Frédérique Louault, Jean-François Soussana, Pascal Carrère and Marina Visarion*.Laboratoire Fonctionnement et Gestion de l'Ecosystème Prairial, INRA-Agronomie, 12 avenue du Brézet, Clermont-Ferrand, France.
* Present address: Institut Agronomica N. Balcescu, Bucharest, Romania.
Within grazed mixtures the balance between clover and grass is affected both by competition for light and by selective defoliation. Selective defoliation could play a major role under continuous sheep grazing, whereas rotational grazing promotes competition for light. Using mixed swards sown in alternate lines, we undertook a comparison between these two contrasting grazing managements.
At spring 1990, perennial ryegrass (Lolium perenne var. "Preference") and white clover (Trifolium repens var. "Grasslands Huia" or "Gwenda") were sown in alternate rows (14 cm between two rows) on a granitic soil at Theix (850 m height) in the Massif-Central. In 1991 and 1992, these swards (8 plots arranged in two complete blocks) were grazed by sheep:
- Under continuous grazing (C) (four 1 000 m2 plots) sward height was kept at (7.5 ±0.5) and (6.5 ±0.5) cm in 1991 and 1992, respectively.- Under rotational grazing (R) (four 500 m2 plots) defoliation occurred approximately every 4 weeks.
From April to July, several biometric parameters were determined on the plant material from randomly sampled cores: tiller and shoot growing point numbers, clover and grass shoot dry-weight components, specific leaf area and specific leaf, sheath and stolon length.
Growth, senescence and intake rates were measured on marked organs (grass: 40 tillers; clover: 40 elongated shoot growing points and 40 axillary growing points pa-plot). These were observed in April-May and June-July every 3 to 4 days (C), or during the start and at the end of the regrowth period (R). Measured changes in leaf lamina length (ryegrass), leaf lamina area (by visual comparison with an area scale), sheath, petiole and stolon length were multiplied by the specific length and specific area coefficients determined through destructive sampling at random. By multiplying these individual turn-over rates (mgDW. organ-1 day-1) by tiller and growing points numbers per m2, tissue turn-over rates (gDW.m-2.day-1) were calculated. Individual clover plant architecture was described using a technique similar to that reported by Brock et al. (1988) on two (40* 40) cm2 cores per plot in March and May 1991 and in May 1992.
Under both grazing managements, clover cultivar did not result in significant differences in tissue turn-over rates or in individual clover plant architecture (data not shown). Therefore, both cultivars were confounded for data presentation.
During the four studied spring and summer grazing periods, growth, senescence and intake rates per tiller or per shoot growing point (GP) were significantly larger under rotational grazing (Fig. 1). Correlatively, tiller and shoot growing-point mean sizes declined gradually under continuous grazing (data not shown). During 1991, sward establishment resulted in mean tiller density increases under both grazing managements. This trend was, however, not observed with GP densities (Fig. 2). Owing, presumably, to the large distance between grass rows (28 cm) continuous grazing did not increase significantly tiller densities. By contrast, mean GP densities were approximately doubled under continuous grazing (Fig. 2).
Growth, senescence and intake rates (g.m.2.day-1) were increased by 50, 56 and 34 %, respectively, under rotational grazing (Table 1). Also, the number of grazing days was 37 % lower with continuous grazing. A similar mean herbage intake of 1.8 kgDM/sheep/day was calculated from these results for both continuous and rotational grazing.
Positive mean net herbage accumulation (NHA) rates were recorded for both grazing managements (Table 1). Further analysis of the data indicated that biomass accumulation occurred mainly in unfrequently grazed plant parts (clover stolon and grass sheath) (data not shown).
Under both grazing managements, apart from early spring (frosts resulted in severe clover leaf damages in April 1991), white clover contributions to the sward leaf biomass and leaf lamina area varied, respectively, between 30 and 50 % and between 40 and 65 % (data not shown).
Since clover and grass were side by side (alternate rows), sheep had the opportunity to graze clover selectively. For continuous grazing, low stocking rates (1 to 4 ewes per 1000 m2) allowed sheep diet selection. In contrast, the high stocking rates (approximately 15 ewes per 500 m2) which were used for rotational grazing impeded diet selection. Nevertheless, white clover contribution to intake was similar for both grazing managements: 42 % under (C) as compared to 41 % under (R) (Table 2). Therefore, under these experimental conditions, sheep diet selection, if any, did not seem to affect significantly the balance between grass and clover intake.
Under both grazing managements, changes in clover plant population led to a decline in the proportion of plants bearing a tap-root, an increase in mean plant complexity and in the mean number of growing points and elongated branches per plant (Table 3). In May 1992, mean total stolon length per plant varied from 9 (C) to 17 (R) cm. Such rather large clover plant sizes could contribute to clover persistency under grazing.
Increasing both the distance between grass rows and sward management height could promote clover persistency under sheep grazing by increasing bare soil colonization by the legume. Rotational grazing enhances the productivity of such mixtures sown in alternate rows. However, continuous grazing is also feasible since, under these conditions, clover rows do not appear to be selectively defoliated.
Fig. 2. Tillers and total growing points densities under continuous (
) or rotational (
) grazing.
Table 1. Tissue turn-over in the mixture (g DM/m2/day)
|
|
Growth |
Senescence |
Intake |
NHA |
|
|
Continuous |
Spring |
7.6 |
2.51 |
5.24 |
+0.2 |
|
Summer |
10.7 |
2.9 |
4.2 |
+3.7 |
|
|
Rotational |
Spring |
11.8 |
2.9 |
7.1 |
+1.8 |
|
Summer |
15.4 |
4.6 |
6.2 |
+4.6 |
|
Table 2. Contribution of white clover to tissue turn-over (%)
|
|
Growth |
Senescence |
Intake |
|
Continuous |
62 |
54 |
42 |
|
Rotational |
62 |
48 |
41 |
Table 3 Individual clover plant characteristics in May 1991 and 1992
|
Plant type |
Grazing management |
|
Density (m-2) |
GP per plant |
Plant rank |
Rooted nodes (%) |
Internode length (mm) |
|
Tap root |
Continuous |
1991 |
420 |
10.1 |
1.36 |
30 |
4.4 |
|
|
1992 |
130 |
11.2 |
1.86 |
35 |
4.0 |
|
|
Rotational |
1991 |
330 |
7.4 |
1.46 |
46 |
6.3 |
|
|
|
1992 |
85 |
10.2 |
1.90 |
31 |
5.5 |
|
|
Fragment |
Continuous |
1991 |
95 |
2.9 |
1.19 |
45 |
5.2 |
|
|
1992 |
320 |
2.8 |
1.23 |
29 |
3.8 |
|
|
Rotational |
1991 |
95 |
2.0 |
1.18 |
40 |
6.5 |
|
|
|
1992 |
300 |
3.4 |
1.47 |
34 |
5.7 |
