FLUCTUATIONS IN WHITE CLOVER CONTENT OF SHEEP GRAZED SWARDS - M. Fothergill, D.A. Davies and C.T. Morgan

Institute of Grassland and Environmental Research, Bronydd Mawr Research Station, Trecastle, Brecon, Powys LD3 8RD, United Kingdom

SUMMARY

Three treatments of a long-term grazing experiment namely: CaPKN, CaPK and no nutrients (nil inputs), were chosen to illustrate the role of white clover (Trifolium repens) in swards of widely differing nutrient status. The balance of two indicator grass species (Lolium perenne and Agrostis capillaris) was also used to describe the swards in terms of low or high input status. Both CaPKN and nil input treatments produced swards consistently dominated by a single grass species and low clover stolon abundance. Lolium perenne dominated the CaPKN treatment whilst Agrostis capillaris was heavily dominant in the nil input treatment. The CaPK treatment produced a much higher presence of clover within the swards but two major declines were identified over the eight-year period 1991-1998. These declines affected the grass species balance indicating that the CaPK treatment cycled between high and low input status. However, the changes in grass species balance were more gradual than the violent fluctuation in clover stolon abundance and caused longer-term cycles of animal production.

Keywords: Agrostis, clover declines, lamb production, Lolium, low inputs, Trifolium repens

INTRODUCTION

The attainment and maintenance of high levels of white clover within grazed swards, over an extended period of time, remains an elusive goal for farmers. Sheep grazing, in particular, seems to generate cyclic patterns of white clover performance and it has been suggested that the periodic fluctuation in legume performance/content and total herbage production may actually be indicative of a grass-clover system that is working well, maximizing grass exploitation of clover nitrogen fixation.

METHODS AND MATERIALS

An experiment was set up in 1990 on a 25-year old perennial ryegrass/Bent (Lolium perenne/Agrostis capillaris) dominant pasture, situated 370-390 m above sea level to study the effects of the elimination of a) N; b) N, P and K; and c) N, P, K and Ca, compared with a treatment which received all four nutrients. The details of the experiment were presented in a complementary paper by D. A. Davies et al. at this conference. The plots were stocked with ewes and lambs from early May until weaning in early August and thereafter with ewes only until the end of October. Sward surface height was maintained at 4±0.5 cm throughout the growing season by regular adjustment of animal numbers. Plant densities were monitored each year during the spring and autumn by lifting and dissecting twenty 5 cm diameter cores from each plot. The white clover was of a semi-natural wild white type.

RESULTS AND DISCUSSION

In this instance, three treatments were chosen to represent swards with different levels of inputs a) nil input; b) CaPKN (150 kg N ha^-1); and c) CaPK treatments.

Plant Densities

Determining the abundance of white clover stolon (metres of stolon m^-2) assessed the presence of clover within the swards. The status of the sward was also determined by assessing the balance of two indicator grass species Lolium perenne (considered to indicate high input status) and Agrostis capillaris (considered a low input species). The data is represented graphically in Figure 1 for the period 1991-98. Combined applications of Ca, P, K and N suppressed the abundance of white clover stolon as did complete lack of applied inputs. However, applications of Ca, P and K, although producing swards with a significantly higher abundance of clover, created large fluctuations in clover content. During the eight years under study, two distinct clover declines were identified. Clover stolon declined by 68 percent during the winter of 1994/95 and by 55 percent during the summer of 1997. The balance of tiller densities of the two indicator species (Agrostis tiller density m^-2 - Lolium tiller density m^-2) clearly indicated the situation in both low (dominated by Agrostis) and high (dominated by Lolium) input swards (Figures 1a and 1b, respectively). In each case the low stolon abundance of white clover suggested that the clover component of both these sward types had little effect on the species balance. However, applications of Ca, P and K created a system that had very high white clover stolon abundance and seemed to combine elements from both high and low input systems. Initially the high clover abundance supported a high ryegrass tiller density but as the clover declined the balance of the grass species swung towards Agrostis dominance. Clover quickly built up again following the decline but this second high clover phase was associated with swards demonstrating characteristics of a low input system. Thus, the second clover decline occurred at a time when Agrostis dominated the sward.

Table 1. Lamb output (kg ha-1) 1991-1997.

Animal production

The animal production data (lamb output from spring to weaning) is given in Table 1. Lamb output from the nil input swards remained low for the duration of the experiment. During the first clover phase lamb output from the CaPK swards rivalled that of the CaPKN treatment. However, the animal production of the two treatments started to diverge even before the clover decline was detected. The CaPK treatment remained significantly less productive not only during the clover decline but also through the next increase in clover and in this instance the productivity of the system seemed to reflect the Agrostis/Lolium balance rather than the clover stolon abundance. Clover abundance attained a second peak in the spring of 1997 and this coincided with a second peak in lamb production from the system.

Figure 1. Tiller difference m^-2(Agrostis – Lolium) together with clover stolon abundance (m stolon m^-2) for: a) nil input

Figure 1. Tiller difference m^-2(Agrostis – Lolium) together with clover stolon abundance (m stolon m^-2) for: b) CaPKN

Figure 1. Tiller difference m^-2(Agrostis – Lolium) together with clover stolon abundance (m stolon m^-2) for: c) CaPK treatments 1991-1998.

CONCLUSIONS

There is little doubt that the cyclic pattern of grass dominance identified in the study is controlled by a fluctuating clover component. The changes in clover stolon abundance are sudden and dramatic yet the subtle interplay of the two indicator grass species showed a much slower rate of change and in this respect created longer-term cycles of animal production. The clover component declined within swards that exhibited different characteristics and thus the mechanism of decline was not obvious. Fluctuations in clover content of swards have been attributed to the changes in grass/clover competition based on the nitrogen fixing ability of white clover (Swimming & Parsons, 1997). However, if this was always the case, clover would decline when swards were dominated by ryegrass. It is well known that clover plants fragment into large numbers of small plant units during the winter and build-up size, complexity and stolon thickness during the latter part of the summer (Fothergill et al., 1997). We suggest that this cyclic pattern of change in clover morphology was driven into a spiral of decline by the intensive grazing regime of the experiment.

Our challenge for the future will be to try to deflect the clover declines, to constantly maintain high clover stolon abundance and a productive grass species balance.

Our tools will include the use of new clover cultivars, with improved persistence and tolerance to nitrogen, and a dynamic attitude towards sward management.

REFERENCES

Davies, D.A., Morgan, C.T. & Fothergill, M. 1998. Effect of nutrient input on the sustainability of sward composition and animal productivity. Ibid

Fothergill, M., Davies, D.A. & Daniel, G.J. 1997. Morphological dynamics and seedling recruitment in young swards of three contrasting cultivars of white clover (Trifolium repens) under continuous stocking with sheep. Journal of Agricultural Science, Cambridge, 128, 163-172.

Schwinning, S. & Parsons, A.J. 1997. Interactions between Grasses and Legumes: Understanding Variability in species composition. In: Younie. D. (Ed). Legumes in Sustainable Farming Systems, Occasional Symposium of the British Grassland Society, No.30, pp 153-163.