Grazing Changes Grazing?

---

Introduction

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 conditions.

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.

Stocking Strategies

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) grazing.

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 grasslands.

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" to produce.

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.

Studies

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 Archer 1990).

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 winds.

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.

Conclusion

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.