Location and characteristics of the study area

The work was done from October 2012 to September 2013 in five cattle ranches with cattle production system in grazing dual purpose, located in the common Punta Arenas, Catazajá Municipality, Chiapas, Mexico. This community is located between the coordinates 17° 46' 50 "north latitude 92° 06 ' 50" west longitude and 17° 43' 13 "north latitude 92° 01' 03" west longitude, with warm humid climate with rain all year (Af) (57.41%) and warm humid with abundant summer rains (Aw) (42.59%), with average annual rainfall of 2 600 mm (^{Tamayo 1985}) and annual average temperature of 26 °C (^{INEGI 2009}), with heights ranging from 9 to 10 meters. The region is characterized by three times similar to those described by ^{Escobedo (1989)} are the days of "norths" (October to January) at low temperatures and cloudy sky well defined year, the rainy season moderate to severe (June to September), and the dry season (February to May) with lack of moisture, increasing sunlight and high temperatures. The predominant forage in pastures are: Paspalum notatum (ranches 1, 3 and 4), Brachiaria humidicola (ranches 2), Brachiaria brizantha and Brachiaria hibrida (ranches 5).

Sampling

The cattle ranches in all forage samples were taken using exclusion cages considering the proposed ^{Mannetje and Jones (2000)} procedure; samples of the available forage in grasslands for grazing animals were also taken by manual sampling directed "Hand Plucking" (^{Le Du and Penning, 1982}) to compare with the nutritional composition of exclusion cages.

Exclusion cages technique: 30 exclusion cages 0.5 m wide x 1m long x 1 m high made of rebar half an inch thick covered with a metal mesh used. The location of the cages considered the height variation forage selecting three layers: low, medium and high, randomly placing two cages exclusion each stratum, for a total of six cages per cattle ranch. Before starting sampling (30 days before), he cuts forage in the area of the cage exclusion at a height of 2 to 3 cm from the floor (^{Pérez-Prieto and Delegarde, 2012}), later the forage samples were cut at the same height every 30 days using a 0.25 m2 quadrant inside the cage, after the remaining fodder at the same height cut in and

around the cage exclusion classroom to avoid interference from adjacent forage sunlight. The forage harvested monthly in cages exclusion was used to analyze the content of FDN and FDA, while forage harvested during the months of September (rainy), January (norths) and may (dry) was used to determine the content of PC and minerals.

Technique hand plucking: this technique was carried out simulating the activity of grazing cattle (^{Fick et al. 1979}) collecting five samples of 400 g of feed on each ranch during the months of September (rainy), January (norths) and may (dry) to determine the content of PC, FDN, FDA and minerals.

The forage samples were placed in paper bag and were weighed on a digital scale. Subsequently, the collected fodder was dried in a forced air oven at 65 °C for 48 h; dry forage samples were weighed to determine the dry each sample weight, ground in a hammer mill and then a blade mill (Wiley) with 1 mm screen (^{Fick et al., 1979}), and stored in plastic bags for determinations PC, FDN, FDA and minerals Cu, Fe, Zn, Ca, Mg, Na, K and P. in addition, rainfall data (PP) and temperature (T°) were taken weather station located in the city of Catazajá, Chiapas by the department of Civil Protection Municipal (Figure 1).

Figure 1 Temperature and precipitation during the study period Catazajá, Chiapas. 

Laboratory analysis

The analysis PC, FDN, FDA and minerals in the forage was conducted in Ruminant Nutrition Laboratory of Animal Production Graduate of Universidad Autónoma Chapingo (UACH). For the equivalent value of PC content, is determined nitrogen content (N) of forage Analyzer 2400 Series II CHNS/O, Perkin-Elmer in CHN mode. Subsequently, the N content was 6.25 times to obtain the percentage of the total sample protein (^{AOAC, 1995}). The neutral detergent fiber content (NDF) and acid detergent fiber (FDA) collected fodder samples was determined by the method described by ^{Van Soest et al. (1991)}.

The concentrations of Cu, Fe, Zn, Ca, Mg, Na and K in the feed were determined by the technique of atomic absorption spectrophotometry using a spectrophotometer of ^{Perkin Elmer model Analyst 700 according to procedures Perkin- Elmer (1996)}. The content of phosphorus (P) was determined by the colorimetric method (^{Fick et al., 1979}; ^{Clesceri et al., 1992}) using the spectrometer ultraviolet-visible light brand Perkin Elmer model Lambda 2. The samples of forage for PC analysis and mineral composite samples corresponded to combine forage samples of each season; samples for the windy season, dry season and rainy season came together. For determination of FDN and FDAforage samples collected monthly they were used.

Statistical analysis

The data PC, FDN, FDA and minerals were analyzed using the general linear model (GLM) of the SAS program (2004) for a completely randomized design. The statistical model for data FDN and FDA of forages harvested exclusion cages considered the effects ranch (Ri) [1, 2, 3, 4, 5], month of the year (Mj) [october, november ... september ], time of year (Ek) [norths, dry, rains], forage species (Sl) [B. brizantha, B. humidicola, B. híbrido, P. notatum] and interactions (R*Mij), (R*Eik), (R*Sil), (M*Sjl) and (E*Sks); PC data while only the effects ranch, season and forage species were considered; mineral data for only the effect of cattle ranch was considered. Data available forage in grasslands harvested by Hand Plucking were used to compare the contents of PC, FDN, FDA and mineral fodder exclusion cages. The Tukey test (^{Steel et al., 1997}) was used to compare treatment means.

Nutritional composition of forages in cages exclusion

There were statistically significant differences (p< 0.05) in the content of PC, FDN and FDA between ranches and seasons, and the content of FDN and FDA between forage species. In the Table 1 shows the values of PC and fibers ranches and forage species evaluated are presented.

Table 1 Protein content (PC), neutral detergent fiber (FDN) and acid detergent fiber (FDA) in four forage species collected exclusion cages in five ranches in southeastern Mexico. 

^abc Medias en la misma columna sin una literal en común son diferentes (p< 0.05); *EEM= error estándar de la media; MS= materia seca.

Regarding PC, lower amounts are presented in ranch 4 and are similar in the rest of the ranches. The low protein content in the ranch 4 with P. notatum as the dominant forage in the pasture may be due to the null fertilization of grasslands of this ranch. ^{Muchovej and Mullahey (1997)} report PC content in a range of 9.80 to 11% by P. notatum that are similar to the contents of PC on ranches 1, 3 and 4, where P. notatum is the dominant fodder. ^{Martin (1998)} for genotype Brizantha spp. reports a range of 4 to 14.9% CP. According to ^{Gates et al. (2001)}, P. notatum is one of the species of greatest economic importance in livestock for their high nutritional value, product of its high proportion leaf/stem, as well as for its ability to tolerate low temperatures thanks to a selection that has sought to improve their adaptation to this type of stress also that this grass allows proper animal production mainly in the windy season (October to February) in southeastern Mexico (^{Juárez et al., 2004}).

The highest contents of FDN was presented by ranch 2, which had a higher MS production throughout the year. ^{Lemaire and Salette (1984)} mention that there is a negative relationship between forage production and protein content so it is necessary to simultaneously consider both variables. The contents of PC ranches for 1, 3, and 4 with P. notatum as the predominant forage in grasslands, are in the range found by ^{Jarillo-Rodríguez et al. (2011)} who report to Axonopus spp. and Paspalum spp. The PC content of 8.2-15.25%, the FDN 61.8-70.8% and 32.1-43.8% FDA in the Mexican tropics; while ^{Ortega-Gómez et al. (2011)} reports PC content of 7.6%, FDN 73% and 42.8% for FDA Brachiarias spp. Vargas et al. (2013) report PC content 10.3-13%, 64.6- 66.4% FDN and FDA 30.4-33.7% for B. hibrida. According to these results, the ranch 5 with B. brizantha and B. hibrida as dominant in the meadow grasses, can be considered the highest quality, with respect to forage other ranches, due to their higher content of PC and lower in FDN, otherwise the ranch2mostly FDN. According ^{Taiz and Zeiger (2002)} and ^{Venuto et al. (2003)}, differences in growth habits, morphology and metabolism carbonaceous determine, among other factors, the variability in the protein content of the pasture. The time affected (p< 0.05) the content of PC. In Figure 2, the PC content is presented in forages studied seasons.

Figure 2 Content of crude protein in forages in three seasons in southeastern Mexico. 

As noted, the PC content was higher (p< 0.05) in the dry season (13.93%) over the windy season (10.80%) and rain (9.26%), representing the fodder in the dry season they were 39 and 40% more PC than forages in the windy season and rainfall, respectively. ^{Arteaga (2014)} found different contents of PC compared to this study in the order of 14.63, 11.05, and 14.35% for the windy season, rainy and dry seasons respectively, however, it agrees that the highest levels of PC are presented in dry season. Thus, it can be suggested that the quality of grass, about their PC content decreases in the rainy season which can be associated with the rapid growth of the grass at this time. ^{Greenwood et al. (1990)} and ^{Marino et al. (2004)} among others have called this phenomenon "dilution of nitrogen (N)", or dilution of the protein (N x 6.25).

Figure 3 Production of neutral detergent fiber (FDN) and acid detergent fiber (FDA) in forages in the months of the year in southeastern Mexico. 

These results agree with mentioned by ^{Lorenzo et al. (2012)} who report that in the rainy a process of dilution of N, in which the ratio of the PC with other components of the MS decreases due to the growth achieved by the grass, the absolute light conditions occurs , temperature and humidity of the rainy season. Thus, in this case the dilution of the protein can be attributed to increased production of MS in the rainy season, which could increase the percentage of stems and increased cell wall content, since they are elements that affect the PC digestibility and pastures (^{Jarillo- Rodríguez et al., 2011}). However, ^{Cuadrado et al. (2005)}) report PC content 9.8% in the rainy season and 7.8% in the dry season in B. Hibrida and 8.3% in the rainy season, 7.2% in the dry season in B. decumbens in Valle del Sinu, Colombia.

Regarding FDA was higher in the months of the dry season (36.12%), followed by the months of the windy season (34.02%) and the months of the rainy season (32.35%), which represents that at the time dry forages were 6 and 12% FDA in the windy season and rainfall, respectively; generally in the months of november (69.20%) and December (68.51%) for the windy season and the months of June (69.70%) and July (73.21%) for the rainy season it was when they showed the highest contents of NDF and minor amounts of FDA. The amount of FDN and FDA are different from those mentioned by ^{Cuadrado et al. (2005)} who reported content of 62.5% NDF in the rainy season and 64.4% in the dry season; and content of 40.1% by FDA in the rainy season and 43.2% in the dry season in B. hibrida; while B. decumbens reported for 52.7% of NDF in the rainy season and 62.4% in the dry season; and content of 46.9% by FDA in the rainy season and 48.3% in the dry season in Valle del Sinú, Colombia. Similarly, ^{Garcés (2005)} also reports values mulatto grass NDF of 62.5%, 40.1% and 9.8% of FAD in Colombia.

These results are similar to those reported by ^{Arteaga (2014)}, who reports lower (p< 0.05), NDF content in the dry season (59.77%) compared to the rainy season (63.39%) and norths (62.45%); however, this author found no difference (p> 0.05) in FDA content in the dry season (32.50%) compared to the rainy season (34.26%) and norths (32.44%). According to the above, it follows that the quality of forages decreases in the rainy season and norths. Regarding the rainy season possibly due to favorable conditions for the further growth of fodder and increased the number of stems with a consequent increase in FDN (cellulose, hemicellulose, lignin and cutin), which are components of the cell wall. As for the windy season it may be due to increased dead material caused by moisture and low incidence of sunlight with the time characteristic cloudy sky.

There were differences (p≤ 0.05) between ranches in the concentrations of Cu, Fe, Zn, Ca, Mg, Na, K and P (Table 2).

Table 2 Mineral concentration in forages protected exclusion cages in five ranches in southeastern Mexico. 

^abc Medias en la misma columna sin una literal en común son diferentes (p< 0.05); *EEM= error estándar de la media; (1)Nivel mínimo en base a los requerimientos delganado bovino (^{McDowell y Arthington, 2005}); (2)Niveles máximos tolerables de cada mineral en la dieta de bovinos (^{NRC, 2005}).

Differences in mineral concentrations of forages among ranches may correspond to the concentrations of minerals in the soil. In Mexico has reported a wide variation of mineral content in soils of tropical regions (^{Cabrera et al., 2009}) and temperate (^{Domínguez and Huerta, 2008}), as well as in other parts of the world (^{Pereira et al., 1997}; ^{Sharma et al., 2003}; ^{Ndebele et al., 2005}) highlighting elevated Fe and low levels of Cu and Zn which is reflected in the mineral composition of the forage eaten by animals (^{Whitehead, 2000}).

^{Muñoz-González et al. (2014)} in the same area study found differences (p< 0.0001) between the seasons in the concentrations of Cu, Fe, Zn, Ca, Mg, Na, K and P, being the rainy season where they have the lower concentrations of Cu, Fe, Zn, Mg and Na. These authors reported in the same study area concentrations of Cu, Fe and Zn of 4.93, 253 and 31 mg kg-1 and Ca, Mg, Na, K and P of 0.31, 0.23, 0.09, 196 and 0.22%, respectively, for the rainy season; concentrations of Cu, Fe and Zn 6.37, 275 and 36 mg kg-1; and Ca, Mg, Na, K and P 0.33, 0.25, 0.14, 1.73 and 0.21%, respectively, for the windy season; and concentrations of Cu, Fe and Zn 6.8,298kg and 44mg Ca, Mg, Na, K and P 0.42, 0.32, 0.15, 1.39, 0.18%, respectively, for him dry season. Domínguez-Vara and Huerta-Bravo (²⁰⁰⁸) reported concentrations of Fe in lower forage during the rainy season (258 mg kg-1) and higher in the dry season (761 mg kg-1), while Vieyra-Alberto et al. (²⁰¹³) report 114 and 149 mg Fe kg of fodder for the dry and rainy seasons, respectively, in the Huasteca Potosina, Mexico.

According to ^{Minson (1990}) the increase in the content of K and P in forage during the rainy season and norths corresponds with the greatest accumulation of these elements during active growth of pastures in conditions of greater soil moisture. These results are also consistent with those reported by Vieyra-Alberto et al. (²⁰¹³) who obtained forages lower concentrations of K and P (0.13 and 0.06 mg kg-1) during the dry season and 0.17 and 0.07 mg kg-1 K and P, respectively, in the rainy season. In Mexico, several studies have reported effects of the interaction between the production unit and the time of year in the mineral profile fodder. Domínguez-Vara and Huerta Bravo (²⁰⁰⁸) on the content of Cu, Zn, Mg, Na and P in temperate forages; ^{Morales et al. (2007}) analyzing Fe, Zn and P also in temperate forages, and Muñoz-González et al. (²⁰¹⁴) in the determination of Cu, Fe, Zn, Ca, Mg, Na, K and P in forages the Mexican humid tropics.

According suggested by ^{McDowell and Arthington (2005)} none of the farms considered in this study meets the minimum requirements of Cu and P in forage for cattle. 100, 28 and 72% of samples of fodder had values below the minimum level of Cu, Zn and P, respectively. Conversely, high levels of Fe were taken into pastures and 7.6% of the samples exceeded the maximum tolerable levels. This could be due to high amounts of Fe in the soil, allowing the forage plants present Fe accumulate more than required by cattle (^{Kabata-Pendias, 2011}). According to (^{Weiss et al., 2010}) more than 250 mg kg-1 of Fe in the diet of cattle increases oxidative stress and lowers the status of Cu, health, production, consumption and digestion of fiber. Furthermore, ^{Genther and Hansen (2014)} concluded that diets low in Cu antagonize with high levels of Fe and Mo, Cu diminishing reserves in the liver. In addition to the variability in the mineral content of forages, interference and natural or provoked antagonism between some elements (such as Cu and Fe), as factors limiting the availability of mineral sources added affecting its nutritional value or promoting potential toxicity excess (^{Suttle, 2010}).

^{Arteaga (2014)} found the same trend in PC content, with higher contents of PC accumulated in fodder exclusion cages compared to the forage available for grazing in eastern Puebla, Mexico. This indicates the advantages provided by the newspaper cut forage, in this case every 30 days, the nutritional composition of forages. Concretely it can be suggested that the nutritional quality of forage increases when cut periodically, avoiding the accumulation of senescent material more cell wall content of lower quality, which can be achieved by appropriate rotation of paddocks for animals consume higher quality forages. According to ^{Dillon et al. (2005)} grazing management is a key factor in determining the efficiency of dairy systems based on forages. It is recognized as the main tool for controlling the use of pasture and production per cow, and achieve the optimum balance between these factors is the main objective of dairy farms trying to achieve maximum profitability.An important question is; however, the lack of control over the quality and availability of food throughout the year.