Technology of Forage and By-Products Preservation
6. The Chemical and Microbiological Qualities of the Whole Plant, as Relevant to Preservation by Ensiling
The chemical and microbiological composition of the plant (residue or by-product) strongly affects the ensiling quality. The most important components are:
- Moisture content.
- Carbohydrates quality and quantity.
- Protein content.
- Buffer capacity.
- Microorganism load.
Moisture content The moisture content of the raw material is a dominant factor in determining the fermentation quality, and there is an optimum content for the development of the desirable lactic acid bacteria (LAB). Too dry a material will encourage the development of yeasts and molds, create compaction difficulties and inhibit LAB. Too wet a material will encourage the development of butyric acid bacteria (clostridia), and will form effluent. The moisture content also affects the concentrations of the other ingredients (such as sugars) in the material. In many cases, especially with forage crops (not direct cut) it is possible somehow to control the DM content, and to bring the water content to the desired level by wilting or by adding water. The recommended DM content in forage for ensiling is not precisely defined; it ranges between 30 and 42%, and sometimes even more widely.
Carbohydrates: Carbohydrates in forage crops fall into two categories:
- Structural - mainly cell wall (cellulose, hemicellulose and lignin), starch, and peptic substances.
- Nonstructural - glucose, fructose, sucrose, etc., collectively referred to as WSCs, they can be fermented rapidly by LAB. Their content in plants may vary, according to species, cultivar, stage of maturity, climate, and time of day. Tropical grasses are generally considered to be lower in WSCs than temperate species; it has been calculated that each degree Celsius of temperature increase in the cultivation area reduces the digestibility of grasses by 0.5%. Sugar content has a very important effect on the ensilability of forages. The usual calculation is that for efficient lactic acid fermentation, a crop should contain a minimum of 3% of WSCs (sugars) in the fresh weight, and over 15% in the DM. This factor is especially important in WSC-poor crops, and in those with high buffer capacity (legumes). The WSC content data can indicate the amount of wilting that is needed.
Sugar requirements for complete fermentation
Minimum initial sugar content (% of DM)
| Dry matter (%) | Alfalfa | Grass | Corn |
|---|---|---|---|
| 17 | 34 | 28 | 20 |
| 20 | 25 | 19 | 14 |
| 25 | 21 | 14 | 10 |
| 30 | 17 | 10 | 7 |
| 35 | 14 | 7 | 5 |
| 40 | 10 | 5 | 4 |
| 45 | 7 | 3 | - |
| 50 | 6 | 2 | - |
| Typical WSC content on DM basis* | 4-6 | 10-20 | 8-30 |
Protein Most of the total protein content of the plant is present as true protein, and in silage much of the nitrogen content is in the form of non-protein nitrogen (NPN) compounds such as ammonia, nitrates, nitrites, free amino acids, and peptides. In the silage, the solubilization of the true protein occurs through the activity of plant enzymes (proteases) that are released when the cell wall breaks down during chopping. Low pH, high DM content, and low temperature decrease enzyme activity and protein solubilization. Legumes, which by their nature are rich in protein, are more sensitive to this process. Plants that are harvested in their early maturation stage, or that received heavy nitrogen fertilization may also have high protein contents. Generally, tropical grasses contain less protein than temperate species. NPN in silage can range between 30 and 50% of the total protein content of the fresh plant. Although protein is an essential component in nutrition, in silage it has a buffering effect that reduces the intensity of pH reduction, therefore protein is considered to be a negative factor in the ensiling process.
Buffer capacity The buffering capacity is a measure of the ability of forage (and any other material) to resist changes in pH during ensiling, the greater the buffering capacity, the greater the amount of acids required to reduce pH. Most of the buffering capacity of forage can be attributed to the anions present (organic acid salts, phosphates, sulfates, nitrates and chlorides), and only 10-20% to plant proteins. The buffering capacity (of forage) is expressed as the number of milli-equivalents (mEq) of alkali required to change the pH of 1 kg DM from pH 4 to pH 6; it can range from 265 in grass to over 600 in legumes. A high buffer capacity prolongs the fermentation process, necessitates the use of more WSC for fermentation, inhibits pH reduction, and increases losses.
Buffering capacity of some forage crops
| Crop | Buffer capacity (mEq NaOH kg-1 DM) |
|---|---|
| Corn, cereals, sorghum | 200 |
| Cocksfoot | 300 |
| Perennial ryegrass | 350 |
| Italian ryegrass | 430 |
| Lucerne | 520 |
| Red clover | 560 |
Microorganism load The assembly of microorganisms that are picked up from the field with the harvested forage affect the fermentation process and the silage quality. The epiphytic microorganisms are present in very large numbers and many varieties, both of which may vary according to climatic conditions and forage type. Temperature, moisture and radiation are among the factors that have a significant effect on the size and composition of this microorganism population, which, in turn, will later affect the fermentation process Fig. 7 . There is little that can be done to influence the epiphytic population, beyond maintaining the hygiene of the field, and avoiding the collection of weeds and organic material residues (from the previous harvest), and preventing contamination with soil. Past experience has provided us with information on the character of the microorganism population. Chopping increases the number of the bacteria that are counted, because chopping releases WSCs, which enables some additional dormant microorganisms to become active.
Epiphytic microflora on standing and chopped corn
(log10 *CFU/g of fresh crop)
| Microflora | Standing | Chopped |
|---|---|---|
| LAB | 4.22 | 6.31 |
| Enterobacteriaceae | 6.87 | 7.49 |
| Yeasts and molds | 6.85 | 7.12 |
| Lactic assimilating yeasts | 6.36 | 6.55 |
| Clostridia spores | 1.97 | 2.88 |
The quality of the forage has a very strong impact on the animals' intake. Animals fed with low-quality forage will have a low intake, which will affect their performance. Dairy cows are particularly responsive to forage quality. Since the input costs of preserving good or bad forage are the same, when forage has to be preserved attention to its quality is especially important.
The effect of forage quality on predicted forage dry matter intake of ruminants*
| Forage quality | % forage NDF (dry basis) | DM intake as % of body weight |
|---|---|---|
| Excellent | 38 | 3.16 |
| Excellent | 40 | 3.00 |
| Excellent | 42 | 2.86 |
| Excellent | 44 | 2.73 |
| Excellent | 46 | 2.61 |
| Excellent | 48 | 2.50 |
| Excellent | 50 | 2.40 |
| Excellent | 52 | 2.31 |
| Poor | 54 | 2.22 |
