- Animals: bovine
- Animal Production: rangeland/pasture management
- Crop Production: water management
- Production Systems: integrated crop and livestock systems
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.