The deserts of Texas, New Mexico, and Mexico have a history of grazing since at
least Spanish rule, in the 1500s. The older native people seemed to be more
hunter than herder, but the chain of land supporting animals supporting man is
the same. These ranching strategies worked reasonably well for hundreds or
thousands of years, but a juggernaut of progress, the railroad, was the
breaking point. Newly accessible lands required new property rights. The
natives had to be removed to make room for more grazing animals. Markets were
demanding products. Little to no attention was paid to the land.
Overgrazing in general, but especially during dry periods, is often seen as one
of the most abusive agricultural practices. We can't forget the removal of
forests for logging and mining. Some land clearing and control, even today, is
done by burning. This article will examine overgrazing and additional causes
that might be driving the overall degradation of the region.
The driving factors of degradation are poorly understood. Grass-shrub dynamics,
livestock grazing, small animal activity, drought, changes in the fire regime,
and changes in the climate are some of the causes that can at least be
recognized (Humphrey 1958; Archer 1989; Allred 1996; Reynolds et al. 1997; Van
Auken 2000). The overall effect is not understood, but the consequences are self
evident - desertification (Schlesinger et al. 1990).
We must also remember that shifts in the environment are natural, but The time
scales we are seeing now are not. After centuries of recorded abuses, the land
was still in "great" shape even 50-100 years ago. In less than one century look
at how much damage has been done. At first it was probably pleasant, but as
pressures increased, the land quality rapidly diminished. In the past to change
from a plains to a scrub land seems to have taken around 3000 years and has
oscillated 3-4 times over the last ~10,000 or so years. This shifting nature is
good evidence that if the proper environmental conditions/balance can once
again be met, the grasslands can be restored.
Reasons aside, the grazing capacity has been reduced within a period of twenty
years, from one head per 2-5 acres to 1 head per 20-25 acres in places like
West Texas, New Mexico, and Arizona. It is widely believed that this reduction
is a result of overstocking.
Overgrazing or ...
Overgrazing has been seen as the main component that has greatly reduced
the carrying capacity of these lands in the last few decades. Overgrazing is
defined as the repeated removal of above-ground biomass and disturbance of the
soil surface leading to reduced vegetation, increased soil erosion, and reduced
potential for native species to re-establish naturally.
Grazing, as opposed to overgrazing, itself is not responsible for the
degradation of the region. It is an important component in a balanced
ecosystem. Before the railroad and subsequent expatriation of the native people
and fauna, it would be obvious that the mega herds of buffalo, numbering an
estimated X-X head, roamed the area. Given the lush condition that attracted
settlers in the first place, it is safe to assume that the bison did not
degrade the land, but possibly helped maintain it. This lends credibility to
Allen Savory's rehabilitation techniques of using fast cycling mega herds to
mimic the more ancient systems.
More recent studies have explored the possibility that the shrub encroachment
was the cumulative result of centuries of events of which overgrazing was only
one prong (Fredrickson et al. 2005). Gibbens and Beck (1988) made similar
conclusions after evaluating 64 years of data from the Jornada Experimental
Range. They reported that compared to fluctuations in annual precipitation
grazing had little impact on perennial grass cover.
The other extreme of overgrazing would be to completely fallow the land. This
is also not effective. When grass dies in such an arid environment it breaks
down slowly, in a process called oxidation, smothering annual vegetation and
allowing woody shrubs and trees to take hold. Herds of animals, constantly
migrating in search of food, would trample and defecate, returning nitrogen to
the carbon rich dry grass, on the dead grasses, hastening their decomposition.
This cycle continued for the last several thousand years, and likely a similar
cycle existed in pre-history. Oxidized grass is often treated by burning, but
this leaves the land bare and more vulnerable to erosion and evaporation.
Was overgrazing the only cause behind our eroding landscape? It most certainly
played a role, but the total rainfall, frequency of rain events, and the season
in which the rain fell appear to be at least as important as overgrazing. One
study concluded that it saw little effect of grazing and that the perennial
grass composition was more a function of abundant rainfall or drought
Nelson concluded in 1934 that overgrazing was damaging during any period, but
more so during droughts. In both cases black grama cover was reduced, but to a
greater extent in the drought periods. Again, rain being the as or more
critical component in the changes.
Settlers utilized surface springs to water their livestock in the mountains.
This limited their herds to relatively few cattle. In the 1880's drill
technology was able to tap water as deep as 400 feet (120m). Havstad notes
that after drilling began "there were 20,000 head of cattle out here. This
place just got hammered." By 1888 the Detroit and Rio Grande Livestock Company,
owned by former U.S. Army cavalry officers from Michigan, was pumping water
from the river to the Jornada Basin, 10 km away, to enable cattle (Bos taurus)
By the turn of the century southern prairie stock-men had noticed swaths of
grass had been fragmented by prickly pear so thick you could hardly drive
cattle through it (Smith, 1899). While warning signs of overtaxing the land
were observed, few heeded them; by 1900 grasses around Big Bend National Park
in Texas, that once supported grasses tall enough to brush the bellies of
horses, and the desert grasslands of West Texas were supporting over 9 million
head of cattle, sheep, and goats, up from just 500,000 head in 1830.
In 1990 the herds between Arizona and New Mexico numbered 900,000 (Fredrickson
et al. 1998). This decline, like everything else, is not the result of one
specific event, but the complex interactions between many events. The
degradation of the land, higher economic costs, and more market competition in
the beef industry from places like Latin and South America have all contributed
to the lessening of stock rates in the US desert southwest. It should be restated
that ranching had been one of the main economic activities at least as far back
as the 1500's, but, again, the railroad allowed such an unprecedented access to
and export from the region, that the stresses were finally too much.
Today 37% of New Mexico's and Arizona's 10 million annual unit months of feed
comes from foraging federal managed rangelands (Torell et al. 1992). Animal
unit months is a way to generalize feed consumption between animals. Each
animal unit month is the amount of feed consumed by an animal over 1 month.
Cows generally consume 5 AU/mo while sheep, goats, or pigs are rated at 1AU/mo.
As national demand for beef increased, fueled by increased railroad
destinations, and the degradation of the land again noticed, the solutions
proposed focused on adjusting carrying capacity and spacial distribution of
grazing (Holechek et al. 1998a) (Jardine and Forsling 1922). These ideas were
founded in the thought that the Chihuahuan Desert grasslands were as resilient
as the other North American grasslands immigrant settlers were used to and that
with proper management, a static carrying capacity could be determined.
Experimental stations established in the late 1800's and early 1900's were
tasked with developing a model for estimating carrying capacity, water usage,
mineral feed placement, and fencing of stock, required to produce the quality
and quantity of meat production. Enclosures were constructed and clipping
trials were undertaken towards these ends.
Most grazing systems employed then, as today, have stocking rates based on a
relatively fixed grazing capacity. Wooton (1915), Jardine and Forsling (1922),
Canfield (1939), and Paulsen and Ares (1962) established guidelines for
carrying capacities of black grama rangelands. No accurate judgement, in
accordance with economic potential, had been found. The variation in stock
rates needed to account for periodic droughts and dry season too negatively
impacts the expected profit margins.
At the Jornada Range Reserve in 1915, carrying capacity and forage usage
research was founded (Havstad and Schlesinger 1996) on the principle ideas were
that in good seasons the excess grass would be left to protect the soil and in
bad seasons the temporary overgrazing would be reversed by the next period of
rainfall; fencing was also utilized to distribute livestock more evenly across
the land. After 50 years of conservative stocking it was seen that "attempts to
adjust stocking rate to this highly variable basis of forage have had
disastrous results. A breeding herd built up to use most of the forage crop in
good or even average years cannot be maintained in dry years" (JER field-day
report 1948 unpublished).
Jardine and Forsling (1922) evaluated large-scale pasture responses on the
Jornada Reserve and adjacent rangeland from 1915–19, a drought period. They
measured basal cover responses of black grama to three coarsely applied
management practices: (1) heavily grazed yearlong until 1918 and lightly grazed
during the 1918 and 1919 growing seasons, (2) grazed yearlong 1915–19, and (3)
reduced grazing during the growing season but fully utilized during the dormant
seasons, 1915–19. Basal cover responses of black grama, compared to an area
protected from livestock grazing, clearly favored treatment 3, and the authors
concluded that light grazing during the growing season was the appropriate
grazing strategy for black grama dominated rangelands.
Jardine and Forsling (1922) recommended the following drought strategies: (1)
limit breeding stock to carrying capacities during drought, (2) add surplus
stock during good forage years depending on market conditions, (3) adjust range
use seasonally depending on growth characteristics of key species, (4)
establish permanent watering points no more than 5 miles apart, and (5)
establish both herding and salting practices that achieve optimal stock
distribution. Strategy #5 may have accelerated shrub expansion into areas
By the 1930's there was growing worry that the grasslands would succumb to the
encroaching shrubs. Cheap labor during the Great Depression was used to
manually remove, or grub, shrubs. As economies and technology advanced, manual
labor was displaced with mechanized labor and herbicides as the main shrub
control methods. Again, economics is driving the land use that requires "force"
Eventually, Herbel and Velson (1969) began suggesting a more extreme rotation
that would opportunistically take advantage of the highly variable ecology of
the region. Using the chance to graze plants at various stages of their
life-cycle (ex: flowering soapweed (Yucca elata)) and to "follow the rains" in
such a path that plant production was maximized was a large step in the right
direction, but the variability was still too high.
A massive 36 year long study, from 1967 to 2002 was undertaken at the Chihuahuan
Desert Rangeland Research Center (CDRRC). The study examined a three-pasture seasonal
rotation approach versus yearlong grazing in a fourth pasture. The study pastures
were located 24 miles north of Las Cruces, New Mexico. The terrain was nearly level,
with slopes less than 2% and an average elevation of ~4,350 feet above sea level.
In the yearlong pasture the cows grazed different plants with the changing
seasons. Similarly the rotated herd would consume the seasonally available
forage first, but would be excluded from taking advantage of the in-season
forage in the other 2 pastures. This led to consumption of lower quality
forage and in turn slightly lower weaning weights. Calves weighed an average
of 477 lb. at 7 months of age on the seasonal pastures and 494 lb. on the
yearlong pasture. This is 17 lb. difference is less than 2% variance.
Long-term, there were no differences detected for calf production or changes in
vegetative composition between the two grazing management systems.
Both herds of cattle were removed from the pastures from late 1994 to early
1997 because of drought conditions
Much more variation can be seen when comparing years of high and low rainfall.
In dry years with limited forage, such as 1994, utilization averaged more than
37%. In high rainfall years such as 1978, with large amounts of forage
available, utilization was near 10% on the pastures. In years of higher
rainfall, the cows were not limited to perennial grasses but ate a large
variety of plants (Mofareh et al. 1997)
The pastures themselves were made up of varied grassland species dominated by
black grama (Bouteloua eriopoda) and mesa dropseed (Sporobolous fleuosus) to
dense mesquite (Prosopis glandulosa). The total annual perennial grass
production average 150 lb./acre across all the pastures. This average is
constructed from a wild swinging range between 3 lb./acre in 2002, the driest
year and 420 lb./acre in 1986, the wettest year.
Annual non-grass production averaged 71 lb./acre. This declined steadily over
the course of the study, from 490 lb./acre to 0 lb./acre during the dry 2002.
Dropseeds were important forage plants during the growing season but were not
important in cattle diets in the dormant season (Mofareh et al. 1997).
Threeawns were eaten more (P≤0.10) in the winter-spring pasture as they were
some of the only things available with leaves. Black grama was grazed
year-round but was utilized more (P≤0.10) in the fall pasture when other plants
began to become dormant.
Stock Can't Stop It
The region is still poorly understood and new information will always be refining
the models, but we can see a few things emerging from the data at this point. The
land was overstocked and stressed even in good years, and during drought years
the carrying capacity was exceeded. There are no major differences between a yearlong
and a rotational pasture approach.
We can see that the cows and grazing has direct positive effects on shrubs.
Particularly honey mesquite recruitment. Cattle eat a large number of mesquite
seeds and many pass through their digestive tracts viable (Paulsen and Ares
1962, Mooney el al. 1977). Seeds are often deposited large distances from
where they were consumed (Humphrey 1958; Paulsen and Ares 1962) and in a
favorable micro-environment provided by the cattles' dung. If the deposition
occurs during the rainy season, seedlings of honey mesquite become established
within 4 months, with a root system reaching depths of up to 40 cm (Brown and
This knowledge is nothing new, in 1929 Campbell concluded that livestock
dispersal of mesquite seeds was exacerbated by poor salt placement, the
locations actually promoting mesquite recruitment. It was speculated that the
cattle would trample the grounds of their favorite feeding spots and those
areas would become less able to absorb water. The rain would now runoff instead
of lingering in the upper soils, instead concentrating into channels and only
allowing the deeper rooted shrubs to access it. Hoof prints furthered the
problem even more, providing small pools of water that would allow mesquite
seedlings to take hold. Lastly the cattle graze the surrounding grasses that
might act as some kind of competition for the young mesquite plants.
Another experiment, started in 1933, constructed a 250-ha cattle enclosure. The
northwest part of this enclosure was dominated by grassland and the southern
part was dominated by honey mesquite. Cattle were excluded from the enclosure,
but freely grazed the surrounding area. Mesquite expansion occurred similarly
in both the enclosure and surrounding land. Seed dispersal in the enclosed area
was attributed to small animals.
This expansion continued, visibly, from 1948 to 1987. At some point between 1987
and 1998 a critical threshold was passed. Mesquite now came to dominate the
enclosure, wind erosion became prevalent and coppice dunes developed in both the
grazed and excluded lands. This is a stark warning that systems are resilient
up until their tipping point, but then changes can occur quite abruptly - about
70 years to go from partial grassland to coppice dunes.
Early on in the 1900's through today it is commonly believed that some
combination of overgrazing and drought has caused the deterioration we
currently see. Recently though, it has been proposed that the changes were the
result of a series of events occurring over centuries and not a response to
livestock grazing (Fredrickson et al. 2005). In short, the region may be drying
since the end of the last ice age because of larger effects, like the
orographic effects of the mountains and the current combination of global
Though this might be hard to believe, there is a lot of data that suggests
grazing strategies have little to no effect on land changes. Paulsen and Ares
showed in 1962 that grazing intensity had little effect on black grama during
even extended drought. The only measurable difference was in recovery rate,
where the conservatively grazed plots recovered faster.
In 1988, Gibbens and Beck added support for this view when they examined 64
years of quadrat data from the Jornada Experimental Range. They reported that
annual percipitation was a far stronger driver of perennial grass cover than
grazing. The amount of rain, frequency of rain events, and the season in which
it fell appeared to be most critical in determining plant composition changes
across the pastures.
The 36 year study previously mentioned also shows that changes in grass cover
was similar across all four pastures. The season of grazing appeared to have
little influence on grass cover. Changes were once again predicated on the
abundance of rainfall. Which, unsurprisingly, also drove utilization. Average
perennial grass utilization ranged from 16% to 26% with an average near 21% on
both grazing systems. In drought years, utilization sometimes exceeded 50% on
some perennial grass species.
We still have incomplete understanding of the complex dynamics degrading these
lands. We can see the damage, continuing, but little can be done as long as you
define grazing stocking rates the way we do currently. One can not plan to have
"full" utilization in years of plenty and expect to make it through a drought
without undoing any progress that may have been made in the good years. Need
highly variable stock rates.
That said, much of the changes seem to originate from long term rain cycles.
These changes can not be managed away with annual stock rates or other small
changes relative to the regional rainfall. When we see mesquite stools
spreading aided by cow dung and trampling of grasses, it is easy to blame
grazing as the sole problem. While grazing management will play a role in the
overall reclamation of the region, it is not at all the primary concern.
Leaving it fallow is not enough either. We must first get the land in a
condition where it is improving before you can fallow. While more active
management, orchestration if you will, is probably needed at this point, action
without understanding can be as, or more, damaging as the problems you are
addressing. The goal should not be to forcibly dominate the land, but to gently
coerce it into a state where it is not degrading. Then we can work on how to
best utilize the land. I believe, and will show in future articles, that the
rain itself can be coaxed into strategically selected locations with swales,
small dams, and similar techniques laying a solid foundation for restoration.