To achieve optimal silage yields, balancing soil fertility is crucial. While grass destined for silage cutting requires a high nitrogen input, it is not the only ingredient required to maximise yield. Teagasc research has shown that where soil P (phosphorus) and K (Potassium) levels are low, grass growth will be detrimentally affected.

Most farmers with good quality perennial ryegrass swards will target an N application of about 90 to 100 units N/acre (110-124kg N/ha) for first-cut silage to be cut in May or June. Where older swards are present, fertilizer applications of 80 units N/acre (98-100kg N/ha) are more common.

Farmers who have applied early N to fields should also be aware of the fact that some of that nitrogen may still be available. For example, a farmer who spread 40 units N/acre (50kg/ha) in February should consider that 20-30% or 8-12 units N/acre (10-15kg N/ha) of this will be still available to grow grass for silage.

The amount of fertilizer applied to silage swards will ultimately determine the cutting date. Grass utilises about two units of nitrogen/day or 14 units per week.

Applying 90 to 100 units of N/acre (110kg to 124kg of N/ha) will mean that the sward cannot be cut for at least seven weeks. Cutting before this will lead to elevated levels of nitrates in the grass, which could affect fermentation.

Applying lower levels of nitrogen will allow the crop to be cut earlier. This will result in higher quality but lower yield. Moving the first cut later to increase yield will result in a smaller window to make a second cut, and lower yields in the second cut.

Phosphorus and potassium

Growing grass for silage results in a large amount of P and K being removed from soil to facilitate the growing of the crop. A farmer making first-cut silage should be aware that 16 units P/acre (20kg P/ha) will be removed by the crop.

In addition, about 100 units of K (125kg K/ha) will be removed by the same crop. A second cut of silage will remove 8 units P/acre and 28 units K/acre (10kg K/ha and 35kg K/ha).

Slurry

Cattle slurry can contain a varied amount of N, P and K. Teagasc research shows that cattle slurry contains about 6 units of N, 5 units P and 38 units K per 1,000 gals.

Given the concentration of nutrients in slurry, about 3,000gals of slurry would be required per acre to replace the P and K removed in silage. Where slurry is more watery in nature, the nutrient levels will be lower.

Grazing

On wetter ground, the heavy rainfall over the spring has resulted in farmers not being able to graze off fields before closing for silage. Teagasc studies have shown that where fields have not been grazed in the autumn or spring prior to closing for silage, the dry matter digestibility of the silage may be 5-6 units lower.

Grazing now with light stock, without causing damage to the sward, may help to increase the quality of silage produced. Where ground conditions are too poor, grazing should be avoided as poaching will result in a reduction of grass growth by up to 20%.

It should also be noted that grazing to 5cm (2in) in the spring before closing for silage will decrease yields by about 1.5t DM/ha.

Quality or quantity?

For suckler farmers feeding dry suckler cows over the winter months to maintain condition, silage quality may not be as big an issue. Average quality 65-68DMD silage fed ad-lib is sufficient for dry cows to maintain condition. However, where cattle are on the farm for fattening, or where cows are at milk, higher quality silage is required.

Feeding a beef animal average quality 65-68DMD silage alone ad-lib will result in a daily liveweight gain of 0.45kg/day. A lactating dairy cow on the same diet will produce 9-10 litres of milk from forage. However, the same animal categories on 80-82DMD silage would gain 0.9kg liveweight per day or a lactating dairy cow would be able to produce 22-23 litres of milk from silage alone.

Quality silage must be cut when no more than 50% of grass has headed out. This will depend on the grass type in the sward and the heading date. After this, silage quality will deteriorate by about 2 DMD units per week.

Conditions at harvest

Everything should be done to ensure top-quality silage is harvested. However, despite many farmers’ best efforts, ground conditions and weather at harvesting play a fundamental role in determining the quality produced.

Poor ground conditions can result in many farmers delaying cutting for days or even weeks in the hope of improved conditions so that machines will not cause damage to fields.

Contractor availability can also affect quality in some cases. During the main silage season, contractors work to a schedule, which in some cases is not very flexible.

This can mean that weather conditions may not be desirable when harvesting takes place but there is little that can be done about this as investing in your own silage making machinery may not be financially feasible.

Farmers cutting silage in mid-May and late June often have the best chance at having flexibility in cutting date as these dates are outside the main silage cutting period.