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The Cotswold Seeds DatabaseDetailed information and articles on grass seed and related matters |
Grass and legume mixtures produce free nitrogen
Organic Farming:
Consumer demand for improved standards of food production and environmental welfare has increased the interest in organic farming and encouraged growth in this sector of the agricultural industry.
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Table 1. Changes in forage production and stocking density at Ty Gwyn during the transition from conventional to organic milk production. |
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Year |
Management System |
Annual Forage Production (t DM/ha) |
Stocking Density (dairy cows/ha) |
1991 |
Conventional (380kgN/ha) |
10.50 |
1.97 |
1992-4 |
In-conversion |
8.31 |
1.55 |
1995 |
Organic Year 1 |
8.37 |
1.49 |
1996 |
Organic Year 2 |
9.49 |
1.76 |
1997 |
Organic Year 3 |
10.03 |
1.8 |
Forage Management
In organic systems, forage provides 60-95 per cent of the feed required by the dairy cow. Depending on the intensity of the system prior to conversion, forage yields and stocking density typically decline by 10-20 per cent when a farm is first converted to organic management. At Ty Gwyn, the change from a system based on high rates of nitrogen fertilizer initially led to a sharp decline in forage production and stocking density (Table 1). Yields then increased as the proportion of clover in the swards increased and as the crop rotation became fully established. Yields in the third year after conversion were 95 per cent of those produced under the previous conventional management.
When perennial ryegrass/white clover swards are grazed continuously to maintain a minimum sward height of 6.7 cm, white clover may contribute over 30 per cent of the total annual yield, and digestibility of the herbage will be maintained at over 70 per cent. Slot-seeding Westerwold ryegrass, Italian ryegrass or rye into the existing swards during the autumn period provides extra yield and early growth. As a result, the dairy herd is able to start grazing 10 to 14 days earlier in the spring and at the same time as conventional herds on fertilized swards. The proportion of clover in the sward increases during the growing season, leading to higher crude protein concentrations in later cuts for both Italian ryegrass/red clover and perennial ryegrass/white clover silages (Table 2).
| Table 2. Analysis of grass/clover silages showing the improved quality of later silage cuts. | ||||
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Silage Type |
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Perennial Ryegrass/White Clover |
Italian Ryegrass/Red Clover |
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Year 1 |
Year 2 |
Year 1 |
Year 2 |
Cut 1: pH |
3.80 |
4.08 |
3.80 |
3.85 |
Crude protein (% of total DM) |
13.7 |
14.1 |
17.8 |
16.2 |
Metabolisable energy (MJ/kg DM) |
10.6 |
11.3 |
10.7 |
10.9 |
Ammonia N (% of total DM) |
4.97 |
2.30 |
5.71 |
6.71 |
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Cut 2: pH |
4.07 |
4.98 |
3.95 |
4.73 |
Crude protein (% of total DM) |
17.5 |
20.3 |
20.7 |
20.5 |
Metabolisable energy (MJ/kg DM) |
10.3 |
10.2 |
11.3 |
9.7 |
Ammonia N (% of total DM) |
7.76 |
9.00 |
4.16 |
7.54 |
Weeds, with the exception of docks, have not been a major problem under organic management. The establishment and maintenance of dense swards and good grazing management have prevented docks increasing in the grazed swards. However, in newly-sown silage fields, the low populations and erect growth habit of Italian ryegrass and red clover produce an open sward that leads to an increase in dock numbers.
Nutrient management
The dairy cow has an annual forage requirement of 5-6 t dry matter. The production of adequate quantities of forage for grazing and conservation is dependent not only on the biological fixation of atmospheric nitrogen in grass/clover fields, but also on the efficient recycling of nutrients in the slurry and manure produced by the herd.
Averaged over the whole farm area, nitrogen fixation at Ty Gwyn was estimated to supply about 100kg N/ha annually, but feed purchased from outside the farm was also an important source of nitrogen. In common with all ruminant-based production systems, whether organic or conventional, only a small proportion of the total nitrogen input was incorporated into the milk and livestock sold from the farm. The recovery in products counted for 21 per cent of the input, leaving a surplus of 140 kg N/ha when averaged over the whole farm area. Some of this nitrogen may have been retained within the farm by incorporation into soil organic matter, but the remainder will have been lost through leaching and as gaseous emissions with risks of damage to the wider environment. However, these losses are likely to be less than from more intensive conventional dairy farms.
In the essence of regular inputs of phosphorus and potassium in fertilizers, the organic system is reliant on inputs of these nutrients in purchased feed and bedding to replace the quantities exported from the farm in agricultural products. At Ty Gwyn, the quantities of phosphorus and potassium bought into the farm as purchased feeds were sufficient to balance the outputs in milk and livestock. Measurements of plant-available phosphorus and potassium in soils also indicated that balances were satisfactory. Contents in soils from Ty Gwyn and from other organic dairy farms that have been monitored in our studies have shown no evidence of decline during six years of organic management.
Animal Health and Performance
Changing to an organic system has increased milk production per cow by 11 per cent and also the fat and protein content of the milk from 3.94 and 3.17 per cent to 4.17 and 3.33 per cent, respectively. Home-grown forage contributed 84 per cent of the annual feed requirements of the herd and 60 per cent of the total milk was produced during the grazing of perennial ryegrass/white clover swards. No major health problems were recorded with the dairy herd, and the withdrawal of long-acting antibiotics did not lead to an increase in the number of cases of clinical mastitis. Alternatives to conventional medicines, including herbal pessaries, liniments, nosodes and phytolacca, have been used successfully to treat specific ailments.
The Future
The objective for the Ty Gwyn farm during the next few years will be to compare two contrasting organic dairy systems. System A will be self-sufficient in terms of feed with the production of home-grown concentrate feeds and a stocking density below 1.5 cows/ha. System B continues to be dependent on purchased concentrates to maintain both a higher stocking density and increased level of production. The two systems are expected to exhibit marked differences in physical and financial performance and in their versatility to react to external market forces. Although the system that has been studied to date appears to be relatively sustainable in terms of nutrient inputs, it is important to recognise that the favourable balances for phosphorus and potassium are dependent upon the import of feed from external sources. Production of these feeds will have depleted the nutrient reserves on the farms where they are grown, but this is excluded from our current assessments of sustainability. The self-sufficient system will provide a measure of the true nutrient requirement of the overall system. The incorporation of an arable component into the management will also increase opportunities for manipulating nutrient flows to reduce nitrogen surpluses and losses from the farm.
With acknowledgements to Steve Cuttle, Evan Jones and Richard Weller. Forage Matters Spring 2000
- by Ian Wilkinson of Cotswold Seeds
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