A S Laidlaw1 and Karen Søegaard2
1 Applied Plant Science Division, Department of Agriculture for Northern Ireland, Newforge Lane, Belfast BT9 5PX, UK2 Department of Forage Crops and Potatoes, Research Centre Foulum, 8830 Tjele, Denmark.
Abstract
Introduction
Defoliation
Plant nutrition
Animal slurry
Water availability
Nutritive value
Intake and feeding value
White clover monoculture
Practical implications
References
The consequences of cutting grass/clover swards for conservation on clover growth, development and contribution to total DM yield in comparison with grazed swards are considered. Increasing the cutting interval or cutting height generally reduces white clover contribution and so the effects of cutting can be manipulated by varying these factors. Clover contribution is likely to be maintained if cutting intervals do not exceed the normal 5-7 weeks practised for high quality silage making. As nutrients are removed in cut herbage, their return via fertilizers or slurry is required. However, this nutrient return can be controlled unlike in grazing. While white clover improves nutritive value of herbage its potential to do so is greatest at the first cut when, unfortunately, it is likely to contribute least to the total herbage. The advantages of inclusion of white clover to intake and utilization of ingested nutrients, typically found in experiments in which herbage is grazed or fed fresh, are less obvious in conserved herbage.
It is concluded that a cutting regime for conservation allows better control of clover contribution than grazing. Nevertheless it should not be considered in isolation from grazing or other management options within a grassland system since a flexible system is likely to have the most beneficial effect on the performance of white clover.
Although white clover is usually associated with grazed swards there is evidence to show that it has a valuable role to play in swards cut for conservation (Frame, 1992). As a consequence of the emphasis on white clover being suited to grazing much of the research in recent years has been directed towards the impact of the grazing animal on white clover growth, development and persistence. However these two managements can impose markedly different conditions for clover, thus influencing the relationship between grass and white clover. Therefore, conclusions drawn from grazing experiments are not always applicable to swards mainly cut for conservation.
In comparison to grazing, cutting involves:
(i) defoliating uniformly and sometimes, less severely.
(ii) longer regrowth intervals.
(iii) control over recycling of nutrients.
(iv) influence of conservation processes on nutritive and feeding value.
(v) potentially less N lost in leaching.
These differences will be addressed in relation to factors which influence clover growth, development and utilization in cut mixed swards.
Cutting height and cutting interval are the two variables associated with defoliation. Experiments carried out over twenty years ago showed that reducing cutting height from 5 to 2.5 cm generally resulted in little or no effect on clover content but could increase yields by 30 to 40%. More recently Acuña and Wilman (1993) found that a high stubble height (10 cm) at 4 weekly cutting intervals resulted in a severe reduction in clover content, almost eliminating it from the sward by the third year. In the west of Scotland reducing cutting height from 8 to 4 cm resulted in an increase in total yield of 16% and in clover yield by 31% (Frame and Boyd, 1987).
White clover in swards with tall stubbles has larger leaves and thicker stolons but fewer total nodes and branch-bearing nodes and less stolon length per unit ground area than swards cut to a low height (Wilman and Acuña, 1993).
When cut for silage height of cut may be governed by the danger of soiling the crop if cut too low but conversely the consequences of leaving a high stubble could be severe for clover.
As with grass swards, the total herbage yield of grass/white clover swards increases but white clover content may decrease at long cutting intervals, although white clover yields may continue to increase at cutting intervals beyond which clover content begins to decline. A cutting interval of about six weeks, i.e. normal practice for making high quality silage, is considered an acceptable balance between maximising white clover and total herbage yield and minimising the adverse effect of long cutting intervals on clover contribution (Orr and Laidlaw, 1978). The impact of lengthening cutting intervals on clover content is, however, dependent on season, stage of development, companion grass or clover variety (Harris, 1987). The adverse effect of longer cutting intervals on clover content is more pronounced under N fertilization (Fisher and Wilman, 1995). Inclusion of two silage cuts in a simulated grazing regime can reduce clover content (Frame and Paterson, 1987).
Both grass and white clover respond to increasing defoliation interval by producing larger leaves - clover also producing larger stolons - and by increasing the proportion of leaves harvested. However, both components suffer from a reduction in leaf-producing sites (tiller and stolon apices) due to long regrowth interva1s.
Suitability of white clover cultivars for cutting systems has been studied by Swift et al. (1992). They found a strong association between those with the largest leaves and high yield under a silage cutting regime, while the smallest leaved types were most suitable for grazing represented by a regime of more frequent cutting.
Response of cut grass/white clover swards to N fertilizer in terms of total herbage DM and white clover contribution have been well documented. In a review of Danish experiments Søegaard (1988) calculated that yield from cut grass/clover swards receiving no N fertilizer averaged 80% that of swards receiving 250 to 400 kg N ha-1 year-1. This comparative yield is similar to the average for 21 sites in England and Wales harvested three times per year over four years (Frame and Newbould, 1984). The Danish study also showed that grass/clover receiving half of the maximum rate applied to the grass produced a similar yield to that of the grass. A similar relationship was found in the UK study, at least in swards sown with the large leaved cultivar, Blanca. Thus well established grass/clover swards can respond to N fertilizer when cut and have a role to play in saving N fertilizer in intensive systems.
There is still considerable debate about the cause of the adverse effect of N fertilizer on white clover growth in mixed swards. It is clear that shading on white clover leaves by grass overtopping the clover in the canopy during the growing season is not the main reason since many of the laminae of leaves of the clover remain at the top of the canopy as the LAI increases during regrowth (Woledge et al., 1992).
White clover is less competitive than accompanying grasses for most below-ground resources e.g. P and K (Dunlop and Hart, 1987) and water (Thomas, 1984). Consequently when N fertilizer is applied the competitive advantage of grasses is accentuated, due to the poor response of clover to N, even in monocultures. The adverse effect of N fertilizer on clover can be partly reduced by K application in some instances (e.g. Frame and Tiley, 1990). Nevertheless the accumulating LAI probably has an impact on clover development and photosynthesis in swards receiving N fertilizer. The rapidly accumulating combined LAI of both grass and clover does reduce the in situ photosynthesis of clover (Woledge, 1988), reduces the red: far red ratio in irradiance at the base of the sward and hence white clover branching (Thompson, 1993) and also the contribution of smaller younger stolons to the upper strata of the canopy thus increasing the likelihood of death of these branches (Laidlaw et al., 1995).
Return of nutrients taken off in cut herbage has to be considered. If a sward produces 8 t ha-1 of harvestable herbage DM comprising 30% clover and the grass DM contains 2.5 g P kg-1 and 20 g K kg-1 while the clover DM contains 3.5 g P kg-1 and 25 g K kg-1 22 kg P ha-1 and 172 kg K ha-1 are removed annually. Coupled with the relatively poor competitive ability of clover for these nutrients replenishment will generally have to exceed these quantities.
In a cutting situation, utilization of the conserved herbage inevitably results in the production of slurry. Slurry has a wide N:K ratio (about 1:2.5) and as it also contains P it is not as damaging to white clover as straight N fertilizer at a similar rate. The K content in slurry can partly offset the adverse effect on clover caused by the N component (Chapman and Heath, 1987). Bax et al. (1993) reported that clover content and persistence was maintained in their grass/clover system for dairy cows even though it received slurry each year for six years.
Nesheim et al. (1990) have shown that clover growth in a mixed sward is less affected by the N in cattle slurry than by a corresponding rate of N fertilizer despite the N in clover derived from N-fixation being lower in the slurry treatment. As P and K were adequately available in all their treatments more work is required on the effects of slurry on clover to explain the apparently less severe effect of slurry than fertilizer N.
Slurry damages the leaf cells of white clover more than those of grass (Wightman and Younie, 1994) yet the content of clover in the slurry treated swards was equal to that in the N-fertilized control suggesting that white clover is capable of recovering rapidly from scorch damage.
White clover is a poor competitor with grass for water so its content in the total herbage declines during drought periods whether it be in the uplands of Scotland (Marriott, 1988) or in Spain (Gonzalez-Rodriguez, 1992). Irrigation strategy for grass/clover swards has been studied in Denmark where there are soil moisture deficits in most years (Søegaard, 1991). White clover content is increased in mixed swards when irrigated at frequent intervals (i.e. at soil water potential of 0.08 MPa) compared with less frequent irrigation (at 0.2 MPa), the increase being in excess of 10% at some cuts, especially in years with prolonged drought periods.
Thomas (1984) argues that conditions which improve white clover's competitiveness will enable it to withstand drought better. As frequent defoliation is one of the factors identified which improves its competitiveness this implies that clover in swards cut for conservation will be less able to withstand drought. Also if N fertilizer is normally applied, withholding at least part of it to improve clover's competitiveness might also help it to withstand drought better. Thomas (1984) also suggests that in the long term compatible grasses (less aggressive water users) and clovers capable of producing more roots may be bred to provide swards in which clover would be less severely affected by drought.
Søegaard (1994) has summarised the herbage quality aspects of grass/white clover swards compared with grass receiving N: generally, protein, pectin, lignin and ash are greater, cellulose, hemicellulose and water soluble carbohydrate are lower while digestibility is usually unaffected when clover is included with grass. However in cut swards the consequences of including clover may differ from the above. For example, in swards with a good clover content digestibility is usually higher at the first cut than in grass swards due to the clover digestibility remaining high as the primary growth accumulates (Thomson, 1984). This is particularly so when first cut yields are high (5-6 t DM ha-1) However at subsequent harvests, especially during summer when inflorescence production is abundant, the low digestibility of these plant parts offsets the beneficial effect of the leaves (Søegaard, 1994). Grass stem content can also be higher in mixed swards and so reduce the digestibility of the total herbage.
In herbage harvested to correspond to grazing or conservation i.e. at two maturity levels, Steg et al. (1994) reported that grass had a lower amount of rumen degradable OM than clover at both maturity levels but the proportion of crude protein which was undegradable in the rumen was lower for clover than grass, the level in both increasing with maturity. However, as clover had a higher crude protein content than grass it still had a higher amount of escape protein which combined with the higher digestibility of this protein in the small intestine than that of grass resulted in a greater amount of intestinal digestible protein in the clover diet, although it was lower in the more mature herbage.
These highly digestible protein fractions especially in the intestine contribute to the high feeding value of grass/clover herbage. However an undesirable consequence of high N content is a high loss of N from the rumen in the form of ammonium-N.
The superior voluntary intake characteristics (and consequential effect on animal product) of fresh white clover relative to grass have been widely reviewed (e.g. Thomson 1984). However less consideration has been given to clover's contribution to the intake and animal production potential of ensiled herbage, the most frequently utilized form of cut herbage. Castle et al. (1983) found that mixing white clover and grass silages in equal parts increased DM intake of dairy cows by 12% and increased milk yield by 7%. In another short term experiment they found no effect on silage intake by including white clover silage in the diet. A positive effect on intake by including clover in the herbage has been recorded by Wilman and Williams (1993) in a short term experiment feeding silage without concentrates to dairy cows. In a systems experiment Younie et al. (1987) achieved daily liveweight gains of steers 16% higher when fed grass/clover silage, with clover content of about 35-40%, than grass silage. Mould et al. (1993), feeding silage from grass/clover (clover content 18 to 28%) or N-fertilized grass to bulls, obtained a 9.5% increase in intake with the grass/clover silage over the grass silage at the first cut. However they did not detect any difference in performance when feeding the silages from either the first or second cut.
In regions where spring conditions are more suited to grass than white clover growth the resultant low clover content in spring may be a disadvantage in a production system when heavy reliance is placed on first cut silage (Bax and Schils, 1993).
Despite the large increases in DM intake and efficiency in utilization of ingested energy and nutrients, and subsequent production, often found in experiments in which white clover has been included in fresh herbage diets, the benefits which have accrued from including white clover in silage have generally been less dramatic.
Potential yields of white clover when grown in monoculture under conditions in the British Isles have been calculated to be about 16 t DM ha-1 (Frame and Newbould, 1984). This contrasts with the calculated potential yield of a grass/white clover sward of 18.5 to 22.5 t DM ha-1. Wider differences between estimated ceiling yields for grass/white clover and white clover monoculture have been quoted for New Zealand conditions viz. 22 to 28 t DM ha-1 and 10 t DM ha-1, respectively (Smetham, 1973).
In practice, in unirrigated cut swards the highest recorded monoculture yields in the UK have been in the range 8-10 t DM ha-1 while the highest recorded mixture yield has been 15.5 t DM ha-1 (Frame and Newbould, 1984).Therefore yield of white clover in monoculture in a cutting system would be expected to be considerably lower than that of total herbage yield in a mixture. Other shortcomings of a monoculture include the difficulty in maintaining it weed-free and the increased likelihood of bloat in animals utilizing the herbage.
In order to gain maximum benefit from cutting grass/clover swards without putting the clover at risk the cut swards should be integrated into a system which not only includes grazed grass/clover swards but also relies partly on N-fertilized grass swards. Flexibility is then maximized so that remedial action can be taken to improve the clover content in the sward, for example, by temporarily changing the management without markedly changing the output of the system as a whole.
If clover content declines due to repeated cutting as a consequence of too tall a stubble height, for example, too long a cutting interval or too much N fertilizer being applied, remedial management can be adopted by strategic use of grazing. While this may reduce the grass/clover area available for cutting temporarily, the shortfall may be redressed by applying more N fertilizer to grass areas. Integrating N fertilizer into the clover system by strategic use on cut grass/clover swards, usually adopted as a means of manipulating the growth profile, may have a role at times to reduce excessive clover content in summer and reduce variability between individual cuts (Søegaard, 1991).
Integrating cutting and grazing has advantages, allowing a cut to be taken when deemed necessary to improve the clover content in swards subject to severe defoliation as would be experienced under continuous sheep grazing (Barthram and Grant, 1995). However, when cutting after cattle grazing problems may arise from dung being picked up with the herbage. In addition to the changes in morphology of white clover when cut subsequent to grazing, which may improve clover content in the herbage, cutting removes N from the system, benefiting white clover at the expense of grass.
All of these strategies are feasible when required if a flexible grassland management system is adopted. This is particularly important if the productive persistence of white clover is a priority.
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