Technology of Forage and By-Products Preservation
8. Wilting
Wilting involves partial drying of the forage in the field in order to raise the DM content to a suitable level for the LAB activity, and to minimize effluent production. Wilting is not necessary or even possible for every forage crop: crops that have thick stems, such as corn, sorghum, sugar cane, elephant grass, etc., are directly cut, whereas winter cereals, grasses and forage legumes usually need to wilt Fig. 11 and Fig. 12.
Effects of wilting:
Positive effects.
- It increases the DM content and so brings the plant to a suitable moisture content for the LAB fermentation.
- It increases WSC concentration – an important consideration, especially for WSC-poor forages and legumes.
- It reduces transportation costs by removing surplus water.
- If it raises the DM content above 30% the ensiling process is not likely to produce effluent – a very important goal.
Possible negative effects.
Potential negative effects that necessitate care include:
- The plant enzymes continue to be active during the wilting period, which increases deterioration and losses. The longer the wilting continues, the greater will be the losses.
- There are likely to be changes in the microflora population.
Effluent
Effluent is the liquid that leaks from the bottom of the silage stack when very wet forage (generally with < 30% DM) is stored as silage. Short chopping and high compaction increase the volume of effluent released. The effluent contains up to 5% of dissolved components, including valuable feed nutrients (soluble sugars, organic acids, minerals and NPN components) which can thus be lost from the silage, therefore, it should be collected and can be either fed to the cattle or spread on fields as a fertilizer. In addition to the nutritional losses, effluent is a very highly polluting material; its biological oxygen demand (BOD) is very high, therefore, in most countries it is forbidden to release effluent. If effluent is produced it must be collected, to avoid pollution, especially of water resources.
The BOD is expressed in milligrams of oxygen consumed per liter of sample during 5 days of incubation at 20°C; it is a measure of the water-polluting potential of organic matter.
Effluent produced and resulting DM loss from bunker silo
| DM content (%) | Effluent produced (Gallons per ton silage) | DM loss (%) |
|---|---|---|
| 30 | 0 | 0 |
| 25 | 5 | 0.4 |
| 20 | 15 | 1.6 |
| 15 (direct cut) | 50 | 7.2 |
The BOD of common agricultural pollutants of water sources
| Pollutant | BOD (mg O₂ l⁻¹) |
|---|---|
| Silage effluent | 90.000 |
| Pig slurry | 35.000 |
| Cow urine | 19.000 |
| Cow slurry | 5.000 |
| Domestic sewage | 500 |
The negative effects of wilting are related to the enzymatic activities, mainly respiration, which is a biochemical process involving the breakdown of plant sugars to gaseous carbon dioxide and liquid water.
(sugar)
(oxygen)
(carbon dioxide)
(water)
Reducing the duration of wilting will decrease these losses; however, in places with unfavorable climatic conditions wilting is difficult, so that a technology to accelerate the wilting process (conditioning) becomes very useful. The respiration rate decreases almost linearly with decreasing moisture content until the DM content reaches approximately 60%, and it increases exponentially with temperature up to about 27°C. Rewetting of the crop by dew or rain (especially during haymaking) reactivates enzyme activity and thus prolongs respiration.
Factors that affect wilting rates
Many factors can affect the speed of wilting; they can be divided into three main groups:
- Climatic factors such as: temperature, relative humidity, wind, rain, dew, soil moisture, and solar radiation. Probably radiation is the most significant wilting factor.
- Crop factors such as: character of the plant (waxy, cuticular), yield, maturation stage, and initial DM content.
- Husbandry factors such as: distribution of the forage in the field, times of turning over, swath fluffing, and cutting height. The combination of all the above factors will determine the wilting speed. Since radiation has the most significant effect, and penetration of sunlight into the forage swath is very limited, it is important to spread the forage over the whole available ground area to form as thin a layer as possible, and to turn it several times in order to maximize the exposure of forage particles to sunlight.
Conditioning
To accelerate wilting, and to eliminate effluent production and respiration losses, conditioning devices have been developed, in which the conditioning process is combined with the cutting operation. Additional knives mounted in the mowing machine crack the plant stem or scratch its outer layer, in order to increase moisture loss. The use of conditioning to shorten the forage wilting time has become a popular technology in several countries, especially in northern Europe where wilting time and effluent production are problems. Drying agents (Na2CO3) are also used to promote drying, but they are mostly used in haymaking Figs. 13 , Fig. 14 ,Fig. 15 Fig. 16 and Fig. 17 .
