Characteristics of the study area

This study was conducted in the Production Unit "The Gargaleote" property of Universidad Autónoma Chapingo, located in the municipality of Tamuin, San Luis Potosí, in the region known as "The Huasteca Potosina in Mexico". The study area is located between the geographical coordinates 24° 29' and 21° 10' north latitude and 98° 20' and 102° 18' west longitude, 40 meters (INEGI, 2005); It presents a climatic condition Aw''o type (e) described as hot subhumid with summer rains, with annual average temperature of 26.8 °C and annual rainfall of 927.7 mm (^{García, 2004}). Soils are vertisoles and fluvisoles (according to FAO-UNESCO classification), deep and well drained, textured silt or silty clay sandy, with alkaline pH above 7.5 due to geological origin.

Features of parcels

The study was conducted on a hectare of land associating star grass and leucaena established during the months of July to December 2010 in an intensive silvopastoral system (^{Solorio et al., 2009}). The experimental plot was divided into 18 batches of 11.11 x 50 m (555.55 m2). Three batches selected randomly were assigned to one of six treatments that corresponded to the age of suckers: 35, 42, 49, 56, 63 and 70 days from the recent pruning of leucaena at the beginning of the rainy season.

Collection and analysis of samples

The sampling consumable foliage of each forage species was performed on each batch in triplicate, in three sampling sites 3.2 m2 randomly selected by the method of simulated grazing Hand Plucking (^{Penning, 2004}) considering the behavior of grazing sheep. The samples were dried in a forced air oven at 65 °C to obtain the dry matter yield and subsequently crushed to 2 mm to determine dry matter (MS), organic matter (MO), ash (CEN), ether extract (EE), total protein (PC) using the methodology of the AOAC (2000); neutral detergent fiber (FDN) and acid detergent fiber (FDA) using the method of ^{Van Soest et al. (1991)}. The concentration of each nutrient is adjusted using the proportions 52:48 to leucaena and star grass, respectively, from dry matter yield obtained from each forage species during the rainy season. Since the inorganic fraction of each forage species content of Ca, Mg, Na, K, Cu, Fe and Zn it was determined by the method of atomic absorption spectrophotometry (^{Fick et al., 1979}) and P by colorimetry (^{Clesceri et al., 1992}).

Statistical analysis

The data were analyzed using the general linear model (GLM) of SAS (²⁰⁰⁴) with the following statistical model:

Where: Yij= value of the response variable corresponding to the i-th age of regrowth in the jth forage species, μ= general mean Eri effect of the i-th age of regrowth (i= 35, 42, 49, 56, 63, 70), EFj= effect of j-th forage species (leucaena, grasses) and εijk= random error ~ NIID (0, σ2e). Tukey test (^{Steel et al., 1997}) was used to compare treatment means.

Effect of age of the volunteers (ER) in chemical and mineral composition of leucaena and star grass

There were no differences (p> 0.05) EE and PC between the ages of regrowth. The age of the volunteers showed differences (p> 0.05) in the percentage of FDN and FDA. The age of the volunteers showed differences (p> 0.05) in the concentrations of Ca, Mg, Na, P, Cu and Fe. In the Table 1 shows the effects of the age of the volunteers (ER) in the variables of chemical composition and leucaena ore and star grass associated in an intensive silvopastoral system.

Table 1 Effect of the age of the volunteers (ER) on the chemical composition and mineral leucaena and star grass associated in an intensive silvopastoral system. 

EE= extracto etéreo; PC= proteína cruda; FDN= fibra detergente neutro; FDA= fibra detergente ácido; ab= medias con diferente literal muestran diferencias significativas entre columnas; y= error estándar de la media; x= nivel de significancia.

Ether extract (EE) and crude protein (PC)

Contrary to what mentioned by Lorenzo et al. (2012) who report that can produce a dilution process nitrogen (N), in which the ratio of the PC with other components of the dry matter (MS) decreases due to the growth achieved by the grass, the conditions favorable light, temperature and humidity. According Jarillo- Rodríguez et al. (2011) diluting the protein can be attributed to increased production of MS, which increases the percentage of stems and increased cell wall content, as they are elements that affect the digestibility of grasses and PC. However, in this case the similar content of EE and PC between the ages of flare- ups can be attributed to a positive response from the leucaena associated with star grass providing fixed nitrogen to the soil and other nutrients from deeper soil layers allowing the tissue plant of both forage species maintained high nutritional quality (^{Sánchez et al., 2007}).

Neutral detergent fiber (FDN) and acid detergent fiber (FDA)

The content of FDA, older increased to 54.8% cut to 70 days; FDN and a higher percentage at 49 days with 69% was observed; however, these increases were moderate, allowing the plant tissue of both forage species maintained a higher nutritional quality (^{Sánchez et al., 2007}). These results agree with those reported by ^{Lara et al. (2013)} who observed an increase in the NDF and FDA of leucaena and star to increased pasture regrowth age, but differ with ^{Maya et al. (2006)} who found no difference between periods of cut for the same components of the cell wall of both forages; however, they differ from those obtained by Maya et al. (²⁰⁰⁵) who found no significant differences for the components of the cell wall between cutting periods. However, they have been reported increases in the concentration of fibers older cutting, because when mature tissues lignification process is given, increasing the fibrous fraction of the cell wall (Martínez and Reyes, 2013).

Minerals

The concentrations of Ca, P and Cu decreased with increasing age of the volunteers, while Fe, Na and Mg increased, showing the highest to 70 days after pruning values, which can be explained because several of these elements remain constant in mature organs and stems of plants (^{Gomide and Zometa, 1976}). According to Minson (1990) there is a greater concentration of P in tender forages mature forages compared with corresponding to increased accumulation of this element during active growth of pastures in wetter conditions on the ground. The Ca:P ratio showed no significant differences due to the age of suckers, however, increased calcium and phosphorus decreased as the plant matured.

The PC levels in all ages of the sprouts were higher or adequate to meet the requirements of sheep and cattle grazing (^{Sánchez and Faria-Marmol, 2008}). The content of NDF and FDA are among the normal standards reported by other researchers comparing legumes and grasses in different age of the volunteers (^{Nouel et al., 2005}; ^{Sánchez et al., 2007}). As for the minerals studied considering the minimum requirements for minerals in the diet of sheep (^{NRC, 1985}), in the order of 0.3, 0.1, 0.1, 0.5, 0.25% and 2.0:1.0 Ca, Na, Mg, K, P and Ca:P ratio, respectively, and Cu, Fe and Zn in the order of 10, 50 and 30 ppm, forages analyzed showed deficiencies of Ca, P, Cu and Zn, and a lower ratio Ca:P.

This may be due to low levels of these minerals in the soil, as well as greater dilution of minerals due to the rainy season. The low concentrations of Cu, Zn and P, as well as high concentrations of Fe and K in forages in the rainy season based on the requirements of sheep, coincides with previously reported by Muñoz- González et al. (²⁰¹⁴) who they reported concentrations of Cu, Fe and Zn of 4.93, 253 and 31 mg/kg and Ca, Mg, Na, K and P of 0.31, 0.23, 0.09, 196 and 0.22%, respectively, in tropical forages the rainy season in the northeastern Chiapas; these authors mention that forages have lower concentrations of minerals in the rainy season compared to the dry season.

Other studies in Mexico ^{Morales et al., (2007)}; Domínguez- Vara and Huerta Bravo (2008) in temperate forages; ^{Vieyra-Alberto et al., (2013)}; ^{Muñoz-González et al. (2014)} in forages tropical climate reported variations in the concentration of minerals between seasons, so the development of further research is needed to determine the composition nutrimental of forages evaluated in this study during the dry season. Supplementation is recommended with Cu, Zn and P during the rainy season to correct any deficiencies in animals that consume the feed in the study area.

Effect of forage species (EF) in the chemical and mineral composition of leucaena and star grass

In the Table 2 shows the effects of forage species on the chemical and mineral composition are shown.

Table 2 Effect of forage species (EF) on the chemical and mineral composition of leucaena and star grass in an intensive silvopastoral system. 

EE= extracto etéreo; PC= proteína cruda; FDN= fibra detergente neutro; FDA= fibra detergente ácido; ab= medias con diferente literal muestran diferencias significativas entre columnas; x= error estándar de la media; y= nivel de significancia.

There were significant differences (p< 0.05), the content of EE, PC, FDA, FDN, Ca, Mg, ratio Ca:P, Cu and Fe between leucaena and star grass, however, concentrations of Na, P, K and Zn were similar for both species.

Ether extract (EE) and crude protein (PC)

The Leucaena showed higher values for EE and PC. The EE was 203% higher in leucaena the percentage EE star grass, while PC was 175% higher in the legume in the grass. In leucaena ^{Clavero (2011)} reported higher contents of PC to this study with 30.4%. This can be linked to that legumes are fixers N with deep roots that increase the availability of nutrients through biological fixation, recycling or pumping nutrients from deeper layers to the surface of the soil (especially in dry areas) and accumulation organic matter in the soil (^{Rao et al., 1998}). In addition, compared with species of tropical grasses, legumes are capable of synthesizing high levels of PC with a relatively low rate of decline in this component as the plant matures (^{Rincón, 2011}).

Neutral detergent fiber (FDN) and acid detergent fiber (FDA)

Leucaena showed lower values (p< 0.05) for the components of the cell wall (FDA and FDN). The content of FDN and FDA was lower in leucaena than star grass (135 and 59%) respectively. The lowest levels of fiber in legumes benefit has been observed nutrient intake in animals. For example, it has been observed that in sheep supplemented with legumes, consumption of organic matter (MO) and PC increase (^{Abreu et al., 2004}), according ^{Weisbjerg and Soegaard (2008)} increased consumption of legumes compared with grasses it may be, inter alia, to differences in the chemical structure and the fiber content.

This is attributed to that legumes have a lower concentration of total FDN, but a higher concentration of lignin than grasses, and this lignin is linked to a low proportion of the indigestible NDF, so there is a greater proportion of indigestible FDN in legumes than in grasses. According to ^{Wilson and Kennedy (1996)} in legumes, lignin is found only in the xylem, so this is entirely indigestible, while in other tissues no lignin and the cell walls are completely digestible.

However, according ^{Buxton and Russell (1988)} in grasses lignin is distributed in all tissues of the plant, except in the phloem, therefore, the least amount of lignin which have grasses protect more of walls rumen cell, causing the rate of digestion of the cell wall is less than in legumes. This may relate to the positive effects that different studies have shown the inclusion of leguminous species and their effect on milk production in different ruminant species (^{Razz and Clavero, 1997}; ^{Steinshamn, 2010}) and weight gain (^{Combellas et al., 1999}; ^{Verdolak and Zorate, 2008}; ^{Benavides-Calvache et al., 2010}).

Minerals

For Ca, Cu and Mg, the content was 46, 86 and 31% higher than in leucaena that star grass, while the Ca:P ratio was 43% higher in star grass. ^{Lara et al. (2013)} reported greater intake of Ca and P in leucaena, while ^{Vivas et al. (2012)} observed in leucaena highest percentage of Ca and Cu compared to other grasses; while ^{Gomide and Zometa (1976)} observed higher percentage of Mg in the legume. The iron content was high in both forage species, even above the critical level for ruminants (^{McDowell and Arthington, 2005}). The Fe concentrations in grasses of this study are consistent with those of ^{Vieyra-Alberto et al. (2013)} who reported an average of 116.5 ppm of Fe; however, reported lower concentrations of K and P in the order of 0.15 and 0.07%, respectively, in grasses of the Huasteca Potosina, Mexico.

Muñoz et al. (²⁰¹⁴) reported concentrations of minerals in different grass species in southeastern Mexico in the order of 0.36, 0.27, 0.13, 1.66, 0.2% of Ca, Mg, Na, K and P, respectively, and 6.18, 274.25, 36.5 ppm of Cu, Fe and Zn, respectively, highlighting high levels of Fe and low levels of Cu and P, with respect to the requirement of cattle. Insufficient levels of P, Cu, Zn and tropical forages were in were reported in other regions of the country (^{Castañeda, 2012}), although Cu deficiency and is usually correspond to alkaline soils and antagonism with high levels of Fe, S and Mo (^{Suttle, 2010}). Even with these problems, the silvopastoral system leucaena-star grass, increases the mineral content and quality of the diet of ruminants grazing.

Minson (1990) mentions that there are differences in forage (dry matter) of different species and different climate, for example in concentrations of Cu in legumes tropical and temperate with 3.9 and 7.8, Zn 40 and 38 mg kg-1, respectively; in grasses of tropical and temperate climate with concentrations of 7.8 and 4.7 Cu, Zn 36 and 34 mg kg-1, respectively. To ^{Suttle (2010)} provide more legumes than grasses minerals which could explain the balance of minerals observed in our study. However, the Ca: P ratio was low which can cause an imbalance between the two minerals (^{Ceballos, 2004}).

As for the minimum requirements of minerals sheep (NRC, 1985) leucaena and star grass presented low concentrations of Ca; however, ^{Huerta (2003)} Ca deficiency in ruminants grazing grasses is very rare and never occurs in legumes. Both forages also deficient Zn also presents the grass P and Cu concentrations under the requirements of ovine.

It is necessary to mention the importance of these minerals are in the body of animals, that Cu is a necessary component of important antioxidant enzymes superoxide dismutase (SOD) and ceruloplasmin (Waldron, 2010) to prevent oxidation and peroxidation of tissue (Spears and Weiss 2010), while Zn is necessary for the synthesis of metallothionein that helps removing free radicals and maintain the integrity of epithelial tissue and the formation of keratin which provides a physiological barrier against infection (O'Rourke 2009). It is important to mention the increased capacity of extraction of nutrients by leucaena, so it is necessary that new will continue investigating what possible benefits from their association with other forages.