Introduction

The grasslands and shrubs of the arid and semi-arid zones of central and northern Mexico are natural resources that comprise approximately 25 % of the national territory¹ and have the ability to provide several products and environmental services to society. One of these products is ruminant meat from extensive livestock farming, while carbon sequestration is one of their most important environmental services. Grasslands and scrublands have undergone major transformations, mainly due to changes in land use, the climate, and overgrazing, which cause a serious deterioration of these resources.

Grassland rehabilitation is a necessary activity for correcting this deterioration. Fire is an ecological, low-cost alternative for grassland improvement^2,3, while adjusting the stocking rate along with practices for improving grazing distribution are recommended to conserve or improve the condition of arid and semi-arid grasslands^4,5. Re-seeding pastures, although a risky and costly practice due to erratic rainfall and to the high cost of land and seed preparation, is still an option for improving pastures.

Studies on the management and rehabilitation of grasslands and scrublands in Mexico began in the 1950s at the La Campana Experimental Ranch of the now extinct National Institute for Research in Livestock (Instituto Nacional de Investigaciones Pecuarias). Subsequently, since the creation, in 1985, of INIFAP -which merged the agricultural, livestock and forestry research institutes-, these studies were intensified in its Livestock Experimental Stations in the north of the country, such as "La Campana" in Chihuahua, "Carbó" in Sonora, "Aldama" in Tamaulipas, and "Vaquerías" in Jalisco. The results of these works are reflected in countless publications and have been transferred to the producers through courses, workshops, and demonstrations. During the 1970s and 1980s, they were supported by livestock organizations, state governments and CONACYT, while in the 1990s and the early XXI^st century, research and grassland technology transfer were promoted by the State Produce Foundations. In the last 20 yr, Federal Programs such as SAGARPA's PROGAN have supported INIFAP in training a large number of producers and technicians in rangeland management. In addition, INIFAP has participated in the promotion of grassland management through the co-organization of Symposiums, Conferences, and Forums with different universities in central and northern Mexico and the Mexican Society for Range Management. The socioeconomic and environmental impact of pasture research carried out at INIFAP and other institutions is palpable in the management of cattle ranches. For example, a large part of the ranches in the north and center of the country, including some ejidos, carry out grazing management practices such as deferred grazing or pasture fallowing, adjustment of animal load, and supplementation of livestock during drought. This has allowed the conservation and improvement of the condition of certain grassland and scrubland areas. Today, there is a large amount of knowledge and technological innovations available to achieve sustainable management of grasslands and scrublands, generated by INIFAP, universities, and research centers. This document presents the current status of arid and semi-arid grasslands and shrubs in relation to certain strategic themes, as well as the main challenges and perspectives which must be faced with endeavor for conserve and sustainable exploitation of these ecosystems.

Grassland and scrubland condition

The semi-arid grasslands of Mexico are distributed in a strip from north to southeast of the country, from Sonora to Guanajuato, while the arid scrublands extend widely from Baja California to Oaxaca⁶ (Figure 1). These ecosystems have suffered reductions by 14 % in grasslands and 26 % in shrublands as a result of their conversion to agriculture in the last 50 years, so that they currently comprise around 9.77 and 40.95 million ha, respectively¹. Unfortunately, 95 % of the grasslands and 70 % of the shrublands are overgrazed, according to official sources (Figure 1).

Figure 1 Intensity of grazing in grasslands and natural shrublands of Mexico. Source: Dirección General de Ordenamiento y Conservación de Ecosistemas, INE, SEMARNAT, México. 2003 

The first studies at the national level during the decades of 1950 to 1970 indicate that the semi-arid grasslands of central and northern Mexico consisted mainly of native grasses of low and medium size, such blue grama (Bouteloua gracilis), sideoats grama (B. curtipendula), hairy grama (B. hirsuta), Rothrock’s grama (B. rothrockii), purple grama (B. radicosa), slender grama (B. repens), black grama (B. eriopoda), sprucetop grama (B. chondrosioides), scorpion grama (B. scorpioides), six weeks three awn (Aristida adscencionis), poverty three-awn (A. divaricata), spidergrass (A. ternipes), buffalo grass (B. dactyloides), while in arid or halophytic grasslands the most common grasses included the tobosa grass (Pleuraphis mutica), alkali sacaton (Sporobolus airoides), and saltgrass (Distichlis spicata)⁶. However, recent studies report some changes in its floristic composition and the appearance of exotic grasses. In northeastern Sonora, semi-arid grasslands are mainly composed of native grasses of the Aristida and Bouteloua genera⁷, while in the central plains of the state, approximately 800 thousand hectares of exotic buffel grass (Pennisetum ciliare) are used for cattle raising and that were originally native scrublands⁷. Unfortunately, the overgrazing caused by beef cattle industry has degraded and reduced condition of short grass prairies, affecting the economic income of families⁸.

In the case of the Chihuahua grasslands, the most common grasses are of the Bouteloua and Aristida genera, and among the arid grasslands, the dominant species are the tobosa, alkali sacaton, and alkali lovegrass (Eragrostis obtusiflora)⁹. In addition, an invasion of shrubs such as the catclaw mimosa (Mimosa biuncifera) and whitebrush (Aloysia gratissima), as well as exotic grasses such as Lehmann lovegrass (Eragrostis lemahnniana) and natal grass (Melinis repens)^9,10. Local studies confirm a high degree of deterioration, as well as losses of 378 thousand to 2.72 million hectares^11,12, attributed to the opening of land to cultivation, overgrazing^10,12, climate change, human settlements, and inadequate public policies in the grasslands of Chihuahua¹².

In Durango, semi-arid grasslands are mainly composed of native grasses of the Aristida, Bouteloua, Elionurus, Eragrostis, and Heteropogon genera, while in arid grasslands, saltgrass, scratch grass (Muhlenbergia asperifolia), alkali grass, chino grama (Bouteloua ramosa), and toboso are the most common¹³. However, overexploitation of grasslands, adverse climatic conditions, and the introduction and invasion of exotic species such as the rose natal grass have caused a significant reduction of the state's grasslands¹³, as well as the change of an association of Bouteloua - Bothriochloa grasses to another of natal grass/tanglehead (Heteropogon contortus)¹⁴.

For the semi-arid grasslands of Zacatecas, the most common associations are Aristida / Bothriochloa / Bouteloua and other exotic grasses such as buffel, weeping lovegrass (Eragrostis curvula), lehmann lovegrass and natal grass, while in arid-halophyte grasslands the most common is the alkali sacaton¹⁵. In addition, most Zacatecas grasslands appeared to be in a moderate health state, although the properties of the soils are acceptable, the vegetation exhibits a high degree of deterioration¹⁶.

In the case of the semi-arid grasslands of Aguascalientes, the dominant grasses are sprucetop grama, blue grama, and wolfgrass (Muhlenbergia phleoides), which exhibit a good condition, with 80% of the original vegetation, especially of the Bouteloua genus¹⁷. As for the state of Jalisco, Bouteloua / Microchloa / Aristida are the most prevalent communities in the semi-arid grasslands¹⁸, which were in moderate to extreme health conditions in 2002¹⁹, the main problem being the deterioration of the vegetation. In San Luis Potosí, the most common grasses are blue grama, wolfgrass, buffalo grass, and the invaders buffel and natal grass²⁰. Unfortunately, a recent study indicates that overgrazing has impacted the composition of grasslands in central Mexico throughout history, but that it is still possible to improve the grasslands²¹.

In the 1970s, the scrublands of the Sonoran Desert were represented by shrubs such as saguaro (Carnegia gigantea), burro-weed (Ambrosia dumosa), elephant tree (Bursera microphylla), ironwood (Olneya tesota), yellow paloverde (Parkinsonia microphylla), and the creosote bush (Larrea tridentata)⁶. Recent studies on the vegetation of Sonora indicate that the predominant shrubs in the central plains region are the ironwood and brittlebush (Encelia farinosa); in the central coast, the elephant tree and Jatropha cinerea; in the highlands, the yellow paloverde, Cylindropuntia , Opuntia, and saguaro, and in the Lower Colorado River Valley, the Larrea / Ambrosia association⁷. In the case of the xerophilous shrubs of the Chihuahuan Desert, the dominant species are the creosote, the tarbush (Flourensia cernua), lechugilla (Agave lechuguilla), ocotillo (Fouquieria splendens), Chihuahua whitethorn (Acacia vernicosa), and others of the Dasylirion, Opuntia, and Yucca genera⁶. At present, viscid acacia (Acacia neovernicosa), honey mesquite (Prosopis glandulosa), spiny allthorn (Koeberlina spinosa), guajillo (Acacia berlandieri) and Warnock’s snakewood (Condalia warnockii) are also common, while in Coahuila there is also Hechtia spp, candelilla (Euphorbia antisyphilitica), leatherstem (Jatropha dioica) in the north, and Allenrolfea sp., Atriplex sp., Suaeda sp., and mesquite in the south²². Other dominant shrubs such as cenizo (Leucophyllum frutescens), mariola (Parthenium incanum), creosote bush, mesquite, lechuguilla, catclaw mimosa (Mimosa aculeaticarpa), guajillo, and Gutierrezia microcephala have also been reported in the xerophilous shrub of northern Coahuila²³. For the more arid area of the Chihuahuan Desert in the south of San Luis Potosí, there are shrubs such as the creosote, tarbush, honey mesquite, and chamiso (Atriplex canescens)²².

In Durango, xerophilous bushes are represented by creosote, tarbush, spiny allthorn, honey mesquite, leatherstem, candelilla, and Opuntia, as well as lechuguilla / guapilla (Hechtia glomerata) / Ocotillo associations, the latter species being dominant in the bushes of the north and center of the state of Durango, and the Henricksonia genus standing out as endemic to the Durango and Coahuila scrublands²⁴. In contrast, the scrublands of Aguascalientes have diverse associations where the most common species are nopal cacti (Opuntia spp), catclaw (Mimosa monancistra), huisache (Acacia schaffneri), and creosote bush / mariola, exhibiting a high degradation and preserving only 20 % of its primary vegetation¹⁷.

The availability of practical tools and methodologies is critical for documenting the changes in grasslands and scrublands due to the effects of management and climate. The use of remote sensors has been shown to be a practical tool for estimating forage / productivity and plant cover^25,26 and the extent and fragmentation of grasslands and shrubs²⁷.

Grass seed production

The main value of the production of grass seeds lies in its use for the reseeding or rehabilitation of grasslands, but it also has a great value for the conservation of soils. However, the commercial production of grass seeds for semi-arid zones has been recently developed in Mexico. From 2001 to 2016, the National Catalog of Plant Varieties⁶⁸ has documented 27 varieties and hybrids of grasses, 12 of which are suitable for semi-arid areas and belong to five species. Therefore, it can be said that there already exists genetic material to produce qualified seed.

Research on production of grass seeds for arid and semi-arid zones date back to the 80s and can be seen in Table 2. These works have mainly determined the volume of production and quality characteristics of the seeds, in some cases with fertilization and irrigation trials. The two most studied species have been buffel grass and sideoats grama.

Grassland seeding in arid and semi-arid zones

Considering the deterioration of grasslands and shrublands, grass seeding is a common practice to reverse this decay, which consists in establishing vegetation through the artificial dissemination of seeds of a single species or in a mixture, using adapted species and appropriate seedbeds. This is done in order to increase the productivity and quality of forage and stocking rate and diversification of diet for livestock, in addition to reducing erosion and invasion of less desired species; however, this practice is expensive and high risk. In general, range seeding is recommended in sites with areas with less than 15 % coverage of native grasses^73,74,75.

It is very important to use species adapted to the soil and climate conditions of the area to be rehabilitated. Native species adapt better to different climate and soil conditions and are more persistent; but they are more difficult to establish. Therefore, it is suggested that the origin of the seeds be not more than 300 km from the site where it is going to be seeded. In general, it is recommended to use a mixture of species, preferably native ones. The advantage of mixing is that the diversity of species can make better use of the variability in the soil conditions of the pasture lands⁷⁶. The main forage grasses used for sowing are: sideoats grama and blue grama, among the native species, and weeping lovegrass, wilman love grass, and buffel, among the introduced ones⁷⁷.

The presence or lack of moisture in the soil is the most important variable for the establishment of grass seedlings. Therefore, the land must be prepared for sowing, which is known as "seedbed"; This has the function of loosening the soil, giving it greater porosity, so that it retains a greater amount of water, and providing more favorable conditions for the establishment of grasses in these environments^76,77.

There are many practices for replanting grasslands, ranging from subsoiling, plowing, disking, level borders, “lister” furrows, and the use of aerator roller, among other options. The latter carries out one of the fastest and most effective processes, especially in desert scrublands, as it increases the water infiltration capacity, reduces soil compaction⁷⁸, and has allowed a good grass establishment such as blue grama, sideoats grama, Arizona cottontop, and buffel^76,79,80 in Sonora, Chihuahua, and Coahuila. However, such practices as subsoiling plus plowing and disking also favor coverage with values up to 80 % and a yield of 5.0 to 13.6 t DM ha^-1 in buffel grass in the states of Coahuila and Jalisco, respectively^81,82. In the state of Sonora, total clearing has had repercussions on loss of soil and plant diversity; therefore, it should be avoided in the preparation of the seedbed⁸³. Since the different sowing methods yield different results, it is considered necessary to emphasize that the practices should be adjusted to particular environmental conditions. On the other hand, it is advisable to turn over and break up the soil when wet and harrow it once or twice in order to make a good seedbed⁸⁴. However, it is better to sow in wet soil at the beginning of the rainy season in order to increase the opportunity for the seed germination and plant establishment. The ground can be prepared in advance, by "scratching" counter slope or with plowing and disking. Dry sowing is more risky; therefore, it would be advisable to sow one or two weeks before the rainy season in order to prevent loss of seeds⁷⁷.

After seeding, during the first year the weeds can compete for light, space and soil nutrients, which will reduce the potential of the grasses. It is recommended to control the weed mainly with selective herbicide. From the second year on, in order to keep forage production in good condition, it is recommended to carry out chemical or manual weed control tasks at the beginning of the rainy season, although this is a costly practice⁷⁷. However, it is recommended for the control of shrubs when their density increases.

Fertilization will depend on the amount of precipitation; in very dry years with rainfall under 200 mm, it is not recommended to fertilize. In the first year, fertilization is carried out with the 20-20-00 formula and only when the moisture condition is favorable. Once the grass is established, it is suggested to fertilize every year⁷⁷. However, fertilizers must be applied sparingly due to high costs and to the temporal and spatial variability of rainfall. Other options with good results in seeding native grasses are the application of biofertilizers⁸⁵ and organic by-products such as biosolids⁸⁶.

Challenges and perspectives

Given the great reduction in the areas of grasslands and scrublands, as well as their high deterioration, it is urgent to stop the advance of the agricultural frontier and avoid overgrazing. Thus, it is urgent to comply with current federal and state laws on conservation and sustainable use of ecosystems, as well as a rural technology transfer program in order to train and educate all ranchers on the sustainable management of grasslands and shrubs in the north-central region of the country. Technologies such as the use of images and geographic information systems are currently available for monitoring and evaluating grasslands and scrublands. However, it is important to continue with the development of technologies that allow finer detection of the processes and structure of grasslands, such as the identification of plants at ground level, in order to facilitate small and large scale decision making.

Regarding the use of grasslands, it is urgent to implement grazing management strategies that adapt to the vegetation and climate conditions of each ranch, but, above all, an adjustment of the stocking rate that will allow a moderate use of the grasslands. It is also necessary to reactivate studies of diet quality, selectivity, forage consumption and grazing management strategies to achieve efficient use of resources and the response of vegetation, in a more ecological approach, and in order to maximize animal production and rangeland conservation.

In regard to the conservation of genetic resources, more than 1,200 ecotypes of native and introduced grasses have been assessed^{48-53,56,57,58}. However, up to date, only eight varieties of sideoats grama, eight of buffel grass, and one of each of the blue grama, weeping and wilman love grasses have been generated⁹⁹. For this reason, it is important to design financing strategies that will allow to continue performing diversity studies and implementing grass genetic improvement programs. In addition, it is necessary to search for other forage species that grow in arid and semi-arid Mexico, since more than 300 species of grasses have been reported to exist in these regions¹³. These actions would allow the generation of new grass varieties, with good potential for establishing and producing forage in the face of the challenges of climate change.

Native grass seeds can be produced with currently available technologies at a lower cost than imported seeds although the main challenges are to reduce production costs, and increase seed yield and quality. In the immediate future, more research should be done on the seed vigor, scarification and coating and fixation of products such as nutrients or insecticides in the seeds in order to improve their performance in the field. Subsequently, the generation of technological guides for seed production of different grasses is essential, evaluating the use of agronomic practices and mechanization of seed harvesting and processing.

For the range seeding, it is recommended to revegetate degraded pastures, preferably with native plants, in order to recover their structure and functioning for the production of goods and services for society. In this activity, the availability of seeds of native species is a great challenge to solve, because, although certain varieties have been released by INIFAP and other institutions, such as blue grama and sideoats grama, the available quantity is insufficient to meet the demand. Establishing seed production lots with cooperating producers is a viable option. Secondly, there is a pressing need to design simple, practical and low-cost equipment for the preparation of land for range seeding.

In regard to carbon capture in grasslands and scrublands, the implementation of moderate grazing in “ejidos” and cattle ranches is urgently needed in order to reduce carbon emissions and increase carbon sequestration. Some important challenges are to generate reliable, fast and simple methodologies for the estimation of carbon sequestration and carbon pools, to include the sustainable management of grasslands and shrublands as an option for buyers of carbon credits in official or voluntary markets, and to promote the advantages of grazing management to increase carbon sequestration through sustainable cattle raising.

Climate change is already affecting grasslands and will directly and much more dramatically reduce the productivity and contribution of environmental services from the grasslands and scrublands of northern Mexico. This will impact the carrying capacity of rangelands with reductions in meat production, loss of biodiversity, decrease in carbon sequestration capacity, and effects on the hydrological cycle of these ecosystems. The new climatic conditions make it necessary to carry out comprehensive research studies in order to find options for the management and rehabilitation of these ecosystems according to these changes.