R.H. Johnson
Institute of Grassland and Environmental Research,
North Wyke, Devon, EX20 2SB, United Kingdom.
Introduction
Materials and methods
Results
Discussion and conclusions
References
Acknowledgement
It is generally advocated that grazed grass/white clover swards should be maintained at target sward heights to optimize herbage utilization (Maxwell and Treacher, 1986). Previous research has shown that the liveweight gain per hectare of beef cattle grazing grass/clover swards was higher when the sward was maintained at an overall height of 6 cm rather than either 4 cm or 8 cm (Morrison, 1988). However, providing a 6 cm sward from the beginning of the grazing season would necessitate an unacceptably late turn-out date on most farms. Late turn-out could be avoided either by making a tactical application of fertilizer N to stimulate herbage growth in early spring (Morrison et al., 1983), or by accepting a sward height of less than 6 cm for the first few weeks of the grazing season.
To investigate the effect of these management options, singly and in combination, a grazing trial was carried out over two successive years.
The experiment was carried out at North Wyke in south-west England on a 3-year-old perennial ryegrass/white clover sward which had previously been managed, without fertilizer N, for silage and then autumn grazing with cattle. The site was south facing at an elevation of 170m. The soil was a clay loam over clay, classified as Halstow series.
A 2 × 2 factorial design incorporated two fertilizer N treatments, 0 and 50 kg N ha-1 (N0, N50), and two sward heights, namely, 4 cm at turn-out rising to 6 cm in June (SH 4-6), and 6 cm for the whole season (SH 6-6). All treatments received P2O5 and K2O, each applied at 50 kg ha-1 per year in the first week in April. Fertilizer N, as ammonium nitrate, was applied to the N50 treatments at the same time.
Swards were grazed by Hereford X Friesian steers of 6-9 months of age and average weight 200 kg at turn-out. Each grazing plot was allocated two core animals, which remained on the plot throughout the season, and sward heights were maintained by moving additional non-core animals in and out of the plots as sward height dictated. Non-core animals were of similar age and weight to the core animals and were grazed on a similar sward when not used on the plots. Sward height was measured twice weekly using the rising plate meter (Michell and Large, 1983). All the core animals were weighed weekly, and non-core animals were weighed when they were moved to and from the experimental area.
Herbage accumulation (Hodgson, 1979) was measured at 3-weekly intervals throughout the grazing season, using four quadrats (100 cm × 50 cm) per plot under exclusion cages. Samples were cut using Sunbeam sheep shears, cutting at a height of 2 cm (Michell and Large, 1983). The samples were hand sorted into grass and clover, oven dried, ground, and analyzed for organic matter.
Data were analyzed using analysis of variance, and the results of animal production and herbage accumulation are presented for the circa 6-week period from turn-out until 11 June, and also for the whole season.
Table 1 summarizes the 2-year mean values for turn-out date, stocking rate, grazing days, net herbage accumulation and percentage clover, for (1) the first 6-week period of the grazing season, and (2) the whole season. There were no significant interactions between sward height and fertilizer N levels for either period.
Table 1. Mean values for sward productivity.
|
Sward Height (cm): |
4-6 |
6-6 |
Signif | |||
|
Fertilizer N: |
N0 |
N50 |
N0 |
N50 |
Ht |
N |
|
From turn-out to 11 June | ||||||
|
Turn-out (Julian days) |
123 |
123 |
133 |
131 |
*** |
NS |
|
Mean sward height (cm) |
4.5 |
4.7 |
6.1 |
6.4 |
*** |
* |
|
Mean stocking rate (steers ha-1) |
10.1 |
12.5 |
6.9 |
10.4 |
* |
* |
|
Grazing (d ha-1) |
403 |
496 |
268 |
408 |
* |
* |
|
Net herbage accumulation (t OM ha-1) |
2.51 |
3.02 |
2.55 |
3.93 |
NS |
** |
|
% clover |
24.0 |
12.8 |
22.7 |
22.2 |
NS |
NS |
|
The whole season | ||||||
|
Turn-out (Julian days) |
123 |
123 |
133 |
131 |
*** |
NS |
|
Mean sward height (cm) |
5.4 |
5.3 |
5.8 |
5.9 |
*** |
NS |
|
Mean stocking rate (steers ha-1) |
6.7 |
6.7 |
6.0 |
7.1 |
NS |
NS |
|
Grazing (d ha-1) |
1150 |
1152 |
1033 |
1223 |
NS |
NS |
|
Net herbage accumulation (t OM ha-1) |
7.74 |
7.61 |
7.71 |
8.68 |
NS |
NS |
|
% clover |
40.7 |
32.2 |
33.7 |
36.6 |
NS |
NS |
Period from turn-out to 11 June. Turn-out date of the 4 cm sward height treatment (SH 4-6) was 9 days earlier than the 6 cm treatment (SH 6-6). Fertilizer N had no effect on date of turnout, but it increased mean sward height, herbage accumulation and livestock production in this period. There were no treatment effects on the proportion of clover harvested.
Period for the whole season. There were no significant treatment effects on either livestock production, herbage accumulation or the proportion of clover.
Allowing turn-out onto a grass/white clover sward at a 4 cm sward height gave the advantage of an additional 9 days of grazing at the beginning of the season, without a penalty in terms of livestock production. Fertilizer N applied in early April, when ground conditions permitted, increased herbage production, and hence carrying capacity, during the first 6 weeks of grazing, without adversely affecting white clover. However, it did not advance herbage growth sufficiently to enable an earlier turnout. This may have been because soil N status was not the main factor limiting herbage production on this site during the critical period in April. Perhaps residual N from the previous year was adequate, or soil temperatures may have been too low.
The acceptance of a sward height of less than 6 cm at turnout can extend the grazing season and so reduce indoor feeding. Fertilizer N can enable a higher stocking rate in the period after turn-out. However, early turn-out, and a high stocking rate in spring, may not be options on heavy textured soils, because of the risk of herbage production being affected by poaching.
HODGSON J. (1979) Nomenclature and definitions in grazing studies. Grass and Forage Science. 34, 11-18.
MAXWELL T.J. and TREACHER T.T. (1986) Decision rules for grassland management. In: Pollott G.E. (ed.) Efficient Sheep Production from Grass, British Grassland Society, Occasional Symposium No. 21. pp. 67-78.
MICHELL P. and LARGE R.V. (1983) The estimation of herbage mass of perennial ryegrass swards: a comparative evaluation of a rising-plate meter and a single-probe capacitance meter calibrated at and above ground level. Grass and Forage Science, 38, 295-299.
MORRISON J. (1988) Grassland Production: Fertilizer-N, water and white clover. In: Wilkins, R.J.(ed.) Nitrogen and Water use by Grassland, Colloquium Proceedings, pp. 6-23. Hurley: AFRC-IGAP.
MORRISON J., DENEHY H.L., and CHAPMAN P.F. (1983) Possibilities for the strategic use of fertilizer N on white clover/grass swards. In: Corrall, A.J. (ed.) Efficient Grassland Farming. Proceedings of the 9th General Meeting of the European Grassland Federation, pp. 227-331.
This research was funded by the United Kingdom Ministry of Agriculture, Fisheries and Food.
