Date of Award

12-2017

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant Sciences

Major Professor

Renata Nave Oakes

Committee Members

Gary Bates, Neal Eash, Travis Mulliniks

Abstract

In the Mid-South, Native warm season grasses (NWSG) provide alternative forage to tall fescue (TF) during hot, dry summers. However, NWSG adoption rates are low. This study will evaluate two NWSG pasture types (big bluestem/indiangrass mixture; BBIG; switchgrass; SG) for alternative characteristics that may induce increased adoption of NWSG. The first experiment evaluated the monthly forage characteristics of NWSG (SG and BBIG) and tall fescue (TF) during fall stockpiling (August-December) and winter grazing (January-April) by proteinsupplemented yearling beef heifers. Both BBIG and SG nutritive value deteriorated during the fall, but did not continue during winter grazing. Tall fescue provided adequate forage throughout winter for livestock maintenance (89 g kg-1 [grams per kilogram] CP; 3,766 kg ha-1 [kilograms per hectare]), while dormant SG had the lowest nutritive value and greatest yields (20 g kg-1 CP; 7,489 kg ha-1). The BBIG paddocks had intermediate forage quality compared to TF and SG (31 g kg-1 CP; 4,928 kg ha-1). The second experiment evaluated seasonal dynamics of labile nutrients between NWSG and TF pastures. Labile pools such as hot-water extractable carbon (HWEC) and nitrogen (HWEN), aromatic content of extracts (Abs254), and potassium permanganate oxidizable carbon (POXC) are potential predictors of “soil health” or future carbon sequestration. Samples were analyzed for 18 mo at two depths. Labile soil pools (HWEC, HWEN) had greater seasonal variation relative to more recalcitrant pools (POXC, Abs254). Models indicated greater HWEN (97.4 mg kg-1 [milligrams per kilogram]; 77.5 mg kg-1) and Abs254 (0.66 cm-1 [per centimeter]; 0.58 cm-1) in TF relative to SG. This is consistent with increased microbial activity associated with root traits similar to TF. The third experiment evaluated a gibberellin inhibitor (trinexapac-ethyl) on fall NWSG growth. Fall NWSG growth provides low-quality, high-mass, forage therefore gibberellin inhibitors may provide a beneficial trade-off. The study applied three concentrations (0, 0.3, 0.6, 1.2 kg a. i. ha-1 [active ingredient per hectare]) to SG and BBIG paddocks. Treatment depressed forage mass and improved CP contents. However, minimal digestibility improvements were observed. Therefore, late season application of gibberellin inhibitors to warm season grasses is unlikely to be useful for pasture managers.

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