Technology of Forage and By-Products Preservation
7. Harvesting
The time to harvest the crop is determined by:
- The stage of maturity.
- Husbandry factors.
From the moment the crop is cut, until it reaches the end of the fermentation process, chemical and microbiological activities continue, and the nutritional value diminishes. Thus, the determination of the appropriate time of harvesting can be crucial for some crops, whereas for others it may affect the quality, yield and/or ensilabilty. Different groups of forage crops have differing criteria for determining the harvesting maturity stage, and the decision has to be taken individually for each type of crop within its specific growth area.
Silage making is a relatively big operation that involves several expensive machines and considerable labor, and it should be done in as short a time as possible. Therefore, in many cases, specialist contractors do the technical work of silage making, and the harvesting time has to be coordinated with them too. The correct maturity stage for harvesting is determined on the basis of obtaining the maximum profit (in return for investment in land, water and/or labor) according to economic parameters which vary with time and location.
Usually digestibility decreases and yields increase with maturation, however, and an excessively mature crop may not be suitable for ensiling (too dry, poor WSC content). Harvesting a forage crop too early may lead to a bad fermentation pattern (butyric); its low DM content will increase effluent production or will necessitate more wilting time. Too early harvesting always leads to loss of potential material. Extension of wilting time will increase enzyme activities, and the consequent deterioration and losses. The physiological characteristic of the plant, and the environmental conditions (humidity, soil moisture, temperature, and day length) are important factors, which can help to decide the correct harvest time, therefore knowledge of all of these is essential.
An additional important factor that has to be taken into consideration when harvesting is the cutting height. Stones, clods, uneven soil, and stubble from previous harvests necessitate higher cutting. Too low cutting can contaminate the forage with soil, which has very bad effects on silage quality: it reduces the nutritional value of the silage, and impairs the fermentation process (by increasing the buffer capacity and introducing clostridial bacteria). The height of the cut also affects the yield and the overall quality of the organic material that can be collected; forage material that is allowed to overlie on longer stubble for wilting will dry faster.
It is important to emphasize that the lower parts of the stem (in corn and sorghum) are less digestible, therefore, from the nutritional point of view it is sometimes preferable to cut higher in order to obtain a better quality of material; cutting can even start from as high as 2-3 inches. For forages, the stage of maturity for harvesting is usually judged according to kernel maturation, or the stage of blooming. For cereals (summer and winter) such as corn, sorghum, wheat, barley, etc., the correct harvesting time is judged according to the grain maturity; it is recommended to start harvesting when the grains are in the milk stage and to end it when they are in the dough stage. The milk stage is defined as "the stage at which the seeds are well formed, but soft and immature" and the dough stage is defined as "the stage at which seeds are of a dough-like consistency".
Corn harvesting should start when half of the kernel is starchy, and the other is still milky. In some varieties it is possible to see a "black line" between these two parts Fig. 8, Fig. 9 and Fig. 10 .
For grasses and forage legumes the recommended stage of harvesting for silage is judged according to bloom intensity: between early bloom and mid-bloom, when 10 to 65% of the plants are in bloom. This range of stages represents the best period for harvesting the forage for silage making. In alfalfa the main changes in quality appears in the stem.
Changes in cellulose and lignin contents of alfalfa as it matures (% in DM)
| Harvesting date | Leaf | Stem | ||
|---|---|---|---|---|
| cellulose | lignin | cellulose | lignin | |
| April 22 | 7.1 | 2.43 | 11.0 | 1.80 |
| April 28 | 7.0 | 2.51 | 10.2 | 2.10 |
| May 5 | 6.9 | 2.83 | 15.2 | 3.76 |
| May 13 | 7.1 | 2.37 | 16.6 | 4.73 |
| May 22 | 7.1 | 2.85 | 22.5 | 6.72 |
| June 4 | 7.6 | 7.82 | 23.5 | 8.79 |
In small cereals, as maturation advances and the proportion of grain increases, the lignin concentration decreases proportionally.
Changes of cell-wall components of whole-wheat plants (% in DM)
| Days after seeding | crude fiber | cellulose | hemi–cellulose | lignin | NDF | ADF |
|---|---|---|---|---|---|---|
| 43 | 17.4 | 18.9 | 20.9 | 45.0 | 3.16 | 23.3 |
| 65 | 21.7 | 24.5 | 21.4 | 3.3 | 51.1 | 30.4 |
| 80 | 25.4 | 28.8 | 20.2 | 4.9 | 54.7 | 35.5 |
| 107 | 29.1 | 30.8 | 21.4 | 6.2 | 58.9 | 39.6 |
| 1125 | 29.0 | 31.0 | 19.2 | 6.6 | 58.2 | 40.3 |
| 2137 | 24.7 | 26.0 | 23.7 | 5.3 | 46.9 | 36.8 |
| 3149 | 20.5 | 21.6 | 21.6 | 3.0 | 48.1 | 27.8 |
