Technology of Forage and By-Products Preservation
5. Raw material - Selected forages for silage making
Summer crops:
Corn and sorghum are the main summer forage cereal crops used for silage.
Corn (Zea mays): In many countries corn is the most important forage crop for silage; it originated in America and is recognized as the "nearly perfect" silage crop. It needs rain or relatively intensive irrigation in the growing season, and much fertilizer, especially nitrogen, to which it responds well. It has a high yield - up to 20 tonne (or even more) of dry matter (DM) per hectare - and the digestibility of the whole plant remains stable during the maturation stage, which means that the harvest can be relatively extended. Harvesting for silage is by direct cut (no wilting), and the DM content in the standing crop is usually between 28 and 42%, which can be suitable for ensiling. Corn has an adequate content of water-soluble carbohydrates (WSC), and low buffer capacity (200), but corn silage is low in crude protein and minerals. It is a high-energy forage crop Fig. 3 .
Nutrient contents of corn silage (on DM basis*)
| Nutrient | Average value | Range |
|---|---|---|
| Crude protein | 8 | 6 - 17 |
| Acid detergent fiber (ADF) | 28 | 20 - 40 |
| Neutral detergent fiber (NDF) | 28 | 30 - 58 |
| Total digestible nutrients (TDN) | 67 | 55 - 75 |
| Net energy-lactating (NEL) (Mcal/lb) | 0.68 | 0.58 - 0.74 |
| Calcium | 0.26 | 0.1 - 0.4 |
| Phosphorus | 0.3 | 0.1 - 0.4 |
In light of the wide range of nutritive ingredients in corn, it is important to select the correct hybrid for nutritional values and ensilability purposes. High-moisture corn is a product of ensiled corn grain; it contains 25-30% moisture. These grains are treated as silage, i.e., they are compacted and sealed. The grain undergoes a fermentation process, and is then preserved as silage. This technology is common in places where corn is grown and cattle are fattened in the same area.
Sorghum (Sorghum bicolor) Sorghum is native to Africa. In comparison to corn it is a strong crop: it demands less fertilization, can tolerate low-quality soil and water, and is relatively resistant to salinity. In subtropical areas it can grow on dry land, watered only by winter rain, if the soil can hold the water. Sorghum has a high yield, its harvesting time can be flexible, and the plants have the possibility to regrow. Sorghum accumulates most of its water in the stem, which remains wet even when the plant is in its mature stage. Therefore, sorghum forage may be wetter than desirable for silage.
The grains in sorghum are located on the top of the plant and are unprotected (in contrast to corn grains); therefore they can be attacked by birds. Also the top location of the heavy grain makes the crop subject to lodging Fig. 4 and Fig. 5 . These two factors must be known and taken into consideration in planning the ensiling process.
The grains of some sorghum hybrids may contain tannin, which makes them bird resistant, but reduces the protein quality. This factor is significant when such grains are fed to chickens or pigs, but insignificant when silage is made from the whole plant and fed to ruminants. In bad growing conditions, especially when water is lacking, or under excessive nitrogen fertilization, the sorghum plant may accumulate nitrate, especially in the lower parts of the stem. However most of the nitrate decomposes during the silage fermentation, and ceases to be a danger.
Young sorghum and Sudan grass that grow rapidly or have been stressed can accumulate prussic acid, which may be degraded by the animal to release hydrogen cyanide (HCN), which is highly toxic. Therefore, the plant heights before harvesting (or grazing) should be at least 40-50 cm for Sudan grass, and 75-80 cm for sorghum-Sudan grass hybrids. The fermentation process does not decrease the prussic acid concentration, but wilting can release about 50-70% of it.
The protein content of sorghum is somewhat higher than that of corn, but its digestibility for dairy cows may be a little less. A new type of forage sorghum has been developed - the BMR (brown middling rib) - which contains less lignin and is therefore more digestible. Sorghum for silage is popular in hot, semi-arid areas; in the USA it is cultivated mainly in the Midwest for beef cattle.
Alfalfa (Medicago sativa L.) Alfalfa originated near Iran, but related forms and species grow wild throughout Central Asia and into Siberia. It is a perennial legume forage crop that can be re-cut several times during the hot growing season, and the number of re-cuttings depends on day length and temperature. Alfalfa requires much water, but can grow with low-quality water. Alfalfa is often called the "Queen of the Forage" but this probably does not relate to ensiling. Legumes in general, and alfalfa in particular, need to be wilted before ensiling, and apart from that, they are not the best forage crops for ensiling; the high protein content, high buffer capacity and the low WSC content result in bad fermentation and silage. Other legume forages that can be ensiled include clovers (red and white), field beans, common vetch, etc. The technology of making haylage (a combination term = hay + silage), a common means of preserving legumes, improves the possibility of obtaining a better preserved product. The DM content of such material ranges between 50 and 60%, and the forage is preserved in anaerobic condition, treated like silage to promote fermentation.
Grasses Many grasses, whose characteristics vary according to the growth area (tropical, subtropical or cool-season), can be ensiled. A very popular grass (at least in Europe) is Italian ryegrass (Lolium multiflorum). Traditionally most grasses were used for pasture, but it is more productive (from the yield point of view) to harvest them for silage. All grasses are relatively easy to ensile; they are ensiled after wilting to ensure an adequate DM content.
Winter crops:
All winter cereals can be used as forage crops and can be ensiled well; the choice of crop (wheat, barley, oat, rye or others) is a purely economic decision. The two major advantages of using winter cereals for silage are the use of winter water, and the possibility of growing a second crop during the summer. An additional advantage of cultivating two crops (summer and winter) is the prevention of wind and water erosion Fig. 6.
Miscellaneous crops Other crops commonly used for silage include sunflower, sugar cane, elephant grass, etc.
Feed value of some small grain and corn silages (% in DM)*
| Silage | DM | Crude fiber | Crude protein | TDN |
|---|---|---|---|---|
| Barley | 38.8 | 27.1 | 9.0 | 64.3 |
| Wheat | 39.4 | 27.9 | 9.6 | 63.8 |
| Oats | 40.2 | 31.2 | 9.8 | 60.7 |
| Rye | 39.8 | 33.0 | 58.5 | |
| Corn | 37.3 | 24.7 | 8.1 | 66.4 |
