Evaluating Digestibility and Toxicity of Native Warm-season Grasses for Equines

Introduced cool-season grasses are dominant in Virginia’s grasslands, but their high digestible energy and non-structural carbohydrate (NSC) levels pose a risk for horses prone to obesity and laminitis. Native warm-season grasses (NWSG) have lower digestible energy and NSC levels that may be more suitable for horses susceptible to laminitis. Although NWSGs have desirable characteristics, they are novel forages for horses. Little is known about NWSG intake or potential toxicity to horses or how grazing by horses may affect NWSG swards. The overall objectives of this research were to 1) assess voluntary intake, toxicological response, and apparent digestibility of NWSG hays fed to horses; and 2) evaluate the characteristics of three NWSG species under equine grazing. For the first objective, a hay feeding trial using indiangrass (IG) (Sorghastrum nutans) and big bluestem (BB) (Andropogon gerardii) was conducted with 9 Thoroughbred geldings in a replicated 3 x 3 Latin square design. Voluntary dry matter intake of IG and BB hays by horses were 1.3% and 1.1% of BW/d, lower than orchardgrass (Dactylis glomerata), an introduced cool-season grass, at 1.7% of BW/d (P = 0.0020). Biomarkers for hepatotoxicity remained within acceptable ranges for all treatments. Apparent dry matter digestibility (DMD) did not differ among hays, ranging from 39 to 43%. Non-structural carbohydrate levels ranged from 4.4 to 5.4%, below maximum recommended concentrations for horses susceptible to laminitis. For the second objective, a grazing trial was conducted comparing IG, BB, and eastern gamagrass (Tripsacum dactyloides) (EG) yields, forage losses, changes in vegetative composition, and effects on equine bodyweight. Nine, 0.1-hectare plots were seeded with one of the three native grass treatments, and each plot was grazed by one Thoroughbred gelding in two grazing bouts, one in July and another in September 2019. Indiangrass had the greatest available forage, at 4340 kg/ha, compared with 3590 kg/ha from BB (P < 0.0001). Eastern gamagrass plots established poorly, and had only 650 kg/ha available forage during the experiment. Grazing reduced standing cover of native grasses in IG and BB treatments by about 30%. Horses lost 0.5 to 1.5 kg BW/d on all treatments. Findings suggest indiangrass and big bluestem merit further consideration as forages for horses susceptible to obesity and pasture-associated laminitis.

Tripsacum dactyloides (eastern gamagrass).

Tripsacum dactyloides is cultivated as a forage crop in areas within and outside its native distribution range. The species establishes slowly and competes poorly with annual grasses and weeds during its first year of establishment, but after that, it competes effectively with most perennial and broadleaf plants. Currently, T. dactyloides is listed as invasive only in Cuba. However, this species is often reported to be growing as a 'weed' in ruderal areas, forest edges, and disturbed and open forests in areas within and outside its native distribution range.

Water-Use Efficiency of Forage Crops in the Southeastern United States

Preparing agricultural producers to cope with volatile weather changes, specifically drought, requires a better understanding of forage water-use efficiency (WUE) potentials. Options to improve farm resiliency to drought may include the use of C4 annual and perennial forages, which have greater production efficiency during drought than commonly used C3 forages. Our objective was to measure WUE through real-time gas exchange measurements of photosynthesis and transpiration in (1) a greenhouse study and (2) under field-grazing conditions. Growth parameters, instantaneous water use efficiency (iWUE), and mass-based WUE (mWUE) data were collected under greenhouse conditions in Study 1 for the following species: crabgrass (Digitaria sanguinalis cv. ‘Red River’), switchgrass (Panicum virgatum cv. ‘Alamo’), big bluestem (Andropogon gerardii cv. ‘OZ-70’), indiangrass (Sorghastum nutans cv. ‘Rumsey’), eastern gamagrass (Tripsacum dactyloides cv. ‘Pete’), bermudagrass (Cynodon dactylon cv. ‘Vaughn’s #1’), sorghum-sudangrass (Sorghum bicolor (L.) × Sorghum sudanese (P.) cv. ‘Greengrazer’), and tall fescue (Schedonorus arundinaceus (Schreb.) Dumort). Study 2 occurred from 2014 to 2016, and evaluated iWUE of crabgrass, switchgrass, bermudagrass, eastern gamagrass, and a big bluestem/indiangrass mix under field conditions. Overall, in situ iWUE of crabgrass, switchgrass, eastern gamagrass, and bermudagrass did not differ, while iWUE of the big bluestem/indiangrass was less than switchgrass and crabgrass, an advantage for these species if the standardized precipitation index drops below zero. Bermudagrass, switchgrass, sorghum-sudangrass, pearl millet, and indiangrass had comparable mWUE values under greenhouse-simulated drought. These results will aid in the development of forage species recommendations for mitigating drought and improving resiliency.

Fodder qualities of heterotic hybrids from intergeneric crosses between maize and eastern gamagrass

Green biomass yield and fodder qualities are the starting point in the choice of forage crops. Maize, as a plant with the C4 pathway of photosynthesis, is highly efficient in transforming the energy of light into chemical bond energies, which ultimately leads to the highest yield per unit area of cultivated land. Its grain and green biomass possess good fodder qualities and are actively used in feed rations for cattle, smaller ruminants, and poultry. Eastern gamagrass (Tripsacum dactyloides L.), a wild relative of maize, is widespread over the plains of North and South Americas. To date, it has received universal recognition among breeders as a forage and fodder crop. More than 10 commercial cultivars have already been developed and cultivated in the United States. Eastern gamagrass is a C4 plant as well, characterized by high yield and high feed value of hay. We decided to combine in our research economically valuable qualities of this plant by developing apomictic intergeneric hybrids. This paper presents the results obtained in the process of assessing fodder qualities of apomictic maize × Tripsacum hybrids, where two maize genomes from the lines participating in hybrid selection for F1 seeds were added to the 36 chromosomes of eastern gamagrass. The maize × Tripsacum hybrid forms, produced during a number of years, persistently demonstrated an apomictic reproduction pattern and heterosis in green biomass productivity. Zootechnical analysis of plant samples showed that the hybrids, in which chromosomes of the lines used in commercial seed production to obtain heterosis in F1 had been added to the 36 chromosomes of eastern gamagrass, exceeded in their biochemical indicators the plants, in which 20 chromosomes from one of the maize lines earlier used in hybridization at our laboratory had been added. A theoretical estimate of green biomass yield calculated per hectare for the maize × Tripsacum hybrids is about 65 tons.

Tolerance of Native and Ornamental Grasses to Over-the-top Applications of Topramezone Herbicide

Research was conducted to determine the tolerance of multiple native and ornamental grass species and one ornamental sedge species to over-the-top applications of the postemergence herbicide topramezone at three locations in the southeastern United States in 2016 and 2017. Fully rooted liners of selected grass species were outplanted into research plots in Apopka, FL; Dallas, TX; and Knoxville, TN in late spring, allowed time to establish (≈1–2 months) and then treated with two applications of topramezone at either 0.05 or 0.10 kg a.i./ha at 6–8 weeks intervals. Results showed that species including Andropogon virginicus (broomsedge), Schizachyrium scoparium ‘The Blues’ (little bluestem), Tripsacum dactyloides (eastern gamagrass), and Tripsacum floridanum (florida gamagrass) exhibited the greatest tolerance to topramezone with <10% injury to no injury being evident after each application of both herbicide rates tested. Chasmanthium latifolium (wild oats), Eragrostis elliottii ‘Wind Dancer’, Muhlenbergia capillaris (pink muhly), and Spartina bakeri (sandcord grass) were significantly injured (50% injury or greater) at both herbicide rates. Average injury observed on Panicum virgatum ‘Shenandoah’ (red switchgrass) (ranging from 39% to 100% injury) and Sorghastrum nutans (indian grass) (ranging from 0% to 40% injury) was higher in Florida than in Tennessee (injury ranging from 23% to 43% on red switchgrass and 0% to 10% on indian grass). Similarly, Pennisetum alopecuroides (dwarf fountain grass) showed higher tolerance in Texas (ranging from 0% to 34% injury) compared with those observed in Tennessee (ranging from 0% to 53% injury). Topramezone injury to Carex appalachica (appalachian sedge) was ≤18% following two applications at both rates tested. Although no injury was observed in appalachian sedge following a single application up to 0.1 kg a.i. in Florida, plants succumbed to heat stress and accurate ratings could not be taken following the second application. Because of variability observed, tolerance of red switchgrass, indian grass, dwarf fountain grass, and appalachian sedge to applications of topramezone deserves further investigation. There is potential for future use of topramezone for control of certain grass and broadleaf weeds growing in and around certain ornamental grass species. However, as there was significant variability in tolerance based on species and differences in cultivars, testing a small group of plants before large-scale application would be recommended.