Final report for GS18-196
Extraction of water from the Ogallala Aquifer in the Texas High Plains (THP) ecoregion for irrigation is driving producers to convert portions of irrigated cropping systems to limited–irrigation small grain pastures for beef cattle, potentially prolonging the life of the aquifer, as well as providing a diverse source of income by stacking enterprises when compared with continuous monoculture of cotton (Gossypium hirsutum L.) (Allen et al., 2005, 2012). Cattle trampling heavily degraded warm-season perennial pastures with in silty clay loam soils at high stocking densities has been shown to increase soil bulk density by more than 0.20 g cm–3 compared to an ungrazed control (Vanderburg et al., 2020). Negative effects of grazing are mostly confined to the top 2.5 cm of the soil profile (Mapfumo et al., 1999) when trampled soils are susceptible to compaction. The grazing of annual forages established using conservation tillage has raised concerns about cumulative soil compaction issues because of no mechanical disturbance of the soil surface, especially in the THP region.
Cumulative compaction has been reported in no-till trials on a silt-loam soil when soils were compacted with a pneumatic compactor weighing 76 kg (Ferrero, 1991). Soil bulk density increased in the top 10 cm from 1.22 to 1.33 g cm–³, and, when compacted again within a season, bulk density increased from 1.33 to 1.40 g cm–³. Freezing–thawing and wetting–drying cycles, burrowing by micro, meso, and macro fauna, and root growth can mitigate effects of compaction (Jabro et al., 2012; Kozlowski, 1999). No-till management strategies allow for roots and crop residues to remain near and at the soil surface, thereby slowing the breakdown of sugars, amino acids, organic acids, mucilage, root border cells, and dead root-cap cells, which are then used as carbon sources by soil microbial decomposers (Philippot et al., 2013). As organic matter is more protected over time, improvements in soil structure and stability, porosity, infiltration rate, water holding capacity, and plant nutrient availability can accumulate (Bronick and Lal, 2005).
No-till cropping has been widely adopted in an effort to conserve soil, water, and costs by leaving the soil mostly undisturbed from one growing season to the next. Additionally, by leaving crop residues on the soil surface, increased water infiltration and retention in the soil could potentially be achieved. Previous studies have confirmed that there is no difference in compaction between annual and perennial grazing systems, but have attributed recovery year over year to cultivation between plantings in annual systems (Mapfumo et al., 1999). Further, water infiltration rates can be sustained with proper grazing management practices by maintaining adequate ground cover (Russell and Bisinger, 2015) and deferment of grazing to allow the soil to recover from grazing bouts. Understanding the required deferment of grazing to mitigate (or reverse) compaction and re-establish the subsequent annual crops will inform producers on how to improve soil health, while maintaining soil water balance and crop productivity.
MATERIALS AND METHODS
Field research was accomplished in 2018 and 2019, 10 km east of New Deal, TX at the Texas Tech University Forage Research Lab (33.045' N, 101.047' W; 993 m elevation) located in Lubbock county. Soils in the site were characterized as a Pullman clay-loam (fine, mixed, super–active, thermic Torrertic Paleustolls), with 0 to 1% slopes, containing a thick layer of CaCO3 (caliche) at 60‒120 cm depths (Brooks et al., 2000). Climate in the THP is variable with daily mean temperatures ranging from 10°C in January to 30°C in July (UDSA‒FS, 2004), and annual precipitation averaging 470 mm with large fluctuations among years (USDA–NRCS, 2006). Most of the rainfall occurs as high intensity thunderstorms during late spring and early fall, with two thirds of the annual precipitation falling during the growing season (Texas A&M AgriLife Extension, 2017). Agriculture in the THP is water-limited, relying heavily on irrigation for high yields, where potential evapotranspiration averages 156 mm per month, exceeding average precipitation each month of the year (Stewart and Steiner, 1990). Freeze-free period averages 225 days in the southern portion of the THP ecoregion (USDA‒NRCS, 2006).
Treatment pastures were irrigated with subsurface drip tapes (Netafim, Fresno, CA, USA) located 0.36 m deep with injection emitters every 0.6 m, set to deliver 1.47 L h–1 at 88.3 kPa during the drier months by deficit irrigation, not exceeding 178 mm per month of applied irrigation. Three 0.81-ha pearl millet pastures were planted each spring with a grazing exclosure placed in the center of each pasture. Once the millet was established, 18, 272-kg steers were placed in the pastures intermittently from early June–August in 2018 and late May–September in 2019. At the end of the grazing trial, stocking rates were doubled and steers were allowed to graze out the available forage before removal from the grazing trial. Rainfall was recorded with a rain gauge at the treatment pastures. Cumulative rainfall for each deferment season was calculated.
After the end-of-season graze-out of the pearl millet, cattle were removed. Every 60 d, 20 soil cores were extracted along two permanent, perpendicular transects to determine soil bulk density until the next year’s planting date. These transects were fixed in location, and samples were taken from the same vicinity every sampling date to test for in-field variation within each treatment pasture. Samples were extracted using 5.1-cm diameter x 7.6-cm deep core sleeves with a 30 cm by 30 cm wide, 5-cm thick wooden board placed on top. A rubber mallet was used to hammer the sleeve into the top 8 cm of the soil until the board was flush with the soil surface. A sharpshooter shovel was used to carefully dig out the sleeve. After removal, a soil knife was used at the top and bottom of the sleeve to scrape off the excess soil. The sleeve was then wrapped with cellophane and labeled.
Soil cores were dried at 100 ºC for 48 h in a laboratory oven. Dry cores were weighed with the sleeve, removed from the sleeve, which were weighed empty, and bulk density was calculated and expressed as g cm–3. The second-year pearl millet was no-till planted into the same pastures and grazing occurred sporadically throughout May–September with a final graze-out in September.
When sampling for bulk density, a soil penetrometer reading was taken beside each soil core using a digital soil compaction meter (FieldScout SC 900 – Soil compaction meter, Spectrum Technologies, Inc. Aurora, IL), which measures soil resistance at 0–2.5, 2.5–5, 5–7.5, and 7.5–10 cm to determine correlations between soil resistance and soil bulk density. The meter was equipped with a data logger that saved data between readings. The datalogger was connected to a computer, and data were downloaded.
The experimental design was a randomized complete block design with 3 replicated blocks. To detect differences in soil bulk density as it related to no-till and trampling, an analysis using the MIXED procedure of SAS was utilized. Least squares means were compared for soil bulk density between grazed and the ungrazed treatments for each deferment period in each year. Pasture was considered the experimental unit, and the model included fixed effects (treatment / trampling), where block was considered random. Differences were determined significant at P ≤0.05.
Penetrometer readings were recorded in kPa, and resistance pressure data at 3, 5, and 7 cm were averaged to represent the depth of the soil core. A Pearson’s correlation was analyzed using the PROC CORR function of SAS to test the relationship of soil bulk densities using the core method and penetrometer readings. Significance was declared at P ≤0.05.
Objectives of this research were 1) to quantify the effect of brief cattle trampling events at moderate stocking densities on soil bulk density, 2) to determine if cattle trampling has a cumulative effect on soil bulk density in no‒till annual cropping, and 3) to quantify the amount of grazing deferment required for bulk density to return to a level parallel to that of the untrampled control. Our hypothesis was that after the first year, since there will be no tillage between plantings of pearl millet, soil bulk density would increase in the second year immediately post-trampling, and the soil bulk density would decrease after the cattle are removed from pastures due to natural recovery processes until the next trampling event.
-Plot area was prepared in early March 2018 by:
-lightly disking to remove weeds.
-applying herbicide prior to planting to allow millet to outcompete pigweed.
-Pearl millet was planted in May 2018 and given time to establish.
-Grazing exclosures were placed in the center of each field for an untrampled control.
-Cattle intermittently grazed millet plots throughout June, July, and August.
-Stocking rates were doubled for the last grazing bout in September, and cattle were allowed to graze down residual forage as much as available.
-Two permanent transects were placed in each field running perpendicular to each other.
-20 soil samples were collected from the top 3 inches using 2 x 3 inch core sleeves, 10 evenly spaced along each transect. Initial samples were taken September 24th, 2018, then repeated every 60 days.
-5 soil samples using 3.5 x 7 cm core sleeves from each small grazing exclosure were taken to compare to the trampled treatment. Initial samples were taken September 24th, 2018, then repeated every 60 days.
-Penetrometer readings were taken next to each bulk-density core to a depth of 12 cm. The Field Scout penetrometer records resistance at 3, 6, 9, and 12 cm. (initial samples were taken September 24th, 2018, then repeated every 60 days)
-Soil samples are taken by tapping core sleeves into the ground using a board placed on top and a rubber mallet. Once the board lies flat across the soils surface, a shovel is used to dig the core sleeve from the soil profile. The soil is scraped from the bottom (so the soil is even within the core sleeve) wrapped with clear cellophane and taken to the laboratory for analysis.
-In the lab, cores are removed from their plastic wrap, dried at 105 degrees C for 48 hours and then weighed. Volumetric bulk density is then calculated using the dry weight of the soil divided by the volume of the core sleeve, and recorded in g-1 cm-3.
-Atrazine sprayed to prevent weeds in March 2019, and pearl millet will be no-till planted into the three paddocks.
-After germination and seedling emergence, seedling was counted along the permanent transects using the Daubenmire frame method.
-Establishment and plant cover will be monitored along each transect in May, June, July, and August until treatments were applied.
-Second year of data collected and analyzed.
-Paper for scientific publication is in review with committee.
-Results from this research were presented at the Texas Tech University field day.
-Research results will be posted in the Texas Alliance for Water Conservation newsletter that is sent directly to producers and other subscribers as well as any extension and outreach social media pages associated with Texas Tech University, Texas Alliance for Water Conservation, and the principal investigator.
-Results are to be posted on a discussion board and left open for questions and further discussion for producers and other researchers to interact among themselves and the principal investigators.
-Previous discussion boards with preliminary results of this project have generated interest and discussion amongst 34 producers on social media from all over the world including Australia, South Africa, and England.
RESULTS AND DISCUSSION
Soil bulk densities for the grazed treatment in September 2018 to May 2019 rest period were not significantly different in the grazed treatment from the ungrazed control, indicating no effect of cattle grazing (P = 0.07 to 0.91; Figure 1). Grazing a second year with no-till planting between grazing bouts increased bulk density in May to September 2019 (P <0.0001 to 0.002; Figure 6.1) when compared to the 2018 grazing season (Figure 6.1). Further, soil bulk density was 1.50 g cm–3 immediately post-trampling in October 2019 and decreased to 1.40 g cm–3 to a level not statistically different from the untrampled control before the next grazing season in May 2020 (Figure 6.1). Soil bulk density was not significantly different from the ungrazed control in May 2020 (P = 0.06); however, the bulk density of the grazed treatment was numerically greater than that of the ungrazed treatment (Figure 6.1).
Soil bulk densities did not fully recover by May 2020 (Figure 6.1), but were not significantly affected by grazing. Immediately post–trampling within the first 60 d of grazing deferment, bulk density decreased from 1.50 g cm–3 in October 2019 to 1.46 g cm–3 in December 2019 (Figure 6.1), while cumulative rainfall increased (Figure 6.1). As cumulative rainfall rose from September to December 2019, bulk density continued to decrease. This pattern of decreasing bulk density with increasing cumulative rainfall was not observed in the 2018–2019 grazing season.
When considering variation along the transect, there were slight increases in soil bulk density in areas where cattle grazed the forage more heavily, namely around water troughs and mineral tubs. These differences were not consistent among dates or years, and were not statistically significant. High-intensity hoof action increases compaction, resulting in less infiltration and higher bulk densities (Fleischner, 1994; Kauffman and Kreuger, 1984; Warren et al., 1986; Willatt and Pullar, 1984); however, any temporary or long-term negative effects of trampling depend on soil texture (Orr, 1960; Van Haveren, 1983), weather conditions (Warren et al., 1986), and soil water content (Assouline and Mualem, 1997; Nawaz et al., 2013; Robinson and Alderfer, 1952) at trampling time.
Sandy soils require more force to compact compared to finer-textured soils with higher clay content (Daum, 2002; Van Haveren, 1983). Sandier soils had no significant impacts on bulk density from grazing under light, moderate, or heavy grazing intensities (Van Haveren, 1983), whereas finer-textured soils showed 11.8 to 13.4% greater bulk density when comparing heavy stocking with moderate and light stocking intensities. Reed and Peterson (1961) reported a consistent positive relationship between grazing pressure and bulk density regardless of soil texture. Research on the Pullman clay‒loam soil by Kharel et al. (2018) at the same site as the current trial found that the actual texture in the top 0–10 cm was sandy clay loam rather than the clay–loam indicated by the NRCS mapping unit of Pullman. Soils were not affected by trampling in 2018, likely owing to the sandier make–up of the topsoil in these treatment pastures.
The ideal bulk density for plant growth in a clay‒loam soil is <1.10 g cm–3, and <1.40 g cm–3 for silty and sandy clay loams (Table 6.1, USDA–NRCS, 2011). The difference between the latter two textures is that bulk densities which reduce root growth are 1.55 g cm–3 for silty clay loams and 1.60 g cm–3 for sandy clays. As bulk density increases above these values, root and shoot growth can be severely diminished (USDA‒NRCS, 2011). Although there were increases in soil bulk densities between 2018 to 2019, bulk densities never reached values that exceeded the NRCS critical values that would theoretically inhibit root growth (Table 1).
Table 1. Differences in effects of bulk density on plants in different soil textures. Table adopted through data from (USDA–NRCS, 2011). Pullman clay loam is under the clay loam soil texture.
Ideal bulk density for root growth
Bulk density that affects root growth
Sandy clay loams
Silty clay loams
Clays (>45% clay)
On the contrary, there was a statistical significance in cattle impacts on soil bulk density after the second trampling event. The difference between years in treatment effects could be due to differences in soil water content at time of grazing because more force is needed to compact a dry soil than a wet soil (Lull, 1959). In 2019, 39 mm of rain fell within the last week of grazing (Figure 6.1). The higher bulk density in 2019 than in 2018 could have been due to trampling around the time of rainfall, whereas in 2018 there were no rainfall events within 2 weeks before cattle had grazed. Soil water content at the time of trampling was not measured. In contrast, Laycock and Conrad (1967) reported no effect in northeastern Utah on soil bulk density with varying soil water contents when grazing loam and clay–loam soils.
Once cattle were removed, soil bulk densities began to decrease, indicating recovery. Soils will recover from compaction via events such as freeze‒thaw, burrowing of soil organisms, and increased root growth exploring the soil profile for water, which all aid in the recovery of the compacted soils over time (Jabro et al., 2012; Kozlowski, 1999). In the current trial, it is likely that wetting and drying cycles aided in bulk density recovery between trampling events (Figure 6.1). This is because there were no more than one or two hard freeze events after the 2019 grazing season. Further, roots create organic residues and hold the soil in place by the creation of stable aggregates, which reduce soil compaction and increase infiltration rate of water into the soil profile (Russell and Bisinger, 2015). Increased root growth from volunteer bermudagrass [Cynodon dactylon (L.) Pers.] roots growing in the treatment plots may have helped with soil recovery; however, these results were not consistent, and likely there was an aid with compaction resistance where bermudagrass had established between 2018 and 2019 rather than a recovery effect.
Livestock trampling without removal of any vegetation has resulted in increases in soil bulk density (Alderfer and Robinson, 1947; Betteridge et al., 1999; Hamza and Anderson, 2005; Kako and Toyoda, 1981; Lull, 1959; Van Haveren, 1983, Warren et al., 1986; Willat and Pullar, 1984), reduced infiltration (Gifford and Hawkins, 1978; Van Haveren, 1983; Warren et al., 1986), increased runoff (Thurow et al., 1986, 1988; Warren et al., 1986), and increased evaporation losses (Knoll & Hopkins, 1959; Wraith et al., 1987). Deferral periods between grazing bouts on soils that are prone to compaction (heavier textured) must be sufficiently long for soils to recover between trampling events so that adequate pore space is re–established to allow water and oxygen flow (Nawaz et al., 2013; Whalley et al., 1998) and diminish evaporation losses (Harivandi, 2002; Sosebee, 1976). Decreased macropore size slows percolation of water, which may cause prolonged periods of saturation (Duiker, 2004).
Penetration resistance is likely a more direct indicator of the inhibitory effects of soil compaction on root growth than bulk density because measurements of the former account for the same resistance encountered by roots, apart from the indirect effects of soil texture on soil bulk density. For example, Taylor et al. (1966) reported that root growth decreases linearly with increasing penetration resistance starting at 689 kPa (100 psi), and root growth was completely inhibited at 2,068 kPa (300 psi). Penetrometer readings obtained from this study ranged from 345 to 2,620 kPa (50–380 psi). There was no significant correlation between penetrometer readings and its soil bulk density counterpart for the combined years. In 2018, there was no significant correlation, but there was a significant correlation in 2019 (P = 0.002). It was also noted by Thompson et al. (1987) that penetrometer readings were not well correlated with soil bulk density measurements in the top profile of the soil.
Soil bulk density is a measurement of pore space whereas penetrometer readings measure soil strength, where the resistance using a penetrometer more accurately reflects what the roots encounter when exploring the soil (Phillips and Kirkland, 1962). This means that penetrometer and soil bulk densities are not always correlated with each other. Soil conditions such as soil moisture seem to be one downfall to accuracy of penetrometer readings (Mulqueen et al., 1977) At higher soil water content, penetrometer resistance has been shown to be insensitive to bulk density. On the other hand, intermediate water content increases internal friction, and compression can be sensitive to bulk density (Mulqueen et al., 1977).
Brief cattle trampling, especially on wet soils, will increase soil bulk density, potentially reaching values that could inhibit root and shoot growth. However, an adequate deferment of grazing following livestock trampling can mitigate or reverse compaction and re–establish subsequent annual crops.
In conclusion, there was no difference in soil bulk density between grazed and ungrazed treatments in pearl millet no-till pastures in 2018. There was a cumulative effect of grazing on soil compaction observed in 2019; however, bulk densities never remained at critical values that would severely inhibit subsequent root and shoot growth for the next millet planting season. Soil bulk densities decreased over time once cattle were removed.
Educational & Outreach Activities
Results from this research will be presented at the next field day at Texas Tech University, and research is in preparation for review for a professional scientific journal. Additionally, research results will be posted in the Texas Alliance for Water Conservation newsletter that is sent directly to producers and other subscribers as well as any extension and outreach social media pages associated with Texas Tech University, Texas Alliance for Water Conservation, and the principal investigator. Results are to be posted on a discussion board and left open for questions and further discussion for producers and other researchers to interact among themselves and the principal investigators. Previous discussion boards with preliminary results of this project have generated interest and discussion amongst 34 producers on social media from all over the world including Australia, South Africa, and England.
As the Ogallala Aquifer continues to decline, the number of producers adopting no-till practices or implementing grazing systems will continue to rise. It is important to understand how management affects the ecosystems and biological functions, which, in turn, shift sustainability outcomes of these crop and livestock systems. Our results suggest that with adequate deferment from trampling, no-till management can be continued over subsequent years and sustain soil structural integrity. Increased structural integrity would increase water infiltration into the soil profile for root uptake rather than losing water to runoff and erosion (Heitschmidt et al., 1987). This process would increase the drought tolerance of the whole soil-crop-livestock system. Furthermore, properly managed grazing systems that do not degrade soil structure can encourage next season’s plant growth (Frank et al., 1998) and even potentially increase carbon sequestration (Franzluebbers et al., 2000), which is important in offsetting impacts of enteric methane emissions while enhancing the biogenic carbon cycle. Future research is necessary to continue monitoring longer-term effects of grazing in no-till annual pastures on plant-soil-water dynamics as well as gas fluxes in relation to structural aggregate stability and compaction.
If soils are managed in a way that promotes surface crusting, then root growth, plant productivity, and drought tolerance would be diminished because surface crusting weakens soil aggregation, which in turn slows infiltration, promotes erosion, and reduces gas exchange. Knowing adequate deferment times is useful for producers to plan how to maintain the soil’s structural integrity and aggregate stability. There are negative cumulative effects of trampling on soil physical characteristics in no-till annual pastures; however, allowing for soil recovery between trampling periods can restore soil health and sustain the soil’s support of plant and animal growth. Results from this project suggest that recovery after an early-fall graze–out until the next planting date is adequate for soil bulk densities to decrease to levels that resemble untrampled controls. The outcomes of this project will help the principal investigator and the collaborators make suggestions and recommendations in their extension and producer outreach programs. Knowing that there should be deferment between plantings and grazing dates will inform recommendations that promote practices that increase aggregate stability of soils. Increased stability is integral for infiltration, root growth, and resistance to wind erosion, which is a large biological stressor in the semi-arid High Plains. Additionally, knowing how to manage soils for the continuation of stable aggregates can facilitate adequate pore size, which is essential for air and water entry into soil, and for air, water, nutrient, and biota movement within soil.
Future research is needed to continue to look at long term effects of cattle trampling in the Texas High Plains Ecoregion on soil health. Further, research is needed to investigate soil health and carbon exchange between ruminants and soils, as well as long term carbon sequestration and microbial communities. This should also be coupled with regenerative agriculture practices, such as pasture cropping, and dryland agriculture. The Ogallala Aquifer region has much research ahead to understand and gain the knowledge needed so that agriculture can persist in the wake of changing climate, namely, sporadic rainfall patterns and drought, in the midst of a declining underground water resource. It is pertinent that scientists continue to investigate how producers can be more vigilant in their sustainable farming practices, where technology and education can potentially help producers adapt to these challenges ahead.