EFFECTS OF TRAMPLING AND COVER ON BULK DENSITIES AND RECRUITMENT OF GRASSES ON RESEEDED PASTURELANDS

A field study was conducted to determine how soil bulk density and plant foliar cover change over time in response to deferment following a high density, high intensity, short-term grazing/trampling event. The research site was in the Texas High Plains on the West Texas A&M University Nance Ranch, in Randall County, USA. Average annual precipitation is 50 cm, annual high temperature is 21°C, and annual low temperature is 6.5°C (NRCS- WCC, 2017). Green Sprangletop (Leptocloa dubia Kunth.) and Kleingrass (Panicum coloratum L.) were broadcasted at approximately 4.5 kg haˉ¹ pure live seed (PLS) on former cropland that had a partial stand of WW-Spar Bluestem (Bothriochloa ischaemum L.) prior to treatments. Four 0.10 ha plots were grazed and trampled following a 4.3 cm rainfall by twenty four 408 kg yearling heifers (Bos taurus, 97,920 kg ha ˉ¹) to enhance soil-seed contact, compact the soil, and put standing plant material in contact with the soil. Four adjacent 0.10 ha control plots were left untrampled. There were no differences in mean soil bulk density among treatments before trampling. Six Daubenmire frames were placed randomly in each plot to determine canopy, basal, litter, and total cover of the plants, and a 5.08 x 7.62 cm core was collected from the center of each Daubenmire frame to determine bulk density. Forage consumption goal was to remove 50% of total canopy. Actual canopy removal was about 70% as a result of the
trampling/grazing treatment. We then deferred grazing to measure the rate of soil bulk density change over time. Sorghum-Sudangrass (Sorghum bicolor L.) hay was spread in strips on the untrampled and trampled treatment plots 195 days post-trampling to achieve 100% soil surface cover to determine if the amount of litter cover affects seedling establishment and total vegetative canopy cover in trampled and untrampled areas. After precipitation events of > 0.254 cm, canopy, basal, litter, total canopy cover, and soil bulk density measures were collected as previously described after soils had time to drain. Trampled treatments had 20% less vegetative cover (P<0.01) and average soil bulk density was 0.20 g cm ˉ³ higher (P<0.01) than untrampled plots immediately after trampling. Bulk density decreased sporadically with deferral until bulk density was no longer significantly different between treatments on day 240 after trampling. Freeze/thaw cycles, and root growth likely mitigated compaction from trampling. Significant recruitment of Green Sprangletop and/or Kleingrass did not occur. However, WW-spar basal cover increased in grazed and ungrazed treatments, but no differences were noted between treatments for basal cover or seedling recruitment between trampled and untrampled plots. Canopy cover of warm season perennial grasses in grazed treatments surpassed that of the ungrazed treatments during the early growing season of 2016 (P<0.01), but was no different after mid-June. Regression analysis indicated that for every hoof print per 0.1 mˉ², bulk density increased 0.018 g cmˉ³ (P<0.0001), and for every 1% increase in the soil surface that was trampled, bulk density increased 0.0026 g cmˉ³ (P<0.001). Trampling at high stocking densities did not affect grass recruitment or establishment, and appeared to enhance growth rate in the early growing season. Bulk density and hydrologic function can be maintained with high stock density grazing by providing adequate deferment to re-establish sufficient cover and allow natural processes to restore porosity.