Mineral elements are essential nutrients for animals and have effects on livestock performance¹. In fact, minerals represent 5 % of the body weight¹. Most of this weight corresponds to seven macro-minerals or macronutrients, which play important roles in the animal body. For instance, Calcium (Ca) and phosphorus (P) are structural components of bones and teeth². Potassium (K) is important in acid-base balance, regulation of osmotic pressure, water balance, muscle contractions, nerve impulse transmission and certain enzyme reactions². Magnesium (Mg) activates more than 300 enzymes³. Sodium (Na) and Chlorine (Cl) are involved in maintaining osmotic pressure, controlling water balance, and regulating acid-base balance². Sulfur (S) is a component of methionine, cysteine, B-vitamins, and other organic compounds². Therefore, ensuring an adequate supply of mineral nutrients to livestock is essential for maintaining their growth, health and reproduction⁴. In that context, forage nutrient target values have been proposed to provide growing cattle and lactating cow requirements^1,2.

There is widely recognized that the main source of minerals for animals is forage. Forage is plant material (mainly plant leaves and stems) eaten by grazing livestock. Forage yield and its mineral composition varies among grassland areas and perhaps within each of them. Forage yield and its mineral composition depend on soil and climate factors, botanical composition, and plants age, among other issues. For instance, low availability of P and Na could be the primary reason for mineral deficiencies in grazing animals⁵. That is, grasslands could provide insufficient quantities of macronutrients to meet animal requirements⁶.

There is known insufficiency of at least one of the macronutrients may affect animal growth, and its health and reproductive functions⁷. This type of situations may be improved through mineral supplementation^2,4. Nonetheless, many managers of grasslands do not know current forage composition and its relationships with limitative factors. This is the case of Zacatecas state, Mexico ranchers. This state has 494,203 ha of natural grasslands⁸, which are mainly managed under the called cow-calf production system. Along these grasslands, dominant species belong to perennial and yearly grasses of the genus Bouteloua, Aristida, Lycurus and Muhlenbergia⁹. Then, knowledge on such a topic might be useful to make decisions towards improving the use of the resources involved in the production system, mainly during summer and early autumn when there is maximum forage yield due to rainfall distribution. Therefore, the aim of this research work was to evaluate the soil mineral contents and their relationships with forage mineral concentrations taking into account three grassland sites located at Zacatecas state, México.

A fieldwork was carried out during the end of the rainy season (October, 2013) throughout three beef cattle sites within the territory of Zacatecas state, Mexico. All three sites were managed under the called cow-calf production system. Site 1 is located between the coordinates 23º 40’ and 23º 39’ N, and between 103º 28’ and 103º 27’ W at an altitude of about 2,250 m; it corresponds to an open medium size grassland; this site is within an area of 406 ha, which maintained 45 animal units during all year. Site 2 is between 23º 18’ and 23º 17’ N, and between 102º 46’ and 102º 47’ W at an altitude of about 2,110 m; this site belongs to an open medium size shrub-grassland associated with cactus; it is within an area of 482 ha, which maintained 35 animal units during the four seasons. Site 3 is allocated between 23º 29’ and 23º 27’ N, and between 103º 42’ and 103º 4’ W at 2,240 m; it corresponds to an open medium size grassland within an area of 170 ha, which maintained 32 animal units during summer and autumn. Climate of all three sites is classified as semi-dry (BS₁kw), with annual mean temperature and yearly mean rainfall of 16 to 18 °C and 400 to 500 mm, respectively¹⁰. In all three sites, dominant grass species included Bouteloua gracilis (blue grama), Bouteloua curtipendula (Sideoats grama), Lycurus phleoides (common wolf tail), Aristida arizonica (Arizona three-awn grass) and Aristida divaricata (poverty three awn), Muhlenbergia porter (bush muhly), and Microchloa kunthii (Kunth’s smallgrass).

For surface soil (0 to 15 cm) samples collection using Rodríguez and Rodríguez¹¹ procedure. Each site was divided in four sections. A total of 40 samples were collected from each section. Then, these 40 samples were mixed to obtain a composite sample. So, four composite soil samples belonged to each site. As a result and taking into account the three sites, 12 composite soil samples were obtained.

Forage samples were taken following the same sections used for soil sampling. In other words, each site was divided in four sections. Forage samples were collected from each site using the “hand plucking” simulation technique¹². As a result, four forage samples belonged to each site, obtaining a total of 12 forage samples.

All 12 composite soil samples were dried at 60 °C during 48 h, and sieved through a 2 mm screen. Afterwards, soil pH was determined through a pH meter using a supernatant suspension of a mixed soil to water ratio of 1:2. Available P was measured by means of two different techniques: if pH was alkaline, Olsen et al¹³ method was performed by using 0.5M of NaHCO₃ adjusted at 8.5 pH; and when pH was acid, the other technique¹⁴ was carried out by using a extraction solution of HCl and NH₄F. Cations Ca, Mg, K and Na were extracted by shaking 3 g of air-dried soil in 30 mL of 1M NH4OAc for 30 min; extracts were centrifuged¹⁵, and the supernatant was decanted and analyzed by spectrometry^16,17. Organic matter of soil was determined through organic carbon content using Walkley and Black approach¹⁸.

All 12 forage samples were dried at 60 °C during 48 h and ground through a 1 mm screen in a Wiley mill. Ash was determined by tissue incineration at 600 °C during 8 h. Hydrochloric and nitric acids were used to degrade resultant ash. Calcium, K, Mg, and Na concentrations were obtained by an atomic absorption spectrophotometry (Varian, AA240FS)¹⁹. Phosphorus concentration was determined by spectrophotometry (UV/VIS Lambda 2, Perkin Elmer). Relationship K/(Ca+Mg) was calculated in miliequivalents, mEq²⁰.

Soil and forage samples were randomly collected at field. Measured variables were soil available P, Ca, Mg, Na, and K contents, and soil pH and organic matter content (OM). In addition, forage concentrations of these macronutrients (P, Ca, Mg, Na, and K) were measured. Soil mineral contents were contrasted with soil reference contents pointed out by McDowell²¹ for P, Ca and K, and Rhue and Kidder²² for Mg and Na. In addition, estimated forage mineral mean concentrations were compared with cattle mineral requirements^1,2.

Soil and forage variables were analyzed under a completely random design. The analyses of variance were performed using the General Lineal Model²³ and the factor site as main effect. The Tukey test (P≤0.05) was used to compare mean effects of sites, that is, Ho: site 1=site 2=site 3. In addition, Pearson correlation coefficients (r, P≤0.05) were computed to identify the degree of linear relationships between soil and forage variables trough the PROC CORR in the Statistical Analysis System²³.

Soil OM contents varied from 2.83 to 3.15 % averaging 2.99 % (Table 1). OM contents from soils of the three sites are within the range linked to plant nutrient deficiencies. Soil OM contents were not different among the three sites. Thus, these three sites have soils with medium OM content as pointed out by others¹¹. The OM values of the mentioned range are higher than that reported by Echavarría et al²⁴ for the case of a natural grassland composed by thorny bushes and cacti in the Zacatecas state, Mexico. This difference could be due to yearly produced forage in the three sites is not entirely used by livestock, then, surplus forage should be incorporated into the soil, whereas the grassland of the cited case was under overgrazing condition.

Table 1 Mineral mean concentrations, pH and organic matter (OM) in soil samples collected from three native rangelands (Sites) used for beef cattle in Zacatecas state, Mexico 

^w P, Ca and K²¹, and Mg and Na²² contents suggesting deficiencies; ideal pH interval for most of the plants and organic matter content range (%) for non-volcanic soils¹¹.

^u For soil samples from the sites 1 and 3, P was determined by means of the Bray y Kurtz¹⁴ approach; and for soil samples from the site 2, the Olsen et al¹³ procedure was used.

^ab Means with different letters within each row indicate difference among sites (P<0.05).

Values of pH corresponding to soils of the sites 1 and 3 are within the range linked to plant nutrient deficiencies but that to soil of the site 2 is higher than the upper limit of such a range. Soil pH of site 2 was higher (P≤0.05) than those of sites 1 and 3. Remarkably, values of soil pH <6.5 were estimated for the sites 1 and 3; then, this soil condition could diminish P, Ca and Mg absorption by plants¹¹. On the other hand, soil pH=8.33 could be too alkaline for most plants in the site 2 case. This situation explains, in part, why soil of the site 2 soil had higher available P than soils of the sites 1 and 3.

Concentrations of P, Ca and Mg from soil of the site 2 were higher than the soil reference content. On the other hand, those from soils of the sites 1 and 3 were lower than the target. In addition, soil P, Ca and Mg mean contents were strongly different (P≤0.001) among sites. Therefore, soil P, Ca and Mg could be limitative factors of plant growth in the sites 1 and 3, which reinforce the result on soil pH.

Contents of Na and K in soils from all three sites are lower than those reported as soil reference levels suggesting deficiencies. Moreover, soil Na and K concentrations were not statistically different among all sites under study. Therefore, soil Na and K concentrations suggest both macro-minerals could be limitative nutrients of plant growth in all three sites.

Notably, P, Ca, Mg and Na concentrations in forage from all three sites were strongly lower (P≤0.05) than those considered as requirements for growing cattle and lactating cows, except Mg content in forage from the site 2 for growing cattle case (Table 2). These results suggest P, Ca, Mg and Na deficiencies in foraging plants from all three sites. In addition, K/Ca+Mg index in forage from all three sites was lower than the references for growing cattle and lactating cows. On the other hand, forage from sites 1 and 2 shown higher K concentrations than requirements for growing cattle and lactating cows. In addition, Ca:P ratio in forage from the site 2 did not surpass the reference value for both growing cattle and lactating cow. Ca:P ratio in forage from the site 1 was higher than the reference value for growing cattle, and Ca:P ratio in forage from the site 3 was higher than both growing cattle and lactating cow reference values.

Table 2 Forage mineral concentrations at native rangelands (Sites) used for beef cattle in Zacatecas state, Mexico 

^w Mineral minimum requirements for growing and lactation beef cattle^1,2.

^v Tetany potential, mEq²⁰.

^ab Means within a row with different superscript differ (P<0.05).

The evidenced P deficiency in all three sites agrees with a marginal P deficiency for range forages (foliage from trees and cacti) from seven locations within the territory of Durango State, Mexico²⁵. It is noteworthy the results on P may be considered as similar to findings of other research²⁶, who reported P contents of 0.13 % for a grassland in Durango state, Mexico. Those agreements can be explained because the sites of our study and that grassland are within the Chihuahua Desert.

Concentrations of Ca in forage from all three sites and those corresponding to requirements are lower than that (0.57 %) found by Murillo et al²⁶. This disagreement is unexplained because in that work did not report soil Ca content and botanical composition. In addition, the case of the resulting Mg deficiency in forage from all three sites is similar to other report²⁷ in the case of oats and ryegrass in Northwestern Florida, United States of America.

Concentrations of Na and K in forage from all three sites and those considered as requirements are lower than those reported elsewhere (i.e. Na =0.15 %²⁶ and K=1.1 %²⁸ in forage) for grasslands at Durango state, Mexico. It is not easy explaining those disagreements because they did not report soil K and Na contents and botanical composition.

Concentrations of P and Mg as well as the Ca:P ratio showed strong differences (P≤0.05) among sites. On the other hand, differences of Ca, Na, K, and the K/Ca+Mg index among sites were not significant (P>0.05). Nonetheless, due that K/Ca+Mg values did not surpass both mentioned reference values, there persist a risk of grass tetany or hypomagnesaemia occurrence in lactating cows grazing, especially at the sites 1 and 3 because of the forage having Ca and Mg at insufficiency levels.

Soil pH was positively correlated with P and Mg concentrations in forage and negatively correlated with Ca content and Ca:P ratio in forage (Table 3). These results suggest as pH increase, P and Mg contents in forage tend to be higher, and Ca content and Ca:P ratio in forage tend to be lower. The alkaline soil pH could explain P, Ca and Mg high availability in the site 2 case.

Table 3 Pearson correlation coefficients (r) between soil and forage variables 

NS= Not significant.

Soil available P content showed significant (P≤0.05) positive linear correlations with P and Mg concentrations in forage as well as negative linear correlations with Ca concentration and Ca:P ratio in forage. These correlations suggest positive effects of soil P content on P and Mg concentrations in forage, and indicate negative effects of soil P content on Ca concentration and Ca:P ratio in forage. These results could be explained because of the restricted quantity of available P in the soil from the sites 1 and 3 (Table 1) and to the fact that Ca fixes P at the interchange sites in alkaline soils.

Available Ca in the soil was positively correlated (P≤0.05) with P and Mg concentrations in forage and negatively with Ca concentration and Ca:P ratio in forage. Those correlations indicate positive effects of soil Ca on P and Mg concentrations in forage, and suggest soil Ca negative effects on Ca concentration and Ca:P ratio in forage. These results suggest plants prefer to take up calcium phosphates and many Ca ions were fixed as its availability increased, whereas soils could had have a low potential to fix natural P. This late idea is supported by Gagnon et al²⁹ finding in the case of alkaline soils.

Soil available Mg content showed a significant (P≤0.05) positive linear correlation with P concentration in forage and negative correlations with Ca concentration and Ca:P ratio in forage. These results mean P concentration in forage increased as soil available Mg content did, whereas Ca concentration and Ca:P ratio in forage decreased as soil available Mg increased. However, due to the lack of correlation between soils available Mg content and Mg concentration in forage, there appear soil available Mg content could be a limitative factor as pointed out ut supra, that is, it is at insufficiency levels in the soil, especially in the sites 1 and 3.

Available Na in the soil was negatively correlated with Ca concentration and Ca:P ratio in forage. These relationships suggest that whereas available Na increased in the soil, Ca concentration and Ca:P ratio in forage diminished. However, due to available Na in soil is at insufficiency level, there remains the idea on improving this situation throughout inclusion of such a mineral in food intake.

In general, results suggest current situation of P and Ca insufficiencies in forage could affect growth of cattle and food use efficiency¹. Consequently, these problems should be solved through increasing P and Ca concentrations by means of soil or foliar fertilization or including these minerals in food intake.

In general, soil of the site 2 showed better conditions for plant growth than those of the sites 1 and 3. Nonetheless, Na and K in soil could be limitative nutrients of plant growth in all three sites. Macro-minerals P, Ca and Na in forage from all three sites were at insufficient levels for growing cattle and lactating cows. Concentration of Mg in forage from sites 1 and 3 were at insufficient levels, and K in forage was at insufficient level in the site 3, mainly for growing cattle. Significant correlations suggest a positive effect of soil P content on forage P and Mg concentrations, and indicate soil P content may affect forage Ca concentration and Ca:P ratio. Other important correlations indicate positive effects of soil Ca on forage P and Mg concentrations, and suggest soil Ca negative effects on Ca concentration and Ca:P ratio in forage. The lack of correlation between soil available Mg content and Mg concentration in forage suggest soil available Mg content could be a limitative factor as pointed out ut supra, that is, it is at insufficiency levels in the soil, especially in the sites 1 and 3. Moreover, available Na in soil of the three sites was at insufficiency level. Then, the evidenced nutrient insufficiencies can be improved through increasing nutrient forage concentrations by means of soil or foliar fertilization, or including these minerals in food intake.