24 November 1995



What do cows say about the use of rapid wilting?

Dr Desmond Patterson, of the Agricultural Research Institute of Northern Ireland, Hillsborough, Co Down, has conducted a trial to find out

Is rapid wilting feasible?

The first question which arises concerns the feasibility of achieving a rapid wilt in a typical British summer. At Hillsborough it has been found that spreading grass after mowing is the dominant factor governing the rate of wilting under UK conditions. Factors such as moderate conditioning of grass at mowing and turning the grass have much less effect on the rate of drying. Research has shown that sunlight is the most important weather factor in drying rate, not windspeed, etc.

This explains the importance of spreading, for that determines how much sunlight is captured by the swath. Though spread swaths collect more dew at night, and any rain, they also recover faster than high density swaths when dry conditions return. Therefore, combining two swaths into one, in a "double-decker" arrangement, gives very slow drying. Table 1 shows the effects of these treatments on drying rates at Hillsborough.

Feeding study with silage

produced by rapid wilting

Having established clear criteria for rapid wilting, during 1993 and 1994, 11 silage harvests were taken to examine the effects of the techniques on cow performance. The grass was mown down and then either ensiled in an unwilted state or wilted rapidly by immediate spreading followed by tedding at intervals before ensiling. The target dry matter content was 30%. The average dry matter of the wilted grass was 32% at ensiling, while the unwilted grass was 16%, with an average wilting period across all harvests of 38 hours.

Results of the harvests show that, on average, wilting improved silage fermentation. Although across the total 11 harvests only three unwilted silages had poor fermentations, all the wilted grass had good fermentations.

Feeding to dairy cows

To assess the effects of wilting on milk production, the silages were fed to 96 dairy cows, spread over two winters.

The intake of dry matter from the wilted silage was considerably higher (table 2), with an average rise of 1.8/kg of dry matter a day (up 18%). This has a big impact on the requirements for silage to cover the winter period of a herd of any given size.

Milk yield and composition

On average, across the 11 harvests, wilting increased milk yield by 3%, which reverses the trend in previous studies of a fall in yield with wilted silage. Wilting also increased the butterfat content of the milk and the protein was significantly higher. Comp-osition improvements combined with the higher yield give an overall 6% rise in fat and protein content.

This contrasted with the reductions in yield with wilted silage which had been observed at Hillsborough and other British centres. But those trials were using wilting systems not specifically designed to maximise crop drying rate.

Responses to wilting are variable

Though all the wilted silages across the 11 harvests were made under rain-free conditions, there was a wide range of animal performance responses to wilting. The increases in dry matter intake ranged from 0 to 35%, milk yield from 0 to 12% and fat plus protein yield from 0 to 22%. The reasons for these big differences are not clear, and this important feature is being researched further.

Stability of silages

Wilted silage is more prone to spoilage by infiltration of air into the silo face after it has been opened than unwilted silage. The spoilage is first detected as a heat front moving into the silage and is not initially accompanied by visual changes such as moulding. This type of heated silage, with no visible mould, considerably lowers feeding value by depressing intake.

The problem worsens if there is visible evidence of moulding. Aerobic spoilage must be kept under control by having an adequate rate of use of the silage.

Economics of wilting and output per unit of land

An estimate of the economics of wilting can be made by comparing the value of the rise in output against the additional costs of silage eaten.

When grass silage is valued at £100/t of dry matter the cost of the additional dry matter consumed after wilting, 1.8kg a day, is 18p a day. This can be set against the value of the 6% increase in fat and protein output. Assuming quota is available, this could be equated to an additional 1.25kg of milk a day in this study, if the composition had not changed.

A main factor in any calculation of the economic benefit of wilting is the prevention of damage to concrete silos by the elimination of effluent. Such structures are a big capital investment and reducing the risk of pollution is an important consideration.

From the silage intake and milk production responses obtained with wilting in the present study it is estimated that wilting cut the output of fat and protein an acre of land harvested by 4% in comparison with a direct-cut system.

The study also showed it gave equal output of fat and protein an acre to an unwilted system based on mowing followed by picking up using a precision chop harvester. &#42

Table 1: Effect of the degree of spreading of swaths on the dry matter content of grass with a dry matter of 16% at cutting under rainfall-free conditions

Dry Matter Content


Ground CoverSwath arrangement24h48h

25%Two swaths in double1921

decker arrangement

50%Unspread single swaths2125

100%Swath spread to cover2533

total ground area

&#8226 Increases dry matter intakes by 1.8kg a cow a day.

&#8226 Lifts milk yield by 3%, reversing previous trends which saw a fall in milk yield.

&#8226 Increases fat and protein yield by 6%.

Table 2: Effects of wilting on performance of dairy cows



wilting (%)

Silage dry matter

intake (kg/day)10.212.0+18

Milk yield (kg/day)20.721.3+3

Yield of fat plus

protein (g/day)15981697+6

Dr Desmond Patterson has shown that wilting increases milk yield by 3% and also increases milk fat and protein percentages.

Grass is mown and either ensiled in an unwilted state or wilted rapidly by immediate spreading followed by tedding at intervals before ensiling.