattle willingly browsed young branches of green alder (Alnus viridis (Chaix) DC.). As long as the height of these plants did not exceed 1.5 m, a grazing intensity of 70 LU days ha-1 was
sufficient to prevent their expansion. The GPS monitoring showed that the animals roamed all areas of the plot, even in zones of high shrub density. The vegetation types known to be of low forage value were heavily visited by cows early in the season.
Keywords: Alnus viridis, mountain pasture, vegetation dynamics, GPS tracking, herbage selection, grazing intensity.
Introduction
The cessation or the reduction of pasture grazing leads to an alteration in the botanical
composition of grasslands and to encroachment. Thus, numerous summer pastures in the Alps have become increasingly overgrown by Alnus viridis. This pioneer species competes very well on the deep and nutrient-rich soils. Due to the symbiotic nitrogen fixation, it is often associated with tall nitrophilic herbs. Freléchoux et al. (2007) described the secondary succession pathways and the loss of species diversity that occur in the case of pasture
abandonment. However, the effects of browsing on the growth of green alder have been rarely investigated and little is known about plant selection by grazing livestock in shrubby alpine pastures. The intention of this study was (i) to describe the herbaceous plant selection by the cows, (ii) to assess the effects of frequent browsing on Alnus viridis, and (iii) to define the relationship between the vegetation mosaic and the spatial distribution of cattle.
Materials and methods
The experiment was conducted between 2005 and 2007 on a summer pasture located in the canton of Valais (Switzerland), at 1,800 m a.s.l. The herd was comprised of 9 dry cows and 9 lactating cows with their calf ('Hérens' breed). Cattle were managed in a rotational system, with two or three utilisations per season. The observations took place in two paddocks (about 2.5 ha each) located on the northern slope of the pasture. Botanical composition and herbage
selection were determined according to the method of Daget and Poissonnet (1969), along sixteen transects distributed among the two paddocks. The relevés were made in 2006 during the first rotation, two or three days after the animals began to graze. At each contact point, the hit species were recorded as 'browsed' or 'not browsed'. Based on these data, the relative frequency in the herbage and the proportion among all consumed plants was calculated for each species. The selection index (SI) for the different species was then obtained by dividing the percentage in the consumed herbage by the percentage in the available herbage. To assess the effect of repeated browsing on green alder trees, thirty-nine of them were marked in two distinct zones and plant height and canopy area were measured over the three year period.
The GPS tracking was conducted in 2007 in one of the two paddocks for a period of three to five days over each of the three rotations. Four animals (two lactating and two non-lactating cows) were fitted with a customized GPS tracking harness. After each observation period, the data were downloaded onto a PC and differentially corrected using virtual reference stations (VRS). The GPS data were crossed with a simplified vegetation map by means of a
geographic information system (GIS). Four main vegetation types were considered: tall herb community, open pastures, half-open pastures and shrubby areas dominated by Alnus viridis.
Lastly, the presence index (PI), defined as the ratio between the occupation time in a vegetation unit (in % of total recording time) and its surface (in % of total surface), was calculated. The statistical analysis of the PI was performed with SYSTAT 12. The analysis was made with a linear mixed-effects model (LME) using maximum likelihood procedures (REML).The raw data were log-transformed to reduce heteroscedasticity. A first analysis was run with a complete model, in order to test the significance level of the different factors.
Three additional analyses were then performed and the non-significant terms successively removed ('rotation', 'cow' and interaction 'cow' × 'vegetation type'). Finally, a simplified model using only the factors 'vegetation type' and the interaction 'vegetation type' × 'rotation', was retained.
Results and discussion
Vegetation in the sixteen transects was dominated by forbs (44%) and graminoids (44%).
Overall, the most eaten herbaceous plants were the graminoids (SI of 1.3), followed by the forbs (0.8) and the legumes (0.7). These results confirm the major contribution of graminoids to the nutrient supply of the cattle. The relatively low SI of the legumes can be explained by the small size of the encountered species, primarily Trifolium repens L., T. pallescens Schreb.
and T. badium Schreb., that protected them from a selective grazing. The most eaten
herbaceous plants were not those known to be the most palatable. Thus, the highest selection indexes were observed for Leontodon hispidus L. (1.9), Agrostis capillaris L. (1.4), Festuca rubra L. (1.4), Luzula sylvatica aggr. (1.3), Anthoxanthum alpinum Á. & D. Löve (1.3) and Chaerophyllum villarsii W. D. J. Koch (1.3). Conversely, Veratrum album L. (0.1) and Trollius europaeus L. (0.1) were not surprisingly the least eaten plants. The method used in the present study proved to be useful in making comparisons between species or functional groups. However, it is important to bear in mind that these values depend heavily on the botanical composition and the developmental stage of the plants.
Repeated browsing of the green alders resulted in a weakening of the shrubs, by diminishing their height as well as their canopy area (Table 1). Their localisation in the paddock had no influence on the results. In this study, a grazing intensity of 70 LU days ha-1 proved to be sufficient to prevent scrub expansion. The grazing, however, only impacted the small shrubs.
When the bushes exceeded 2.0 m in height, the expansion process was not reversible: the cattle are too small and the growth of the alders too fast.
Table 1. Evolution of the height and the canopy surface of the green alders after repeated browsing. ANOVA with repeated measurements; n = 39.
Year Significance
2005 2006 2007 Year Zone Y × Z
Height (cm) 122 119 109 *** ns ns
Canopy area (m2) 0.40 0.43 0.32 *** ns ns
*** P < 0.001; ns = non significant.
The presence indexes (PI), calculated on the base of the GPS-monitoring, are presented in Figure 1. The tall herb community was heavily visited early in the season (June), when the quality of the forage was still acceptable. This remains a hypothesis, nevertheless, since it is difficult to infer grazing activity from occupation time with exactitude. The main selected species in this vegetation type were Poa chaixii Vill. and Peucedanum ostruthium (L.)
W. D. J. Koch. The PI of the open zone were significantly higher than the average value. This is mainly due to the presence of many appreciated species (e.g. those mentioned above) but also to the fact that the resting places in this area were overrepresented. The half-open pastures remained attractive during the entire season. Typical cattle feeding patterns showed an alternation between herbaceous plants and young branches of small green alders. The shrubby areas dominated by Alnus were the least visited of the four areas.
-0.50 -0.25 0.00 0.25 0.50
1 2 3
Log(PI)
Rotation
Tall herb community Open pastures Half-open pastures Shrubby areas
**
*
ns
***
*
*
ns
***
ms
ns
ns
***
Figure 1. Effect of the vegetation type and the rotation number on the presence index (PI).
The data have been log-transformed. The zero value corresponds to the mean occupation time.
The values significantly different from zero are indicated on the graph; ns not significant; ms 0.05 < P < 0.1; *P < 0.05; **P < 0.01; ***P < 0.001.
Conclusions
The results confirmed that the selection process in shrubby subalpine areas is important.
Cattle consume a large spectrum of plants. New light of the reforestation process in terms of cattle feeding opportunities have been gained from this investigation.
References
Daget P. and Poissonnet J. (1969) Analyse phytologique des prairies, applications agronomiques, Document 48, CNRS-CEPE, Montpellier, France, 67 pp.
Freléchoux F., Meisser M. and Gillet F. (2007) Succession secondaire et perte de diversité végétale après réduction du broutage dans un pâturage boisé des Alpes centrales suisses. Botanica Helvetica 117, 37-56.
