Karen SøegaardDepartment of Forage Crops and Potatoes Research Centre Foulum 8830-Tjele, Denmark
Abstract
White clover content
Digestibility
Cell wall composition
Protein
Minerals
Intake
References
White clover content is normally constant in regrowths after defoliation, but in the middle of the growing season, when flowering, the flower fraction make up a considerable part (up to 50 %) of the white clover. The digestibility of the leaves and petioles are very high. On the other hand, the digestibility of the inflorescence and flowering stem are low. Addition of white clover to grass did not affect the digestibility when comparing at the same herbage mass, and also the content of low-digestible cell wall was unaffected.
The lower content of structural fibre or cell wall seems to be a main reason for the higher intake relative to grass when feeding with white clover, despite the lower digestibility of the cell wall. White clover contains a higher protein concentration than grass, and the protein seems to be less digestible in the rumen than grass protein.
White clover contains a higher amount of minerals than grass in general but concentrations of specific minerals are affected by a number of factors.
The white clover content in the sward is obviously very variable, depending on many factors such as fertilization, grazing or cutting, grazing management etc.
In the regrowth after defoliation, however, the white clover content seems to be constant (Woledge, 1988; Davies and Evans, 1990). A nearly constant clover content above stubble level after re-establishment is also shown in Fig. 1. The overtopping of the clover in the last part of the regrowth period does not seem to affect the clover content.
The typical progress with the highest clover content in the middle of the growing season at invariable treatment is also shown in the figure. The defoliated part of white clover consists of 'leaves' (leaves and petioles) and 'flowers' (inflorescences and flowering stems). The stolons can only be defoliated to a reduced extent, e.g. at sheep grazing. The flowering takes place primarily in the middle of the growing season, and there are only very few flowers in spring and autumn. In mid summer the 'flower' fraction can attain 50 % of the white clover (Fig. 1).
The digestibility of pure grass compared to grass mixed with white clover is reported differently, e.g. decreasing (Bax and Thomas, 1992) or unaffected (Thomson et al., 1985) digestibility when white clover is added. How white clover affects the digestibility of the sward must depend very much on the conditions, the grass species, the defoliation management etc.
When comparing at the same herbage mass, with a relatively large number of samples, no effect on digestibility was found by mixing grass with white clover (Table 1). The clover content was 25-45 %. The relationship between the digestibility and the herbage mass was the same for the two different crops in the single growth cycle. At 2 t DM/ha above stubble level the IVOMD was 85, 82, 79, 76 for 1., 2., 3., 4. growth cycle respectively.
The content of grass stem was increased by addition of white clover (Table 1). This may affect the digestibility of the grass negatively. The effect was greatest in the mid summer, when the clover content was highest.
In the same mixed sward there was no difference in the digestibility of the grass leaves and the white clover (Table 2). However, the clover consisted of both 'leaves' and 'flowers'. Gibb and Theacher (1983) and Wilman and Altimimi (1984) found a low digestibility in the flowering stem, and especially in the flower. The leaves and the petioles had the highest digestibility.
Table 1. Chemical composition of pure perennial ryegrass (G) and perennial ryegrass mixed with white clover (CG) at two different amounts of herbage mass, 1988-91.
|
|
1-3 t DM |
3-5 t DM |
|||||||
|
C |
CG |
sig. |
n¹ |
G |
CG |
sig. |
n1 |
||
|
(G, CG) |
(G,CG) |
||||||||
|
t DM/ha |
2.01 |
1.99 |
n.s. |
(422,470) |
3.82 |
3.78 |
n.s. |
(154, 149) |
|
|
% DM |
17.6 |
15.6 |
*** |
(422,470) |
17.3 |
15.7 |
*** |
(154, 149) |
|
|
IVOMD |
79.2 |
79.1 |
n.s. |
(396,431) |
77.3 |
77.9 |
n.s. |
(139, 133) |
|
|
% of DM |
|
|
|
|
|
|
|
|
|
|
|
Clover content |
- |
36.3 |
- |
|
- |
30.8 |
- |
|
|
|
Grass stems |
|
|
|
|
|
|
|
|
|
|
and sheaths |
19.3 |
22.2 |
n.s. |
(205,247) |
46.4 |
50.5 |
** |
(84, 90) |
|
|
NDF |
39.7 |
33.9 |
*** |
(114,128) |
42.8 |
37.3 |
*** |
(40, 39) |
|
|
ADF |
23.8 |
23.6 |
n.s. |
(114,128) |
26.0 |
26.0 |
n.s. |
(40, 39) |
|
|
Lignin |
1.9 |
2.4 |
*** |
(59,76) |
2.4 |
2.9 |
*** |
(22, 23) |
|
|
N |
2.73 |
3.25 |
*** |
(396,431) |
2.21 |
2.55 |
*** |
(139, 133) |
|
|
WSC |
11.5 |
10.5 |
*** |
(120,133) |
11.9 |
12.2 |
n.s. |
(55, 55) |
|
|
Organic matter |
91.0 |
90.3 |
*** |
(396,431) |
91.3 |
91.2 |
* |
(139, 133) |
¹ Number of observations
Table 2. Chemical composition of plant fractions in perennial ryegrass mixed with white clover fertilized with 200 kg N/ha. 25-40 % clover of DM. 1991, 17 obs. mean of the growing season.
|
|
Perennial ryegrass |
White clover |
LSD 0.05 |
|
|
Leaves |
Stems and sheaths |
|||
|
N (% of DM) |
3.1 |
1.9 |
4.1 |
0.3 |
|
NDF (% of DM) |
37.8 |
43.3 |
26.2 |
3.6 |
|
ADF (% of DM) |
23.7 |
26.6 |
22.5 |
6.6 |
|
Lignin (% of DM) |
1.7 |
2.1 |
3.9 |
0.6 |
|
IVOMD |
78.0 |
73.8 |
77.3 |
2.6 |
Corresponding results are shown in Fig. 2. The digestibility of the clover 'flowers' was lower than the grass 'stem' and the digestibility of the clover 'leaves' was higher than the grass leaves.
It is often pointed out that white clover maintains a high digestibility in the spring (Frame and Newbould, 1986; Møller and Hvelplund, 1991; Ulyatt, 1981). This is also confirmed by the results in Figs. 1 and 2, because there were no 'flowers' in the spring growth, and the digestibility of the clover 'leaves' maintained a high level. But over the largest part of the summer the clover is flowering and the 'flowers' lead to a lower digestibility. As the 'flower' content increases during the regrowth period (Pig. 1), this also explains why the digestibility does not increase by addition of white clover in the regrowth periods.
Normally white clover has a considerably lower content of total cell wall or neutral detergent fibre (NDF) than grass and a higher content of lignin (Tables 1 and 2; Thomson et al., 1985; Ulyatt et al., 1988). The lowest NDF content was found in the leaves of white clover. In the petiole the content was higher, whereas it increased in the flowers with advancing maturity (Wilman and Altimimi, 1984).
The content of cellulose or the content of acid detergent fibre (ADF) can in some cases be the same (Thomson et al., 1985; Ulyatt et al., 1988). At the same herbage mass addition of white clover did not affect the ADF-content either (Table 1).
Fig. 2. In vitro organic matter digestibility of different plant fractions. Corresponds to the results shown in Fig. 1.
As a consequence of the lower content of NDF and the same digestibility when comparing at the same herbage mass, it is tantamount to a lower digestibility of the clover cell wall. Pectin is not, however, included in the NDF fraction, but it is highly digestible. The pectin content is higher in clover than grass (Thomson, 1984) and not so digestible (Hatfield, 1989).
There is normally a close agreement between lignin concentration and cell wall digestibility (Buxton and Russell, 1988; Wilman and Altimimi, 1984). The higher lignin concentration in white clover could be one of the reasons for the apparently lower digestibility of cell wall. However, the lignin composition differs in grass and clover, and the effect on the digestibility seems to be different in the two crops (Buxton and Russell, 1988; Jung, 1989).
White clover has normally a higher protein content than grass (cf. Tables 1 and 2). Sometimes the protein concentration in mixed crops is unaffected by different N-fertilization on account of compensation of the clover content. In grass grown with white clover the protein concentration can also be increased compared to grass grown without clover (Wilman and Hollington, 1985). The N content decreases with increasing herbage mass (cf. Fig. 3). At the same herbage mass the N content was a little higher in the mid summer, where the clover content was highest (Fig. 3). The higher protein content in clover does not seem highly digestible in the rumen. Sanderson and Wedin (1989) found that concentration of N in ADF in clover was twice the concentration in grass, which may be due to the greater lignin concentration. Maybe this is one of the reasons for relatively higher N digestion in the intestine in sheep fed with white clover than in sheep fed with perennial ryegrass (Ulyatt, 1981).
Fig. 3. Concentration of N in perennial ryegrass mixed with white clover in relation to herbage mass in 2-3 growth cycle (-----) and the other (1, 4, 5, 6) growth cycles (- - - - -), 1989-91.
The concentration of some of the minerals are normally higher in white clover than in grass. It varies which minerals are most influenced. In one experiment e.g., the concentration of K and Ca increased, whereas Mg, Na and P decreased (Whitehead et al., 1985). In another experiment the concentration of both P, Ca and K increased (Bax and Thomas, 1992). In Table 3 results from a fertilization experiment are shown. The concentration of Ca and Mg was the most positively affected by decreasing N fertilization and by this increasing clover content. On the other hand the K concentration was only slightly affected. In this case the risk of grass tetany decreased by addition of clover in the sward.
Table 3. Chemical composition and clover content (% of DM) in pure perennial ryegrass (G) and perennial ryegrass mixed with white clover (CG). Sandy soil, mean of the years 1985-87.
|
|
kg N/ha |
Clover content |
N |
K |
Ca |
Me |
P |
|
CG |
0 |
75 |
3.57 |
3.57 |
1.18 |
0.24 |
0.39 |
|
CG |
175 |
37 |
2.70 |
3.48 |
0.86 |
0.21 |
0.40 |
|
CG |
300 |
16 |
2.49 |
3.49 |
0.70 |
0.20 |
0.41 |
|
CG |
425 |
8 |
2.61 |
3.51 |
0.68 |
0.20 |
0.41 |
|
G |
425 |
- |
2.34 |
3.49 |
0.58 |
0.19 |
0.40 |
|
LSD0.05 |
|
6 |
0.15 |
0.08 |
0.07 |
0.01 |
0.01 |
(Søegaard, 1990)
When experiments show different results concerning effects on minerals, it may be due to the fact, that many parameters affect the concentration of minerals, e.g. temperature, availability, soil type and application.
White clover addition to grass increases the intake by ruminants both at silage feeding and under grazing even at the same digestibilities. This fact has been accepted for a number of years (Bax and Thomas, 1992; Castle et al., 1983; Thomson, 1984; Ulyatt, 1981). Thus white clover has a lower retention time in the rumen, and a relatively higher amount enters the small intestine (Ulyatt et al., 1988). The apparent digestion can therefore be reduced with white clover in the diet.
The higher rate of passage is especially explained by the lower content of cell wall or structural fibre (Thomson, 1984), lesser resistance to mechanical breakdown by chewing (Ulyatt, 1981) and, in addition, an increasing amount of viable bacterial populations in the rumen (Theodorou et al., 1984).
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