Rotational horse grazing and dry weather maximize infiltration into soil macropores

Abstract

Growing livestock populations have intensified the potential for detrimental effects of grazing on grassland soils globally. Grazing management techniques can mitigate these effects but they are livestock-specific and studies on horse grazing are rare. The objective of this work was to compare the effects of rotational grazing (i.e., livestock graze sub-sections of a pasture in sequence) with the continuous approach to grazing on (1) rates of water infiltration (i) at slightly negative pressure potentials (h, −15, −10, −5, −3.5, −1 hPa), (2) saturated hydraulic conductivity (Ksat), and (3) bulk density (BD). At a site in New Jersey, USA, one pair of pastures was managed with rotational grazing while another pair experienced continuous grazing. Twelve Standardbred mares were grazed for two years at a stocking rate of 0.52 horses ha−1. Over that period, i (n = 79, each at 5 h values) and BD (n = 154, from depth ranges 0–10 and 30–40 cm) were measured multiple times and Ksat values were derived from infiltration measurements. Also, the standardized precipitation-evapotranspiration index (SPEI) was calculated with meteorological data from a nearby weather station. We found that: (1) i values inclusive of the largest pores tested (h = −1 hPa) were greater in rotationally- vs. continuously-grazed fields (geometric means ± GSE were 80.2 ± 1.15 and 42.3 ± 1.13 cm d−1, respectively), (2) Ksat values were consistent with those of infiltration at h = −1 hPa but were too variable to isolate the effect of pasture management, and (3) near-surface BD was similar under rotational and continuous grazing (arithmetic means ± SE were 1.32 ± 0.02 and 1.37 ± 0.02 Mg m−3, respectively). Further, during periods of water surplus (i.e., when SPEI was positive), infiltration was strongly reduced in the smaller soil pores (h = −10 and −15 hPa) of the rotationally-grazed fields. This reduction was likely the result of pore colonization and blockage by pasture grass roots growing in these fields. This study suggests that rotational management allows for a larger macropore system than continuous grazing, which could lead to a reduction of water deficits and contribute to the sustainability of grazed ecosystems, with positive effects accruing through time.