**Abstract**

Brazil is one of the largest meat producers. Meat along with other animal products have been responsible for its larger contribution as source of selenium (Se) for human. However, Se deficiency remains a concern because researches have indicated that this nutrient is found in low levels in Brazilian diet. Cattle in Brazil are fed basically from pasture, but there are strong evidences that soils contain low availability of Se; consequently plants and animals incorporate low Se levels. Pastures, Se fertilized, bring benefits to nutrition and health of animal consequently to humans already known in some countries. In contrast, Se fertilization on tropical weathered soils and tropical forages is little known. However, Se management as fertilizer in tropical environments requires researches involving field experiments, especially with animals, for establishing of safe and effective Se recommendations as fertilizer due to the Se toxicity potential and complexity in system of soil-plant-animal-human.

**Keywords:** agronomic biofortification, grazing animal nutrition, selenium fertilization, tropical forages, weathered soils

## **1. Introduction**

Selenium (Se) benefits for health in human and animal have provided popularity for this chemical element. Selenium is a nutrient for animals since 1957 [1]; thus, it must be part of their diet. However, there are large agricultural areas containing low levels of Se in soil or it is present, but as chemical forms unavailable for plants, consequently these areas are producing vegetables or animal products with low Se contents.

In Brazil, there are some researches indicating that large agricultural areas are located in soils with low Se levels, that is, daily diet Se intake evaluated in people groups from São Paulo, Brazil, presented values below to estimated average requirement values, which shows deficiency of Se in the diets from this region [2]. A research at Rio Grande do Sul, Brazil, also resulted in marginal deficiency of Se in cattle [3]. Both data indicated the possibility of Brazilian soils contain low available Se levels.

The agronomic biofortification of food through field fertilization with Se could be a solution to provide this micronutrient for animals and humans through plants. Plants are able to absorb and incorporate Se to organic compounds as seleno-amino acids. Thus, inorganic Se is converted to organic Se compounds through the plants

which can be easily absorbed by the human body and to be available where needed in the body [4]. In some countries, Se fertilization is well established, and it is annually done in New Zealand in which Se along with phosphorus fertilization is applied in pastures [5].

Selenium essentiality for plants is not convinced, but its availability for plants, as well as for the animals, could improve their performance and consequently the human health. For both animals and plants, this element acts as defense through its influence in glutathione peroxidase controlling oxygen reactive species from stress situations [6].

Large knowledge about specific rates, sources, Se dynamic in soils and plants, and even behavior of animal intake in pasture is required for a safe Se fertilization, to ensure food and environmental security. High Se levels available in soils can cause toxicity for plants and animals. Thus, for the beneficial of Se application as fertilizer commonly are required low rates, which raises concerns about risks of super dosages, what can be aggravated by complex dynamic of Se in soil and plants.

### **2. Selenium fertilization in tropical weathered soils**

Se availability in soil is the first requirement for Se application as fertilizer. Most soils contain low Se levels, including in tropical environments, while the highest contents are found at arid areas characterized by the presence of accumulator plants [7].

Low Se levels were observed in the main eight soil types of Brazil (**Table 1**). These soils were collected at São Paulo state as well as in plant of *Urochloa decumbens* grown in them.

The contents up to 500 μg dm<sup>−</sup><sup>3</sup> characterized low Se soils [8], and confirming the relation between soil and plants, the samples of *Urochloa decumbens* comprised contents of 10.4–79.7 μg kg<sup>−</sup><sup>1</sup> Se in dry matter (unpublished data).

Besides the Se presence in soil, its availability for plants depends on oxidation state. Selenium is chemically similar to sulfur, but it occurs naturally in four oxidation states, −2 (selenide), 0 (elemental Se), +4 (selenite), and +6 (selenate) [9]; however, sodium selenate is the source recommended for Se fertilizations due its


#### **Table 1.**

*Chemical parameters of fertility in tropical soils and selenium levels in sampled soils at São Paulo state, Brazil.*

#### *Selenium Fertilization in Tropical Pastures DOI: http://dx.doi.org/10.5772/intechopen.89423*

high solubility. Unlike selenate, the mechanism of selenite uptake by plants remains unclear [10].

In weathered soils, there are low nutrient levels; high contents of Fe, Al, and Mn; and high acidity, according to soils analyzed (**Table 1**). Thus, it is necessary to know the Se dynamic in these soils. A profile of weathered soils analyzed from São Paulo observed low Se levels in soils with higher sand contents (**Figure 1**) that could indicate leaching potential.

A study of Se adsorption and desorption in soils from Cerrado, Brazil, verified low values of distribution coefficient in soils; thus, Se tended to be more in solution than in the solid phase, and in the most weathered soils, with higher clay and Al and Fe oxide contents, there are the highest affinity for Se, while in sandy and loamy soils, Se tends to be less adsorbed and can therefore be taken up by plants or easily leached, damaging the ecosystem [11].

The low natural levels of Se in soils and its absence in fertilization to crops explain the low contents in food from vegetables [12] and consequently in Brazilian diet, except for northern areas [2]. Although, in a study of hemodialysis patients from north and southeast of Brazil, both patient groups presented low Se plasma levels when compared to recommended values; independently of the region, all patients presented Se deficiency [13].

This information is an alert for necessity to Se fertilization in Brazil. It was incentivized in the 1980s, but its requirement was unsuccessful, while in some countries, it is well stablished already. Applications of Se in areas of low Se bioavailability have been an option with good results to supply this element to plants and, consequently to animals, improving animal performance and nutritional quality of food produced as milk and meat [11], even in environments with no deficiency symptoms [14].

However, toxic potential of selenium requires caution as fertilizer due its complex dynamic nature in soil and plants. Selenium as selenate (SeO4 <sup>2</sup><sup>−</sup>) is commonly found in alkaline and oxygen soils under high redox conditions (pe + pH > 15), and this oxidation state is predominantly absorbed by plants, while under low and milder redox conditions, species as selenite and selenide predominate [9].

Selenium fertilization must be controlled through safe doses and soil monitoring due to the possibilities of the Se dynamics in soils. Selenate can be easily absorbed by plants; thus, the doses for fertilizations must be carefully calculated, but also it could be leached with possibilities of water contamination.

Low selenate doses required for fertilization is a challenge due to concern for homogeneous application. According to the Selenium-Tellurium Development

#### **Figure 1.**

*Selenium levels in tropical soil profiles and, respectively, sand contents in sampled soils at São Paulo state, Brazil (unpublished data).*

Association, the best way for Se application is along with other nutrients [15]. There are positive effects in Se application along with phosphorus fertilization [13].

Some technologies involving Se application along with macronutrients as coating became a technique to easy and high quality of application. Urea coated with a mix of boric acid (0.4% B), copper sulfate (0.14% Cu), and sodium selenate applied to *Urochloa brizantha* carried out in pots with weathered tropical soil resulted to desirable enrichment of plant with rate of 34.5 g ha<sup>−</sup><sup>1</sup> Se [16].

Seed pelletization seems to be a promissory tool to increase Se content in plants. Beneficial effects were observed in the evaluation of seed pelletization with increasing selenite doses on three ryegrass cultivars; however, the authors recommended it to be evaluated under field conditions in Se-deficient soils [17].

Another technology is the slow release of Se fertilizer as Selcote Ultra; however, its application in rates of up 20 g ha<sup>−</sup><sup>1</sup> Se on an Ultisol soil of Puerto Rico did not increase in the foliage Se concentrations of Guinea grass pastures [18]. According to the authors, the soil and plant interrelationships may be affecting the foliage of Se absorption potential requiring that the effects need future consideration in terms of Se movement in tropical soils.

The establishment of effective and safe rates in tropical environmental still is required and unknown, regardless of the technical method applied to plant enrichment on Se. High Se availability in agriculture soils can cause toxicity to crops, but it is still more concerning if a crop shows accumulator character, i.e., if a crop has capacity to absorb high levels of selenium with no symptoms of toxicity; this could increase the possibilities to cause toxicity for animals or human.

Forage plants are classified as passive accumulators due its ability to contain 10–30 mg kg<sup>−</sup><sup>1</sup> of Se in dry matter; however, high-quantity animal intake of Se through dry matter intake could induce intoxication [19]. Although the research with *Urochloa brizantha* grown in a weathered tropical soil containing 1.8– 4.6 mg dm<sup>−</sup><sup>3</sup> Se resulted in high levels of Se uptake affecting biomass production, regardless of the soil type, plants showed high levels of Se in leaves [20].

Depending of the soil, excess of Se can be in unavailable forms to plant uptake over time. Soil influence in dynamic and availability of Se was observed by isolation of Selenium rates applied followed by comparing among soils, Arenic Hapludult, Rhodic Hapludox and Typic Hapludoll, which verified differents Selenium content remaining in soil, respectively, 22, 11, and 37 μg dm<sup>−</sup><sup>3</sup> and 55, 4, and 38 μg dm<sup>−</sup><sup>3</sup> after *Urochloa brizantha* and *Stylosanthes capitata* cultivated, respectively (unpublished data).

Besides the uptake ability among plants, the difference among soils was evident. This element in soil can be fixed along with iron, complexed in organic matter, or it can be in many oxidation states depending on the pH, oxygen, and microbial activity [7]. These data confirmed different soil capacities to Se adsorption, but it can turn available for plants in pH changes, as frequently occurs with limestone application, a common practice from tropical agriculture areas.

High contents of selenate in soil, for natural or anthropogenic action, can be establish or reestablish for agricultural or livestock, avoiding the poisoning risk of plants, animals, and humans [20]; in these cases, the areas must be isolated for remediation. The use of plants to clean up contaminated soils is a technique known as phytoremediation that offers a less expensive alternative to stripping pollutants directly from the soil [21].

The management of high Se soil also can include sulfur source application. The similarity between Se and S indicates the competition for sulfate transporters of the root plasma membrane [7]. Sulfur application at 600 kg ha<sup>−</sup><sup>1</sup> as soluble sources such as ammonium sulfate and ferric sulfate reduced damages in productivity and Se uptake by Urochloa grass, while the lower solubility of calcium sulfate resulted

in lower effectiveness in reducing Se uptake [20]. Plants used for phytoremediation can be used for mixture in diets of animals or used as composting to application in low Se areas.
