G Swift, J E Vipond, A T Cleland and J FitzSimons
Scottish Agricultural College, Bush Estate, Penicuik, Midlothian EH26 0PH, UK
Introduction
Materials and methods
Results
Discussion and conclusions
Acknowledgements
References
Despite an increase in the proportion and duration of permanent grassland in the UK, few long-term grazing comparisons of perennial ryegrasses, with or without clover, have been undertaken. In one such experiment in the uplands of Wales, lamb output in years 4-6 averaged 12% more on pure-sown tetraploid ryegrass swards (cv. Meltra) than on diploid S 23 swards (Davies et al., 1992). In the first three years the clover content (cv. Aberystwyth S 184) of the grass/clover swards declined but increased when fertilizer N application ceased (Fothergill and Davies, 1993). Clover content (cv. Grasslands Huia) can decrease without applied N (Orr et al., 1990) but this did not occur over a 5-year period, with swards of tetraploid Condesa ryegrass and the small leaved S 184 clover (Swift et al., 1993). Results over 6 years of this latter experiment are presented here, with particular reference to years 4-6 and to a change in management in year 6 from hard to lax grazing.
Duplicated 1 0 ha swards of Contender late heading diploid perennial ryegrass (D swards), Condesa late heading tetraploid ryegrass (T swards) and 1.4 ha swards of Condesa + S 184 white clover (TC swards), were established in 1987 on a sandy loam soil at an upland site (200 m a.s.l.) near Edinburgh (longitude 3° 14' W, latitude 55° 51' N). The swards were continuously stocked by ewes and lambs from April to September, 1988-93. Scotch mule ewes rearing Suffolk-cross twin lambs were used in 1988-91 and both single and twin rearing ewes in 1992. In 1993 the grazing resource was used to compare the performance of Blackface ewes and May-born lambs of various British and European breeds. Similar sward heights within years on each sward type were maintained by variable stocking on a 'put and take' basis. Lamb output was estimated from the number of lamb grazing days per plot and daily liveweight gains of the 'core' lambs always present on the plots and weighed weekly.
Annual fertilizer N usage on the grass swards averaged 180 kg ha-1 during 1988-92, and 120 kg ha-1 in 1993 when stock carry was lower. None was applied to the grass/clover swards. All swards received annual dressings of phosphate and potash to maintain a 'moderate' soil P and K status. Soil pH was maintained above 5.7 by applications of magnesian limestone at the start of the experiment, in 1987, and again in autumn 1991. Sward surface heights (HFRO sward stick) were recorded weekly. Sward composition was assessed monthly from ground level herbage samples, and clover stolon density in August using 10 cm diameter cores.
In the 1988-92 period, sward heights over most of the 24-week grazing period, from early April to mid September, were maintained within a 4-6 cm target range. The 5-year average was 5.2 cm on all three sward types. By contrast, in 1993 a combination of inadequate numbers of sheep due to quarantine restrictions, and good grass growth, resulted in mean sward heights over 10.0 cm through May and June, and around 8.0 cm thereafter to average 9.2 cm over the season. This degree of lax grazing was associated with a marked increase in unsown species (mainly annual meadow grass, Poa annua) in each sward type compared with previous years, while clover content and clover stolon density of the TC swards fell by 50% (Table 1).
Table 1 Mean sward height and content of unsown species of three sward types, and the mean clover content and in August, stolon density of the grass/clover swards, (years 4-6, 1991-93).
|
Sward type |
1991 |
1992 |
1993 |
|
Sward height (cm) |
|||
|
Diploid ryegrass (D) |
5.3 |
5.3 |
8.7 |
|
Tetraploid ryegrass (T) |
5.2 |
5.3 |
9.2 |
|
Tetraploid-clover (TC) |
5.3 |
5.3 |
9.6 |
|
|
Unsown species (%) |
||
|
D |
3.3 |
6.5 |
12.4 |
|
T |
5.1 |
6.2 |
9.3 |
|
TC |
5.3 |
4.4 |
8.9 |
|
|
Clover content (%) |
||
|
TC |
17.6 |
18.2 |
8.5 |
|
|
Stolon density (m m-2) |
||
|
|
82.0 |
120.0 |
58.0 |
In years 1-5 lamb gains were consistently higher on the TC swards than on the D or T swards, averaging 280 g head-1 d-1 compared with 230 and 234 g head-1 d-1 on the D and T swards respectively. With mean stocking rates achieved of 12.7, 15.6 and 16.6 ewes ha-1 respectively this resulted in a 12% higher lamb output from tetraploid Condesa than diploid Contender, both given fertilizer N, and only 10% higher than from the TC swards given no N (Table 2). In 1993, over the first 11 weeks when a comparison could be made, the T swards carried 13% more stock than the D swards and 38% more than the TC swards.
Table 2 Lamb output on the three sward types (years 1-5, 1988-92).
|
Sward type |
1988 |
1989 |
1990 |
1991 |
1992 |
Mean relative to diploid |
|
Lamb live weight gain (kg ha-1) |
||||||
|
D |
925 |
826 |
945 |
1232 |
1046 |
100 |
|
T |
1052 |
950 |
1112 |
1286 |
1158 |
112 |
|
TC |
804 |
994 |
1130 |
1247 |
868 |
101 |
The first significant deterioration in sward composition occurred in year 6 under lax grazing. An ingress of unsown species is widely recognised as occurring with inadequate stocking, while the marked reduction in clover at a mean sward height of 9.6 cm compared with approximately 5.3 cm in previous years has been recorded in a cutting experiment with defoliations of 8 cm and 4 cm (Frame and Boyd, 1987). The higher lamb output on the Condesa than the Contender swards of 12%, mainly through a higher level of achieved stocking rate at the same sward height, is greater than a 4-6% difference found in cutting trials Lamb gains were also slightly but not significantly (P > 0.05) higher on the Condesa swards each year. Apart from the lower output of the TC swards in the first grazing season, when clover content was still low (13% of herbage dry matter), and in 1992 when the single lambs, unlike the twins, showed no response to the clover pre-weaning, output was similar to the tetraploid ryegrass swards given 180 kg N/ha.
These results highlight the importance of a low sward height for the maintenance of clover, and the rapid reduction in clover and ingress of unsown species that can occur from undergrazing. A consistently high lamb output was achieved from the grass swards with 93% perennial ryegrass, and from the grass/clover swards when the clover content was around 20% (i.e. years 2 to 5) and when grazed with twin lambs capable of benefiting from the high nutritive value of white clover.
The work was funded largely by SOAFD, with support from MLC, the EU and the Royal van der Have Group.
DAVIES D.A., FOTHERGILL M. and MORGAN C.T. (1992). Assessment of contrasting perennial ryegrasses, with and without clover, under continuous sheep stocking in the uplands. 4. Animal performance in years 4-6. Grass and Forage Science. 47, 249-258.
FOTHERGILL M. and DAVIES D.A. (1993). White clover contribution to continuously stocked pastures in association with contrasting perennial ryegrasses. Grass and Forage Science. 48, 369-379.
FRAME J. and BOYD A.G. (1987). The effect of fertilizer nitrogen rate, white clover variety and closeness of cutting on herbage productivity from perennial ryegrass/white clover swards. Grass and Forage Science. 42, 85-96.
SWIFT G, VIPOND J.E., CLELAND A.T. and HUNTER E.A. (1993). A sustainable grass-clover sward for sheep. In: Hopkins A and Younie D (eds). Forward with Grass into Europe. Occasional Symposium of the British Grassland Society. No. 27, pp 170-172.
