The consolidation of smaller farms into larger farms along with a shift to forestry-based applications and pine production has led to a decline in the number of farms in the southeastern US (Marois et al., 2002). This situation provides an opportunity and necessitates the development of robust cropping systems through environmentally friendly agricultural practices that are profitable and sustainable. These cropping systems must be profitable for small farms by increasing the yields and by maintaining or decreasing production costs. Improved financial benefits from new cropping systems will enable small farms to be competitive (Wright et al., 2007). Integrated crop-livestock production systems provide opportunities for greater diversification and yr round utilization of resources thereby mitigating risk, and providing environmental benefits such as nutrient cycling and energy efficiency impacting overall farm profitability (Clark, 2004; Entz et al., 2005; Hanson and Franzluebbers, 2008; Tanaka et al., 2005). Several studies integrating crop and livestock have indicated an increase in total nitrogen, soil organic C (carbon), soil microbial biomass C, water-soluble aggregate stability, and a decrease in penetration resistance (Acosta-Martinez et al., 2004; Maughan et al., 2009).

Bahiagrass (Paspalum notatum Flueggé) has been utilized in reduced tillage systems (sod-based rotations) in the Southeast Coastal Plains (Wright et al., 2005). Deep-rooted bahiagrass leaves soil channels after the decay of bahiagrass roots for the subsequent summer crop to develop roots in these channels (Elkins et al., 1977). Soil quality is also improved by presence of yr-round soil cover, greater water infiltration, reduction in NO₃^- leaching, and soil erosion (Reeves, 1997). Additional benefits of including bahiagrass, a non-host for most of the row crop pests, in the cropping system includes reduction of weeds, nematodes, and diseases and reduced pesticide applications (Katsvairo et al., 2007a; Liebman and Dyck, 1993; Marois, 2003; Sumner, 1982; Tsigbey et al., 2009). Establishment of bahiagrass in the cropping systems has several positive impacts on the soil physical and chemical properties and may often lead to improved crop yields (Brenneman et al., 2003; Katsvairo et al., 2006; Toth, 1998). Also, an increase in earthworm populations (Katsvairo et al., 2007b), organic matter and improved soil structure (Swift and Noordwijk, 2004) has been observed in this system.

Growers are able to benefit from additional income during the yr bahiagrass is present in the rotation through utilization of livestock to graze these areas. Land associated with crop production is rented at a higher price than pasture land. As 80% of the crop land in the U.S. is rented, adoption of sod-based practices strictly for grazing was hindered. However, a row crop production system including a sod-based rotation with grazing as a component may provide benefits of bahiagrass along with the economic benefits.

The University of Florida, North Florida Research and Education Center (NFREC) has been evaluating benefits of grazing by cattle (Bos taurus L) in the sod-based rotations to evaluate the economic sustainability of these systems. Peanut grown in the sod-based rotation system with cattle grazing has shown higher yields with reduced costs (Marois et al., 2002). Grazing in a sod-based rotation system has also shown increased root development (Loison et al., 2012), enhanced microbial and enzyme activity and nutrient recycling (Anguelov et al., 2011; Vendramini et al., 2007; Wright et al., 2010; George et al., 2013).

However, grazing of cattle can adversely impact the soil profile by increasing soil compaction which may negatively impact root development of the subsequent summer crop. Wright et al. (2010) observed an increase in soil bulk density of the top 10 to 15 cm soil profile due to grazing in comparison to the non-grazed plots. An increase in the bulk density of top 10 to 15 cm soil profile due to cattle grazing was also reported by Bell et al. (2011). However, negative impacts of animal traffic on soil physical characteristics can be mitigated by increase in organic matter content due to perennial grass use and conservation tillage (Franzluebbers and Stuedemann, 2008). Franzluebbers (2007) in his review of the status of integrated crop-livestock systems in the southeastern U.S concluded that there were great opportunities for crop-livestock integration in this region and that research emphasizing the consequences of these systems was limited. Agronomic and economic benefits for the growers can be expected with the adaptation of crop-livestock systems despite the low adoption and management challenges (Franzluebbers and Stuedemann, 2008). We integrated cattle grazing with sod-based rotation systems (2 yr of bahiagrass followed by peanut then cotton) to understand the short- and long-term effects of grazing on soil properties and crop yield.

The objective of this study was to analyze the impact of prior grazing on subsequent peanut root development in a sod-based crop rotation. Peanut crop grown subsequent to bahiagrass may develop a better root system but the negative impacts of soil compaction caused by livestock traffic may counteract some of these benefits. Therefore, it became imperative to quantify the impacts of cattle grazing on the root architecture of peanut.

Significant differences in root traits from yr to yr were observed but no yr x grazing treatment interaction was observed for root length, root diameter, and root surface area. However, significant yr x grazing treatment interaction was observed for root volume. Significant treatment effects (P ≤ 0.05) were observed on root length, volume and surface area in 2007 and all root parameters in 2008 (P ≤ 0.001). However, in 2009, the trends were similar but the differences in root parameters were non-significant.

The mini-rhizotron method allowed in situ measurements of these root parameters at several depths over time as compared to other methods which were disruptive, labor intensive, and destructive in nature (Ingram and Leers, 2001). Root length across all the depths and dates was greater in the grazed plots by 71% and 180% in comparison to the non-grazed plots in yr 2007 and 2008, respectively (Table 2). Similarly, root volume increased by 82% and 300%, root surface area increased by 72% and 229%, and root diameter increased by 16% and 78% in the grazed plots in comparison to the non-grazed plots in 2007 and 2008, respectively (Table 2). These root parameters were also numerically higher in the grazed plots in 2009 but the difference with the non-grazed plots were not significant (Table 2). These results confirm that grazing impacted the subsequent peanut crop's root characteristics in the two of three study yr.

Grazing did not impact the root characteristics in the 0-15 cm and 15-30 cm depth (Table 3). However, significant differences in root characteristics were observed in grazed and non-grazed plots at lower depths. Root length and root diameter were significantly higher in the grazed plots when compared to the non-grazed plots at depth of 45-60 cm and 60-75 cm (Table 3). Root length increased by 204% and 179% and root diameter increased by 117% and 95% in the grazed plots when compared to the non-grazed plots at depths of 45-60 cm and 60-75 cm, respectively (Table 3). Root volume and root surface area values were higher in the grazed plots as compared to the non-grazed plots at the soil depths of 30-45 cm, 45-60 cm, and 60-75 cm (Table 3). In the grazed plots, root length, root volume, and root surface area were greater at the soil depths of 30-45 cm and 45-60 cm as compared to the shallower depths. However, in the non-grazed plots such differences in the root parameters at different depths were not observed (Table 3). As previously noted in cotton by Loison et al. (2012) grazing did not negatively impact root traits in the top soil due to soil compaction. In a related study on peanut in 2009, a slight increase in bulk density was observed at 0-5 cm in the grazed plots as compared to the non-grazed plots. The bulk density was 1.21, 1.44, 1.58 g/cm³ in non-grazed and 1.26, 1.33, and 1.48 g/cm³ in grazed plots at 0-5, 15-20, amd 35-40 cm , respectively. Root growth in the grazed treatments may have been encouraged by more efficient nutrient cycling due to cattle grazing. Wright et al. (2010) demonstrated a 3 to 5 fold increase in the nitrate content in the top 20 cm of the soil in the grazed plots.

The development of the peanut root system in the grazed and non-grazed plots was evaluated by assessing root characteristics at different sampling dates. Root characteristics averaged over different depths and yr showed significant effects of prior grazing on subsequent peanut crop roots at each sampling date. The root length increased linearly and was significantly different from June to August in grazed plots, and from June to July in non-grazed plots (Table 4). However, significant differences in root length were observed from June to August and June to July in grazed and non-grazed plots, respectively (Table 4). A significant increase in root diameter was observed in the grazed plots from June to July but not later sampling dates. No significant increase in root volume and root surface area in the grazed plots were observed at different sampling dates. A rapid root development in the grazed treatments seems to be responsible for the non-significant increase of the root characteristics over time. Root length and root volume in the grazed treatment were significantly higher than the non-grazed treatment at sampling dates of June, August, and September (Table 4). Root diameter and root surface area were significantly higher in grazed plots in comparison to non-grazed plots from July to September and from June to September, respectively (Table 4).

The average peanut yield over the 3 yr period of this study was 5316 kg/ha and 5592 kg/ha in the grazed and non-grazed treatments, respectively, indicating that grazing does not have an adverse effect on crop yield. Increased root growth may have resulted in increased vegetative growth without influencing harvest index, although this was not investigated. Junjittakarn et al. (2014) indicated that root traits are not significantly correlated to harvest index or peanut pod dry weight. Moreover, a water limiting situation would perhaps have increased the significance of enhanced root development on crop yield.

We have also reported the effect of grazing bahiagrass and winter annuals on soil physical and chemical properties such as increased volumetric water content, reduced or comparable bulk density and greater N, P, and K cycling in the top 30 cm depth in the grazed treatments as compared to non-grazed treatments (George et al. 2013). Organic matter in grazed treatments was also greater than non-grazed plots (George et al., 2013). Efficient nutrient recycling as well as decomposing soil organic matter may be more readily available as sources of nutrients in a grazed treatment relative to non-grazed treatments, thereby driving greater root growth and development. Watt et al. (2005), McNeill et al. (1999), Khan et al. (2001), and Nuruzzaman et al. (2005) have all suggested the theory of root-root interaction where actively growing crop roots gravitate toward dead roots from the previous crop seeking nitrogen and phosphorus. A greater root residue in peanuts from grazed plots will therefore contribute to nutrient availability to subsequent crops, usually cotton, in the cropping systems prevalent in the region.