2. Influence of drought on the production of the crops in the semiarid regions

Drought is the meteorological event when there is inadequate water availability in the soil or rainfall, including quantitative and qualitative, during the life cycle of a plant, limiting full expression of the gene of the plant potential and preventing the maximum yield from a culture [4].

The planning activities of animal production in drought regions should take into consideration some factors, such as production yield, drought resistance, and water-use efficiency by plants, for crop production. Although a culture presents high production yield, this may not be compatible with higher drought resistance or increased water-use efficiency [5].

In rainfed situations where crops depend on unpredictable seasonal rains, the maximum use of soil moisture is a crucial component for drought resistance, that is, water-use efficiency allows the production yield even in the situations of water deficit [5].

The variability of rainfall provides the diversity of fauna and flora species in the semiarid region. The soil and climate conditions are associated with the characteristics of species, such as solar radiation, sunshine, and air temperature. The climatic variations suggested that animal production systems operate according to the availability of resources and controlled princi-

In semiarid regions, annual rainfall is irregular, low, and highly variable in space and time, with permanently high evapotranspiration rate. Therefore, the agricultural systems used in the semiarid region should be based on plants that develop efficiently and quickly by using the resource availability of pulses [2], because the water dynamics is the main variable for control-

Tropical regions, such as semiarid regions of Brazilian, may have a high capacity for forage production, but climatic variables make it difficult for the development of the animal production system. The quantity and quality of forage are key factors for animal production. The management also influences the forage characteristics and animal production. During the dry season, a significant reduction in native vegetation occurs, and this affects animal forage

The shortage of forage during the dry season and low nutritive value of forage may compromise the animal production, resulting in decreased productivity. In this situation, the producers become dependent on the availability of the preserved forage, hay, and silage,

For the efficient production of forage in the semiarid regio, it is essential to know conditioning factors inherent to soil, climatic condition, and plant interaction mechanisms to drought and production capability. Adapted crops, due to their efficiency in the accumulation of green matter in these climatic conditions, are available as a more viable option to the semiarid region. Among other considerations. the forage conservation practices, silage is a better alternative to reduce the qualitative and quantitative fluctuations in the supply of forage to

As a literature review, this chapter presents scientific reports on ensilage of forage with productive potential for the semiarid regions. This study presents the main crops for ensilage

Drought is the meteorological event when there is inadequate water availability in the soil or rainfall, including quantitative and qualitative, during the life cycle of a plant, limiting full expression of the gene of the plant potential and preventing the maximum yield from a culture [4].

The planning activities of animal production in drought regions should take into consideration some factors, such as production yield, drought resistance, and water-use efficiency by plants, for crop production. Although a culture presents high production yield, this may not be

compatible with higher drought resistance or increased water-use efficiency [5].

2. Influence of drought on the production of the crops in the semiarid

cultivated forage crops, and crop residues to feed cattle in the semiarid region [3].

in the semiarid regions and their fermentation characteristics.

pally by the availability of water, adopting rational strategies for production.

ling the transformation process of individual nutrients available for plants.

production.

66 Advances in Silage Production and Utilization

the animals.

regions

Drought resistance is the ability of a plant to produce with minimal loss in a water-deficit environment. Drought resistance mechanisms can be classifieds into the following three categories: drought escape, drought avoidance, and drought tolerance [6].

Drought escape is the ability of plants to complete the life cycle before there is a serious water deficit for plant and soil. The phenological development of plants is fast with early flowering and maturity and the duration of the growing season depends on water deficit [6]. The success of these species depends on the efficient reproduction before a more intense water stress. With short life cycle and high growth rates and storage, this process uses reserve for seed production [7].

Drought avoidance is the ability of plants to maintain some potential of water in the tissue even with low moisture in the soil. The better absorption of water, mobile water storage, and reduction of water losses are some of the processes that are used for these plants. The balance between turgor, increased depth of rooting, higher absorption efficiency, and lower losses of water allows the survival of plants under dry conditions [6]. Furthermore, older leaf senescence reduces the energy cost of the plant [7] directing all the energy to dry the adaptive mechanism.

Drought tolerance is the ability of plants to resist water deficit with low potential of water in the tissues [6]. In water-limited environments, the plants can produce forage mass using water maintenance mechanisms in the plant. One of the processes is the osmotic adjustment. Osmotic adjustment is an adaptive response to cellular stress that in some cultures increases the avoidance dehydration and supports the production yield under stress [5]. Osmotic adjustment maintains turgor and resists to dehydration through solute accumulation in the cell, an increase in cell elasticity, and a decrease in the cell size [6].

In drought, these plants maintain the water content accumulating several nontoxic solutes that do not interfere with metabolism; these are compatible solutes such as fructan, trehalose, polyols, glycine, proline, betaine, and polyamines [4, 8].

Although the drought resistance is important for crop production in semiarid, the adjustments resulting from this drought tolerance have disadvantages because of lower production output. The stomata closure and reduction in leaf area result in lower carbon dioxide assimilation and higher osmotic adjustments that can have a negative effect on the plant energy requirement [4].

Cultivate crops are grown using more than one mechanism to resist drought [9]. Thus, it is interesting to note that the adaptive mechanisms of crops grown in the semiarid region have a balance of escape, avoidance, and drought tolerance, maintaining the yield production as much as possible. Through conventional breeding or biotechnological methodology, the development of superior genotypes resistant to drought is possible [4].

Most of the crops produced in the world are sensitive to water deficit. Even cultivative crops, such as pearl millet, sorghum, and pigeon pea, in semiarid regions are affected by drought during the reproductive stage [4].

The C4 plants are considered to be dominant in resistance and drought tolerance because they are capable of maintaining photosynthesis with closed stomata. Even with the small reduction of photosynthesis under water stress conditions, the C4 crops such as sorghum and panicum have the ability to grow in dry region and are considered to have a great potential for enhancing forage production and food security in the world [7]. The C4 plants provide competitive conditions of low availability of water, high temperatures, and high light intensities [10]; they have high water-use efficiency and mechanisms for CO2 concentration [7].

of pH that inhibits the growth of some deleterious microorganisms, maintaining the nutri-

Ensiling of Forage Crops in Semiarid Regions http://dx.doi.org/10.5772/101990 69

Before the ensiling process, aerobic and facultative anaerobic microorganisms are able to grow in high pH and predominance. As long as pH decrease and oxygen is consumed, the anaerobic

• Aerobic phase: It occurs during filling of silo and extends until a few hours after the packing of silo. The aerobic phase is undesirable because all obligatory and facultative aerobic microorganisms (yeasts, molds, and bacteria) are active in this phase, but it is an inevitable phase. As it is associated with the fermentable substrate and energy losses, it is important to reduce the duration of this phase. It recommended that the forage be chopped, compacted, and rapid packing of the silo [13]. The final stage of the phase includes exhaustion of oxygen in silo.

• Active fermentation phase: After exhaustion of oxygen in silo, there is a decrease in silage pH because of organic acids production from WSC. In initial, enterobacteria and heterofermentative lactic bacteria grow in ensiled mass. With the larger decline of pH, homofermentative lactic bacteria dominate the anaerobic environment. In this phase, there is the more production of organic acid, such as acetic and lactic acids, and also ethanol and CO2. The major growth of lactic acid bacteria (LAB), and consequently, larger lactic acid formation inhibit the development of other microorganisms, principally due to lowering of pH. This

• Stability phase: It a phase with low biologic activity, since it does not penetrate air in the ensiled mass. The pH permanence is stable in 3.8–4.2, inhibiting microbial activity. Only some acid tolerant enzymes maintain activity [20]. The acid pH and anaerobic conditions maintain

• Discharge phase: It occurs at the opening of the silo and expose the ensiled mass to high oxygen concentration, which favors the growth of enterobacteria, molds, yeasts, and other microorganisms. Yeasts are the first microorganism to develop in silage after the opening, causing deterioration of the conserved forage [13]. There are heat and CO2 production due respiration, which results the decrease in lactic acid and residual WSC, and increase in silage

The ensiling process is complex and variable. It consists, basically, in conjunct action of the large number of microorganisms and may be considered a metabiose because it occurs at simultaneous and successive development of different microorganisms that depends on spe-

The microorganisms present in plant before ensiling may be aerobic and anaerobic, desirable and undesirable to fermentation. Table 1 presents the most common types of microorganisms

pH [13]. The appropriate management may minimize the losses after opening of silo.

and anaerobic facultative acid tolerant bacteria grow in the environment.

phase extends to the stability and reduces excessive microbial activity.

the ensiled mass stability to the silo opening.

3.1. Microorganism involved in the ensiling process

and their presence in plants.

cific pH, substrates, and potential redox in environment of silo.

Ensiling is divided into four phases with different time and intensity [19, 20].

tional values of forage.

Other factor that influences the nature of response of plants to drought is the thermal stress. Thermal stress can reduce transpiration and can dehydrate the plant cells, reduce the availability of nutrients, and cause osmotic stress together with the drought. In the plant growth stage, water stress can interfere with the final yield production of the crop [7]. Corn yield, for example, is a culture that is extremely sensitive to water stress during the period of the previous life cycle of flowering. Crops such as sugarcane may have a greater impact of water stress when its leaves are establish than in the initial period, which may affect the final yield [7, 11].

The adaptive responses are based on complex changes to cope with stress, primarily to maintain water potential in main tissues. Crops such as sorghum and pearl millet are drought tolerant and cultivated on a large scale in the semiarid region. These crops are able to maintain photosynthetic activity under water stress conditions and thus increase the final yield [12].

The osmotic adjustment required for drought tolerance forage can increase the solute values as fructan [4, 8] increases the values of soluble carbohydrate in these forage.

The concentration of water-soluble carbohydrates (WSC) in ensiled materials influences the fermentation profile because the WSC concentrations are used for the production of lactic acid [13]. The minimum content of WSC to appropriate fermentation of good silage varies between 6 and 12% [14]. In contrast, a large amount of WSC concentration may predispose to undesirable occurrence of fermentation realized from yeasts because of the excessive lactic acid production, which leads to losses resulting from the alcoholic fermentation [15].

In the semiarid region, there is a tendency that the forage contains a higher WSC content. The forage sorghum, pearl millet, and buffel grass show a WSC concentration (DM basis) of 13–20, 9, and 3.1%, respectively [16].
