Arthropod and soil microbial communities in forage-livestock systems
Bhandari, Krishna B.
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The rapid decline in water supply for irrigation in the Texas High Plains is encouraging some growers to convert their irrigated cropland to dryland forages for livestock production. The vitality of the Ogallala Aquifer, a source for water supply for irrigation in the semi-arid Texas High Plains, is severely threatened owing to withdrawal rates that far exceed recharge rates. ‘WW-B.Dahl’ Old World bluestem [OWB, Bothriochloa bladhii (Retz) S.T. Blake] is a drought-tolerant grass in dryland and limited-irrigation conditions. Adoption of low water requiring perennial forages such as WW-B.Dahl OWB in forage-livestock production systems could potentially extend the economic life of irrigated row crops. This grass is reported to deter some insects such as red imported fire ant (RIFA, Solenopsis invicta Buren); but its effect on broader arthropod taxa and on soil microbial communities is unknown. Fewer horn flies (Haematobia irritans L.) have been casually observed on cattle (Bos taurus L.) grazing this grass compared to other grasses; however, observations were non-tested testimonials, and definitive data are unavailable. Visual ratings (1-5 scale) were compared for horn fly densities on cattle grazing two forage systems: grass-only (predominantly OWB) vs. grass-legume (OWB and alfalfa). Arthropod and soil microbial communities occurring in OWB, OWB-alfalfa (Medicago sativa L.), alfalfa, native mixed grass, and teff (Eragrostis tef (Zucc.) Trotter) pastures were characterized over 3 yr. I compared soil chemical and microbial properties in a Pullman clay-loam soil among OWB, OWB-alfalfa (Medicago sativa L.), alfalfa and native mixed-grass pastures at 0‒5 cm and 10‒15 cm depths in June and December of 2016. Qualitative analysis of chemical composition of essential oils of OWB was performed by gas chromatography (GC) and GC-mass spectrometry (MS) methods. I make the following major conclusions: • There was not a clear deterrence effect of WW-B.Dahl OWB on horn flies on cattle when compared to cattle grazing a grass-legume pasture system, which entailed spending less time on OWB. There is still a need for insecticidal treatment to control horn flies grazing OWB. • WW-B.Dahl OWB provides an unfavorable habitat for colonization by RIFA and harvester ants. Pastures containing OWB are expected to deter cattle health threats and grass stand decimation owing to absence of RIFA and harvester ants, respectively. • OWB appears to exhibit minor inhibitory effects on bees and other insect pollinators when compared to native mixed grasses and alfalfa pastures; however, pollinator populations are likely not threatened. • OWB did not clearly suppress the canopy-dwelling arthropods, while still hosting some arthropods of ecological significance; it appears that there is low risk of reducing overall arthropod diversity by wide adoption of OWB in the Texas High Plains. • The consistently greatest number of soil microbial communities and favorable soil biological properties in the OWB-alfalfa pastures compared to other forage types indicated that OWB had no suppressing effect on soil microorganisms, thus OWB-alfalfa is a promising pasture combination. • The consistently highest concentration of a single essential oil compound, Spiro [4.5] dec-6-en-8-one, 1,7-dimethyl-4-(1-methylethyl)-, out of 143 detected, prompts the need for further study of this compound to test its biological activity against RIFA and harvester ants. Much lower abundance was naphthalene, a known insect deterrent; however, its role as an insect deterrent in OWB is unknown without further study. In summary, results of these studies add to a growing body of work supporting the value of WW-B.Dahl OWB as a well-adapted perennial forage grass in the Southern Great Plains thanks to ant deterrence while maintaining overall insect diversity and hosting desirable soil microorganisms.