A S Laidlaw¹ and Françoise Vertès².¹ Department of Agriculture for Northern Ireland, Newforge Lane, Belfast, BT9 5PX, Northern Ireland, UK
² Station d'Agronomie INRA, 4 rue de Stang Vihan, 29000 Quimper, France.
Abstract
White clover content
Digestibility
Cell wall composition
Protein
Minerals
Intake
References
The effect of components of grazing on white clover growth and development are reviewed. Short defoliation intervals, removing high proportions of leaf material, reduces branching and leaf size and shortens internodes. However, in mixture with grass long defoliation intervals reduce branching due to shading at the canopy base, small leaved varieties being particularly damaged by long intervals. Treading directly damages stolons but also results in burial of stolons which in combination with defoliation reduces branching rate and hastens auxiliary bud death; but the damage may not be permanent. Return of urine reduces stolon population density and N2 fixation and indirectly by stimulating grass growth. Rejection around dung pats lengthens defoliation interval. Dung pats also create patches which may be colonised by stolons from the surrounding rejected sward.
Grazing management can control clover content, declining levels being controlled by strategies which reduce competition by grass and improve clover's aggressiveness (out-of-season grazing, continuous stocking in spring followed by rotational grazing or continuous stocking interspersed with a silage cut in summer). While these strategies improve stolon branching, a limit is reached beyond which individual plants weaken.
It is concluded that while effects of grazing components on clover are well quantified, factors influencing stability of mixed swards under grazing need further study.
Due to heterogeneous defoliation, return of dung and urine and treading damage by animals on a grazed sward, the long term behaviour of clover under grazing cannot be predicted from cut swards. Grazing increases the importance of genotype, management and edaphic and climatic conditions in determining white clover persistence. For example in Brittany interaction between soil and climate results in clover persistence being poorer in wet than in well drained soils on farms due to increased risk of treading damage (Vertès, 1991). This paper considers the effects of the components of grazing on growth and development of white clover and their relative importance in determining persistence of white clover.
Generally white clover responds to defoliation by producing leaves with shorter petioles and smaller laminae and shorter internodes than in undefoliated plants. Small leaved varieties are less productive under long defoliation intervals (Wilman and Asiegbu, 1982a). While leaf appearance rates are relatively insensitive to cutting interval unless it is extremely short (King et al., 1978) or long, stolen internode length and diameter increase with defoliation interval (Wilman and Asiegbu, 1982b).
Severe defoliation of clover plants, ie rapidly repeated low defoliation, reduces branching (Jones and Davies, 1988), presumably due to limitation in the availability of assimilate; so axillary buds being weaker sinks than the apical meristem are inhibited from developing. Incorporating a longer rest period among weekly defoliation intervals at 3.5 cm has improved leaf appearance and branching rates in clover and reduced them in grass when the weekly defoliation intervals have resumed, compared to the control without a rest period (Grant and Barthram, 1991).
Relative rate of removal of clover and grass by grazing is an important facet of defoliation. The complexity of selective grazing of clover in a grass/clover sward has been demonstrated by Parsons et al. (1991). Under hard, continuous grazing, even if an animal has a strong preference for clover, eventually the composition of the diet will reflect the relative rates of growth of the two components. Therefore rate of clover removal by grazing cannot be considered in terms of one co-efficient for selection or preference (especially for sheep continuously grazing).
Treading damage is a consequence of stocking rate, soil conditions and season. Relative to perennial ryegrass white clover is slightly less tolerant to treading (Curll and Wilkins, 1983) although the difference is dependent on stocking rate. In addition to leaf production per unit area stolon length per unit area is affected, the depression again being related to stocking rate.
While this effect may be due, in part, to direct damage burial of stolon by grazing animals (especially cattle), tramping stolons into soft ground can also influence stolon development. In New Zealand as a consequence of earthworm casts and treading Hay et al. (1987) have recorded about 80% of total stolon weight to be below the ground surface level in mid winter in sheep-grazed plots.
The impact of burying stolons under a shallow layer of soil, with or without defoliation in a pot experiment has been assessed by Grant et al. (1991). They found that conditional on the stolons not being defoliated, stolon extension, leaf appearance and water-soluble carbohydrate concentration in the stolons remained unaffected. However, distribution of axillary buds and inflorescences differed to that in the unburied controls, a higher proportion being on the new aerial segments of stolon but less on the secondary stolons. By combining defoliation (removal of all expanded leaves weekly) and burial almost half of the secondary branches died after l½ months. Fewer axillary buds on secondary stolons developed and although a higher number developed on new segments of primary stolons these were insufficient to compensate.
Clover can recover quickly from autumn treading after a mild winter (Vertès, 1989), stolon fragments developing into separate plants and earthworm activity encourages adventitious root development. Grazing in a wet winter may hinder earthworm activity, delaying recovery of soil structure (Cluzeau et al., 1992). Damage due to treading in spring may be greater than in autumn as grass recovers quicker than clover and so competes vigorously with clover.
Even in the absence of N fertiliser, N returned in faeces and urine to a grass/clover sward may be considerable. Table 1 shows that more N is returned in dung and urine to a grass/clover sward fixing 120 kg N/ha than to a grass sward receiving 250 kg N/ha.
Table 1 Nitrogen returned annually in faeces and urine in a grass and grass/white clover sward grazed with heifers in Brittany (1991).
|
|
Ryegrass |
Ryegrass/white clover |
|
N applied (kg/ha) |
250 |
|
|
N fixed (kg/ha) |
|
120 |
|
Faeces (kg/ha) |
6900 |
6200 |
|
N% |
0.32 |
0.46 |
|
N returned (kg/ha) |
22 |
28 |
|
Urine (l/ha) |
13500 |
18500 |
|
N% |
0.70 |
0.79 |
|
N returned (kg/ha) |
94 |
147 |
The direct effect of dung and urine on clover growth and development has usually not been separated from the effects due to stimulated grass growth. However, Curll and Wilkins (1983) interpreted a reduction in clover content in grass/clover swards due to excretal returns to be caused mostly by stimulated grass growth, stolon length per unit area being affected more than clover growth. In contrast, applying urine to grass/clover, Marriott et al. (1987) found that while reduction in clover growth and branch number was considerable the stimulatory effect on grass growth was modest. Under cattle grazing, urine also reduces N2-fixation but increases both grass and white clover growth (Vertès and Simon, 1992). However, scorching (during dry periods) and loss of N by leaching (during wet periods in summer and towards the end of the growing season) modify this general effect.
An additional effect of faeces return on grazed grass/clover swards is the rejection of herbage around dung pats resulting in changes in grass and clover morphology. In mixed swards continuously stocked with cattle stolen elongation rate was faster (Teuber, 1993) but branching rate slower (Teuber and Laidlaw, 1992) in rejected compared to grazed areas the major effect being due to shading by the accumulating ungrazed herbage (Teuber, 1993). The patches of bare ground resulting from dung smothering the sward are potential sites for clover stolons to colonise, although the relatively high fertility of the area may be a constraint.
The impact of the above components is influenced by type of animal, grazing system and grazing intensity and so system and intensity can be manipulated to some extent to control clover development.
Generally, growing point density will increase with stocking rate, within limits, in swards continuously grazed by sheep (Orr et al., 1991) or cattle (Patterson and Laidlaw, 1990). While this might seem to contradict the point that clover branching is reduced by defoliation it can be explained by the adverse effects of shading on branching by the accumulating herbage in the tall swards (Thompson and Harper, 1988; Simon et al., 1989) (death or inhibition of secondary axillary buds, lower leaf appearance rates and leaf weight ratios). Some cultivars of clover are able to avoid continuous defoliation by reducing the size of new leaves e.g. Kent wild white compared with Huia and so continue branching (Davies and Jones, 1987) and aid persistence.
Comparison between grazing systems generally shows continuously stocked swards to have a higher stolen population density than rotationally grazed swards (eg. Brock et al., 1988; Carrere et al., 1993) but there are exceptions. Different defoliation patterns between grazing systems obviously influence the morphology of clover, regrowth allowing larger leaves and more leaves per stolon to develop (Brock et al., 1988, Chapman, 1986).
Manipulation of grazing intensity or system at strategic times can improve white clover stolon density eg. grazing to a low height in late autumn or early spring (Laidlaw and Stewart, 1986) or continuous stocking in spring (Hay and Baxter, 1989) followed by rotational grazing throughout the remainder of the growing season or interrupting continuous stocking in summer with a silage cut (Grant and Barthram, 1991).
Out of season grazing also increases white clover potential photosynthesis (Laidlaw et al., 1992) presumably due to lower competition from accompanying grass. Interrupting continuous stocking with a silage cut in summer results in a high proportion of both grass and clover leaf being removed but the laminae in new clover leaves remain large when grazing resumes conferring a competitive advantage on clover (Davies, 1992).
While success of clover in a sward is often assessed in terms of stolon population density, a maximum can be reached beyond which total plant size and complexity declines followed by a marked decline in plant number (Fothergill, 1992).
The effect of the components of grazing on morphology and physiology of white clover is quite well understood. However, it is still not possible to predict white clover behaviour in the long-term under grazing conditions. This is due to insufficient being known about the complex interaction between the grazing animal, clover and accompanying grass.
The interaction between grazing behaviour of the animal and the growth rate of clover and accompanying species has been studied by Parsons et al. (1991) using a mathematical model. However, in the long term factors other than defoliation by the grazing animal will influence stability of the system and modify vegetation dynamics. Such interactions require further study and quantification before white clover's persistence and long-term contribution to a pasture can be predicted.
To be of practical benefit, research is required to overcome farmer objections to white clover use. This should include devising a) indicators to aid management decisions e.g. when to employ remedial action to improve clover content and which method, b) edaphic and climatic criteria to identify areas where white clover could be exploited and c) strategies to control weeds, e.g. docks, in mixed swards.
Irrespective of the approach taken an overriding objective should be to make white clover more predictable in production systems.
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