This Is AuburnElectronic Theses and Dissertations

Effects and Sustainability of Clover Inclusion within Warm-Season Turf Swards




McCurdy, James

Type of Degree



Agronomy and Soils


Efforts to decrease supplemental nitrogen (N) applications to turfgrass justify alternative fertility strategies such as legume inclusion. Legumes such as clovers (Trifolium spp.) are present within many turfgrass scenarios. Legume persistence is partly due to an ability to biologically fix atmospheric N, which is incorporated into the plant as proteins and other compounds. N is subsequently shared with associated grasses through the decomposition of legume -roots and -foliage. For this reason, turf health is often improved rather than diminished. There are very few guidelines for white clover (T. repens) establishment and maintenance within warm-season turfgrasses. In fact, much of what we know is from clover inclusion within forage and pasture scenarios. Research was conducted to answer serious knowledge gaps preventing the implementation of white clover inclusion within warm season turf swards. Four studies were conducted to evaluate seeded white clover establishment within dormant bermudagrass (Cynodon spp.) turf as affected by 1) pre-seeding mechanical surface disruption, 2) establishment timing, 3) seeding rate, and 4) companion grass species. White clover establishment was improved by scalping prior to October seeding, but these effects were not further enhanced by the addition of verticutting or hollow tine aerification. Un-scalped turfgrass yielded nearly 50% lower white clover densities than those scalped prior to seeding, possibly due to decreased seed to soil contact and increased bermudagrass competition. January and February establishment dates generally yielded the lowest spring clover densities, while October timing yielded superior establishment. Clover densities resulting from six seeding rates (0 to 6.0 g live seed m-2) were fit to the linear model y = y0 + axb, where y equals trifoliate leaves m-2 and x is equal to initial seeding rate. An important feature of this model was that it accurately represented the diminishing response of increasing seeding rate. Clover establishment was negatively correlated with companion grass densities, with the largest densities occurring when planted with tall fescue and the smallest when planted with annual ryegrass. Weed control within turf-clover swards is often hampered by the lack of effective herbicides that are safe on clovers. Furthermore, differential tolerance of legume species to common row-crop and pasture herbicides has previously been reported. Field and greenhouse studies conducted in Auburn, AL indicate varying herbicide tolerances of Trifolium species to common turf herbicides. In field experiments, imazaquin controlled hop clover 91% but controlled white clover only 50%. Imazaquin reduced hop clover height 87%, which far exceeded height reductions measured among other clovers (< 46%). Although visual estimates of 2,4-DB control (35%) did not differ due to species, differential height reductions were significant. 2,4-DB failed to reduce the height of crimson and ball clovers, while white clover was almost 50% shorter than the non-treated. In contrast to field experiments, 2,4-DB control during greenhouse experiments was less than all other clovers (3% versus > 40% for other clovers). These differential herbicide tolerances are novel but must be refined in order to be adapted in real-world scenarios. On a practical level, our results demonstrate potential herbicide options for maintaining biodiverse turf-legume swards. Candidate herbicides include bentazon, MCPA, 2,4-DB, imazaquin, and imazethapyr. The relative tolerance of clover species to these candidate herbicides is further evidence of their utility within certain mixed turf scenarios. Little is known of the N contribution and carbon (C) sequestration from decaying clover foliage. An in situ decomposition study was conducted in Auburn, AL to quantify C and N -release from the decomposition of white clover (T. repens L.) foliage within a bermudagrass lawn. Fresh white clover was applied during March, June, and December and was retrieved periodically after application. Four parameter double exponential decay models were used to describe clover mass as well as N and C -loss. These models reveal important features of white clover decomposition; mainly that white clover is composed of a quickly decaying labile fraction. White clover litter applied at 0.5 kg fresh weight (FW) m-2 potentially contributed from 2.9 to 4.2 g N m-2, with more than half available for mineralization between 10 and 73 days after application, depending upon time of year. Given that clover populations are regenerative, litter deposited during mowing events may be considered a viable N source to sustain healthy turf. Knowledge of the decomposition of clover within turf swards will enable turfgrass- researchers and professionals to more accurately predict nutrient contribution to associated grasses and help optimize supplemental fertilizer recommendations. A 3-year study evaluated the effects of white clover inclusion within a hybrid bermudagrass lawn. Supplemental N (0, 0.5, 1, 2, 4, and 8 g N m-2) was applied monthly, April to August, in order to evaluate the effects of supplemental N upon biomass composition, N fixation, N transfer, and soil carbon. Mixed grass plus clover swards yielded higher clipping biomass than grass-alone swards, which was evidence of enhanced bermudagrass growth due to biological N fixation. Likewise, grass biomass of mixed swards was increased relative to that of grass-alone swards at supplemental N rates ≤ 10 g N m-2 year-1 but was decreased at higher supplemental N rates. N fixation was estimated to be 6.6 g m-2 year-1 during the 3-year study, with an apparent increase in fixation as years progressed. Results indicate that N fixation was suppressed at the lower and upper extremes of supplemental N rates. N transfer to the associated bermudagrass sward was estimated to be 24% during the latter two years of the study. Soil carbon levels were similar among treatments.