Carol Marriott1 and Titus Barthram2
Macaulay Land Use Research Institute,
1 Craigiebuckler, Aberdeen AB9 2QJ, Scotland, UK
2 Hartwood Research Station, Shotts, Lanarkshire ML7 4JY, Scotland, UK
Materials and methods
Discussion and conclusions
Farmers are likely to respond to changes in EU environmental and agricultural policies by adopting less intensive animal production systems. Fertilizer N application may be reduced or stopped completely and numbers of grazing animals may be reduced. An experiment was set up to determine the effects of cessation of fertilizer and reduced grazing on soil nutrient status, the floristic composition of the sward and the characteristics of the grazed herbage (Marriott and Grant, 1990). Dynamics of leaves of the dominant species in the sward are presented here. Since the performance of individual plant species affects competitive ability, such knowledge will help to explain the changes in species composition which develop over time in extensive systems.
The experiment was set up in 1991 on a perennial ryegrass/agrostis (Agrostis capillaris)/white clover sward on a dry peaty podsol near MLURI's Glensaugh Research Station in NE Scotland. The six treatments, each replicated twice, had different combinations of seasonal grazing and fertilizer application:
1) sward height of 4 cm and 140 kg N ha-1 year-1 (with 40 kg N ha-1 year-1 as 20:10:10)- treatment 4 cm N1.
2) sward height of 4 cm and no fertilizer - 4 cm N0.
3) sward height of 8 cm and no fertilizer - 8 cm N0.
4) sward height of 4 cm until end of September, then 8 cm in winter and no fertilizer.
5) sward height of 8 cm until end of September, then 4 cm in winter and no fertilizer.
6) no grazing and no fertilizer.
Treatments were continued each year from 1991. In 1993 measurements of leaf dynamics of ryegrass, agrostis and white clover were made on 5 occasions, commencing 11 May, 15 June, 27 July, 7 September, and 5 October. Individual tillers of grasses and stolons of clover were marked and leaf number and length of green and senescent material recorded at the time of marking and again one week later. Rates of leaf appearance, senescence and clover petiole and grass lamina extension were calculated for each measurement period.
Data from treatments 1-3 only are presented and these show that there are differences in leaf dynamics between species and their responses to treatments. Mean leaf appearance rates over all treatments were similar for the three species and did not differ with sward treatment (Figure la). Seasonal differences in leaf appearance rates (P < 0.001) were such that the rate of new leaf production for all species increased until July. Thereafter production of new leaves on clover remained high until September, while that for the grasses was significantly less. By October there were no longer any species differences in the rates of leaf production.
Grasses and white clover maintained different numbers of green leaves (P<0.001); agrostis had a higher number of leaves than ryegrass from June onwards (P<0.001) and, on average, white clover had around one green leaf per growing point less than the grasses (Figure 1b).
Rates of leaf extension differed significantly between all species (P<0.001) and were highest (P<0.001) at the measurement dates in June and July. In all treatments ryegrass had a higher leaf extension rate than agrostis (Figure 1c). There were seasonal differences in leaf senescence rates (P<0.001) such that leaf senescence on agrostis was faster than that on ryegrass in June while the reverse occurred in July and September. Petiole extension rate of clover was less than half the rate of leaf extension of ryegrass and, as shown in Figure Id, there was little senescent tissue present compared with the grasses (P<0.001).
Grasses can have a higher component of leaf material in the sward by having faster initiation and extension of new leaves, retaining more leaves per tiller and having slower senescence rates. In swards maintained at 4 cm, both with and without fertilizer, agrostis tended to produce leaves faster, held a greater number of leaves per tiller and had a slower rate of leaf senescence. These results suggest that leaf tissue may persist longer on agrostis than on ryegrass. With regard to nutrient conservation, Thomton et al. (1994) found that more N was retained in roots and stubble after defoliation in Highland bent (A. castellana) than ryegrass. Such conservative strategies would give an advantage to agrostis, particularly under conditions of low fertility. We found that ryegrass had a higher rate of leaf extension; although there was no difference in the proportion of grazed leaves per tiller, grazed leaf lengths were greater for ryegrass than agrostis (unpublished data). Clover petiole extension rate meaned over the season was less than the extension rate of grass laminae, but in mid summer similar rates were found for agrostis and clover (unpublished data). These results agree with the observations of Barthram and Grant (1994) that differences in surface heights of ryegrass and white clover were less in mid summer.
Between 1991 and 1993 the percentage contribution of agrostis to the sward increased by between 11 and 27%, while ryegrass showed a similar decline; clover showed little change and rarely exceeded 5% of the sward (unpublished data). Our preliminary data suggest that differences in leaf dynamics may contribute to the change in species composition.
We thank Geoff Bolton for help in making measurements. The research was funded by the Scottish Office Agriculture and Fisheries Department.
MARRIOTT, C.A. and GRANT, S.A. (1990). Plant species balance in sown swards in low input and extensive grazing systems. Macaulay Land Use Research Institute Annual Report for 1989-90. pp. 24-30.
THORNTON, B., MILLARD, P. and DUFF, E.I. (1994). Effects of nitrogen supply on the source of nitrogen used for regrowth of laminae after defoliation of four grass species. New Phytologist. 128, 615-620.
BARTHRAM, G.T. and GRANT, S.A. (1994). Seasonal variation in growth characteristics of Lolium perenne and Trifolium repens in swards under different managements. Grass and Forage Science, 49, 487-495.
Figure 1a Leaf appearance rate
Figure 1b Number of green leaves
Figure 1c Leaf extension rate
Figure 1d Leaf senescence rate