Article: Effects of Livestock Grazing on Infiltration Rates, Edwards Plateau of Texas

Authors: McCalla, G.R. Blackburn, W.H. Merrill, L.B.

Published: Journal of Range Management, Vol. 37, No. 3. (May, 1984), pp. 265-269.

This paper presents the results of a 20-month study of the effects of different grazing treatments on water infiltration rates at the soil surface.  The research was conducted between 1980 and 1981 at the Texas Agricultural Research Station, located near Sonora in the Edwards Plateau of Texas.

To provide context, the authors assert that “livestock grazing can alter infiltration rates of rangeland soils by removing protective plant cover and by trampling.  Vegetation and mulch cover serve to protect the soil surface from raindrop impact and influence soil surface properties such as bulk density, organic matter content, and aggregation.  Reduced infiltration rates as a result of livestock grazing have been attributed to: (1) loss of vegetation cover, (2) decreased mulch cover, (3) decreased amounts of vegetation standing crop and mulch, (4) increased bare ground, and (5) increased bulk density as a result of trampling.”

Treatments were applied to two different plant communities: shortgrass (sodgrass) and midgrass (bunchgrass).  Three grazing “regimes” were used as treatments: heavy continuous grazing (HCG), moderate continuous grazing (MCG), and short duration grazing (SDG).  Treatment paddocks were each 6 hectares in size.  An ungrazed control site was also established, which was 3.5 ha in size.

Infiltration rates were measured on eight randomly assigned half-meter runoff plots using a drip-type rainfall simulation.  Herbaceous material was sampled and weighed; estimates of ground cover were also determined.  Soil samples were taken to measure soil bulk density, soil water content, soil organic matter, aggregate stability, and soil texture.  Samples were taken 11 times at regular intervals throughout the years 1980 and 1981.

The paper summarizes the results of the data collection: “Infiltration rates of the HCG pasture were significantly lower than the MCG or SDG pastures in January, March and May of 1980.  There was a nonsignificant trend for infiltration rates in the SDG pasture to be greater than the MCG pasture through August 1980.  However, this nonsignificant trend reversed in November 1980 when infiltrates in the SDG pastures were less than those of MCG pasture.  The trend remained for the remainder of the study with infiltration rates in the SDG pasture being significantly lower than the MCG pasture in January and July 1981.”

At first glance, it would seem that short duration-high intensity grazing can destroy the soil characteristics that we seek to create.  However, there are some major shortcomings in this study that only careful analysis will reveal.  First, is the issue of stock density.  For the SDG paddock stocking rates varied from 3.2 ha/AU/yr to 4.9 ha/AU/yr.  Recall that treatment paddocks were 6 ha each.  This then means that around one or two AU (animal units) were placed in each paddock for each grazing event!  This is hardly the type of density required to achieve herd effect, nor is this the type of management intensive grazing one would find on a ranch that practices Holistic Planned Grazing.

Recovery periods are also an issue not directly addressed in the paper.  Again, careful analysis reveals a flawed study.  The paper states that a 4-day and 50-day graze/recovery cycle was used for the SDG treatment.  Yet it also states that this same paddock was under a high-intensity, low-frequency (HILF) grazing system prior to the SDG treatment.  According to the authors: “A 119-day rest period associated with a prior treatment on the site probably delayed and reduced infiltration rates exhibited by SDG…”  This suggests that recovery periods for the region are more likely 100 to 120 days, which the authors concede possibly may have helped infiltration rates, thus the residual effects of the prior treatment.  Cutting recovery periods in half to 50 days is most likely what caused the collapse in soil infiltration rates in this study; however, this variable was not controlled for in the research design.

Studies like this one, though couched in difficult scientific terminology and esoteric statistical methods, deserve greater scrutiny on the part of the Holistic Management community.  One is lead to believe that this study validates the claims made against Holistic Planned Grazing, but a careful look at the actual research reveals major flaws in the overall design, thus rendering the research conclusions only moderately useful at best.

Relevant points from the research do emerge, particularly those variables that exercise the greatest influence on infilitration rates as per strongest simple correlations.  These are, in order of correlation value: total vegetation cover, grass standing crop, midgrass cover, surface roughness, bare ground, bulk density, aggregate stability, soil organic matter, and rock cover.  With the exception of soil bulk density and rock cover, the use of Holistic Planned Grazing has the potential to improve all of these variables, and thus increase soil water infiltration rates.  More recent research by Dr. Keith Weber seems to validate this claim.

Interestingly, the parcel under total rest “had large enough site difference to prevent direct comparison with the grazed pastures.  The non-grazed pasture subsequent to the 1980 drought had a general increase in infiltration rates in both the midgrass and shortgrass communities.”  It is possible that oxidizing plant matter and soil capping were responsible for the declines in infiltration rates under total rest.  Unfortunately, this was not measured.  More research is required to more fully understand the effects of total rest on the soil surface and perennial grass physiology in different environments.