Introduction

Chiapas ranks fifth nationally in corn production with 1.7 million tons and has an inventory of 2 491 226 head of cattle (^{SIAP, 2012}). Meanwhile, the region Frailesca south southwest of the state, also known as the "breadbasket of Chiapas" is the largest producer of corn statewide with 53 238 cultivated hectares (^{SIAP, 2012}); besides having a population of 229 003 head of cattle (^{INEGI, 2012}). Therefore, both activities are considered of great importance for the region and the state, which, in turn, are closely through the direct use of crop residues as an important source of food for livestock, especially in the drought period.

The abundance of stubble, a byproduct of the corn harvest coincides with the start, properly, the critical time for the regional livestock, just as there is a shortage of pasture due to lack of rain and explains why, in the area, is widely used in cattle feeding. It is clear that their use cannot be delayed, either by the results received by producers in animal production, the prevailing need at the time or by the custom of the activity.

^{Guevara et al. (2014)} report that 53% of producers in the region use corn stover to feed their livestock, an activity known locally as "rastrojeo", 27% use it for top dressing fertilizers leaving him on the floor and 20% burn it. This indicates the importance of this activity and how interesting it is to conduct studies that can support the development of a strategy for the rational use of resources.

Although research has been conducted whose aim has been to improve the nutritional quality of corn stover (^{Fuentes et al., 2001}; ^{Yesca et al., 2004}; ^{Sánchez et al., 2012}), there are no data to indicate how cattle make use of these products through direct grazing, so in order to develop strategies more correct use, is necessary to obtain data that will contribute to propose best management practices stubble and animals. Therefore, the objective of this study was to characterize the direct use of corn stover for cattle to generate a strategic proposal to improve its use.

Materials and methods

Results and discussion

In the Table 1 shows the presence of 14 different components in corn stover, covering both structures of the corn plant as weeds and grasses is observed, all available for consumption by cattle. The high diversity of plant components in stover, gives importance in terms of improvements in the quality of the animal's diet (Table 5), the product of a greater chance of selection of your diet (Table 4).

The components bracts, leaf stalk of corn, corn leaf, balsamite and yellow flower statistically different (p< 0.05) in different time’s rastrojeo animal. In the case of weeds, their availability was related to animal preference (Table 4) because of rastrojeo as time progressed, their availability and presence in the selected by the animal diet decreased (p< 0.05); however, in the case of the components of corn, the opposite happened.

In Table 2, the degree of utilization of corn stover, identified as the consume stubble by the animals at different periods throughout the experimental time expressed by weight of dry matter, according shown with the amount of that weight, the degree of use of stover in terms of percentage (^{Huss, 1972}) is expressed. The bracts components, grain corn and thatching grass are those with the highest percentage of use, which may indicate your preference by animals.

Whereas the average degree of stubble by the animal throughout the evaluation period was 60.36% utilization (Table 2) it is considered that 40% of the biomass remains in the soil for conservation purposes. In this regard, proper use of crop residues is essential for the appropriate use practices could maintain coverage and therefore soil conservation, knowing that only by covering 30% of the soil surface erosion is counteracted (^{Guevara et al., 2014}); however, it is necessary to note that the highest daily production of meat or milk per animal, are obtained when there is a use of light to medium (50%) of pasture production by hectare is lower with this degree of use, due to lower number of animals present per unit area.

In the Table 3 shows the variation of animal load of a hectare of corn stover throughout the period of use of the esquiline by the animal is presented, it is evident the decrease in stocking over time grazing, as unlike prairies where the stocking can be maintained in areas of no growth beddings this vegetation. In consideration 100% availability of corn stover, at the beginning of grazing support a stocking rate of 604.95 UA ha^-1 d^-1 to decrease to 291.16 compared to the end of the grazing. With the above values, on average, it can be calculated that corn stover can withstand 313 UA daily or 10.4 UA in a month, which coincides with the number of animals used in the experiment (12 AU).

This proposal could indicate a balance between the amount of stover available and adequate intake of this by livestock; at this point, gains would be achieved, both animal and per area, but the important thing is obtained sustainably without soil degradation by overgrazing and consequences of bare soil (^{Humphreys, 1980}).

In the Table 4 shows the components that formed the diet selected by cattle corn rastrojeando shown. Corn components in the diet were higher (p< 0.05) at the end of rastrojeo, while the share of weeds and African star grass were higher (p< 0.05) at baseline rastrojeo. This could be explained by changes in the behavior of diet selection, by the animal, given the heterogeneous conditions of the pasture, that is, the animals preferred to initially weed and African (65.34%) star grass in your diet, species better nutritional value (^{Pinto et al., 2004}) and end with a similar percentage (62.35%) but corn components. It should be noted that the components of the corn plant were the ones who contributed the highest percentage of the selected diet at all evaluation times, which could be associated with high availability (Table 1).

Weed average values in the diet of animals rastrojeando during the study period (17.12%) are slightly below those reported by ^{Pinto et al. (2014)} in cattle grazing during the dry season a conventional pasture (19%), indicating the importance of weeds in animal diets, which could help change the perception of producers on the disadvantages of them (^{Guevara et al., 2014}).

The choice of diet is the means by which the grazing animal seeks to meet their nutritional requirements from a heterogeneous pasture as the case of the stubble, and can be considered as a behavioral adaptation to variations in the average nutritional environment. Under natural conditions, these variations resulted from two sources: the diversity of the pasture, which offer a range of nutritional value and growth habits, which affects their preference as fodder, and the magnitude of defoliation to be imposed, what influences the seasonal pattern of digestibility and accumulation of senescent material pasture (^{Provenza, 2006}).

The important in the selection process of diet on corn stover, lies in the impact it has on the ecological system of the same and that the possibility or not to select affects the total nutrient intake by varying the amount and quality of existing components in that stubble.

The choice between different species or parts of the same plant is determined by the response to chemical stimuli perceived by the senses of smell, taste and touch the animal. The only signals that can activate this system, are molecules which chemically react with nerve receptors, to transmit information to the brain. The animal responds integrating these messages with others such as nutritional information or the presence of a metabolic disorder. For example, the hungry animal may respond by reducing its rejection threshold of taste or smell; that is, it reduces the preference level. There are probably differences in the number and type of receptors, both between and within animal species, and therefore the selection will be different (^{Provenza, 2006}).

As for quality of selected cattle diet (Table 5), statistically significant differences (p< 0.05) in the content of crude protein, ash and organic matter were found. An increase in the values of protein and minerals at the end of rastrojeo is observed, which could be related to a greater proportional contribution of the components of corn to the diet (62.35%, bracts mainly) in relation to the contribution of weeds and grasses (37%) (Table 4), although the better quality of the latter (^{Pinto et al., 2004}). According to the nutritional requirements of beef cattle (^{NRC, 1989}), the average levels of crude protein supplied in the diet at the beginning of rastrojeo, are below those recommended for maintenance (6.5%) needs, which involves considering strategies supplementation to improve ruminal fermentation of food consumed and nutrient supply (^{Leng, 1986}); however, in the end, the quality of the selected diet suggests that the animal has improved its selection and supplementation strategy could be different (^{Leng, 1986}).

The average values of crude protein found in the selected diet (6.07%) in animals rastrojeando is similar to the values found in animals grazing African star in the dry season (6.3%) in the same study area (^{Pinto et al., 2014}). This supports why producers indicate that the stubble keeps animals like pastures during the dry season (^{Guevara et al., 2014}).

There have been many attempts to relate preference to animal proximal composition analysis of forages. However, the animal cannot recognize soluble carbohydrate, energy, etc., since there are no free at the molecular level in plants. When found correlations between these characteristics and preferences, it is because they are related to some specific entities or physical properties of plants. They may be associated with differences in hardness of the structure of the leaf and stem, young and mature tissue and turgor characteristics between live and dead tissue (^{Sosa et al., 2000}).

Evidence shows that preference animal can be, not only but also induced innate. Such that the selection may depend on previous experience nutritional. This implies that a young animal need not learn by trial and error that forage is appropriate, but it can learn from the mother. Therefore animals that grow in different media have different preferences, even within the same species. So you can also teach them to reject a particular food if for example consuming it, causes an upset (^{Dumont et al., 2002}).

However, the selection will depend on the level of preference that has the animal for certain plant components. But this selection capability will be modified for the opportunity you have to select, and the composition and structure of vegetation cover. Consequently, how they can be intermingled the different parts of the plants and its spatial distribution in the vertical and horizontal plane, will determine the chance of selection (^{Ortega et al., 2009}).

In the Table 6 shows the activities of diurnal behavior (12 h) of cattle using corn stover, showing statistical difference (p< 0.05) between times evaluated activities rastrojeo (grazing), foraging are presented and the walking, which it is related to changes made by the animals in their grazing strategy in response to the level of coverage, as well as changes in the availability of forage present in the grazing areas (^{Burus and Sollenberguer, 2002}).

It is noted that the time spent rastrojeo could be directly related to the availability of forage present in the plot, contrary to what is commonly reported in the case of grazing grasses, in which an inverse relationship is indicated, because when there is a reduction of forage production grazing time (^{Acciaresi et al., 1994}; ^{Hess and Lascano, 1997}; ^{Patiño et al., 2008}) is increased. In this work it was found that at the end of rastrojeo, when the availability of components decreased (Table 1), the time spent rastrojeo was lower (p< 0.05), dedicating the animals, as long as the search and selection of forage and I walked involved in it (p< 0.05). On the other hand, at the beginning of the evaluation, the animals spent more time rastrojeo (p< 0.05) in response to the increased availability of forage present on the plot (Table 1).

Considering rastrojeo activity as the most important, the data obtained show that the hours devoted to this activity by the animal in the stubble is lower than those reported as pastoralist in tropical grasslands (8-9 h) (^{Arnold and Dudzinski, 1988}) and even lower than those reported for the dry season (6.3 h) where no grass growth (^{Pinto et al., 2014}).

The answer may be associated with physical limitations in the consumption of animal due to high fiber diets (Table 5) or to the animal streamlines its consumption rate given by the number of bites and the individual weight of it (^{Chacon and Stobbs, 1976}).

Conclusions

The results under the conditions in which this study was conducted show that high diversity of plant components present in a stubble, gives importance in terms of improvements in the quality of the animal's diet, the result of a greater chance of selection the same, it is important to define more efficient supplementation strategies for animals rastrojean directly.