Predominant populations of indigenous soybean-nodulating Bradyrhizobium japonicum strains obtained from organic farming systems in Minnesota.

AIMS: Bradyrhizobium from organic fields in Minnesota were isolated and genotyped to assess diversity of soybean-bradyrhizobia in organic farming systems that can be used to improve soybean productivity. METHODS AND RESULTS: Soil samples were collected from 25 organic fields in Minnesota during May to July 2012. Soybean (cv. Lambert) was used as a host to trap indigenous bradyrhizobia in each sample. Genetic diversity of Bradyrhizobium strains (n=733) was determined using the horizontal, fluorophore-enhanced, repetitive extragenic palindromic-PCR (HFERP) DNA fingerprinting technique and the soybean-bradyrhizobia were classified into 79 different genotypes. Of these, 15 dominant genotypes were found and were highly similar (>92% fingerprint similarity) to serotypes USDA 127 (40.4%), USDA 4 (31.8%) and USDA 123 (15.5%), which were the three main populations of soybean-bradyrhizobia in organic fields. CONCLUSIONS: Bradyrhizobium japonicum serogroup USDA 4 strains were found to make up a previously unrecognized, predominant rhizobial population in the organic farming soils examined. The relative abundance of strain USDA 4 was negatively correlated with that of USDA 127 and this relationship may be influenced by the levels of NO3 -N and other soil edaphic factors. SIGNIFICANCE AND IMPACT OF THE STUDY: The local community of bradyrhizobia can be affected by applying inoculant bacteria to organic fields. Based on these results, soybean production in organic farms may be improved by displacing strains similar to USDA 4 with those better at nitrogen fixation and competitive ability than indigenous strains.

Effect of hay steaming on forage nutritive values and dry matter intake by horses.

Management strategies for horses with respiratory disease include soaking hay before feeding. Hay steaming is an alternative to this practice; however, little is known about its impact on forage nutritive values or intake. The objective was to determine the effect of steaming on forage nutritive value and intake by horses. Two alfalfa (Medicago sativa L.)-orchardgrass (Dactylis glomerata L.) mixed hays were evaluated: a low moldy (NM) and moderately moldy (MM) hay. Six mature horses were used in a 10 d crossover design. Three horses were assigned to each hay type and treatments were switched on d 6. Each day, one bale of each hay was sampled (pre- and poststeaming) and steamed for 90 min using a commercial hay steamer. Two flakes of steamed or unsteamed NM or MM hay were weighed and offered simultaneously to each horse in individual hay nets. Horses were allowed access to hay for 2 h, orts were collected, and 2 h DMI was calculated. Six additional bales of NM and MM were used to evaluate the effect of steaming on total suspended particulate (TSP). Flakes of unsteamed or steamed hay were agitated in an electric cement mixer, and TSP were recorded every min for 30 min using a tapered element oscillating microbalance sampler. Paired t tests and PROC MIXED of SAS (SAS Inst. Inc., Cary, NC) were used to compare steamed and unsteamed hay nutritive values, mold concentration, TSP, and 2 h DMI. Steaming increased hay moisture and therefore reduced DM to 77 and 81% for NM and MM, respectively (P < 0.001). In NM and MM hay, steaming reduced P content by 16 and 17%, respectively (P ≤ 0.007). Steaming reduced water-soluble carbohydrates (WSC) and ethanol-soluble carbohydrates (ESC) by 13% (P = 0.001) and 27% (P = 0.003), respectively, for MM but had no effect on NM (P > 0.05). Steaming reduced mold concentrations in both hays by ≥ 91% (P < 0.001). Total suspended particulate of MM hay was reduced by 55% (P = 0.043), but TSP in NM hay was not affected by steaming (P = 0.445). Dry matter intake of NM was increased by steaming; horses ingested 0.64 kg of unsteamed and 2.02 kg of steamed hay (P < 0.001). Dry matter intake of MM was not affected by steaming (P > 0.05). For NM hay, steaming decreased P and mold concentrations and increased DMI of the hay but had no effect on TSP. In MM hay, steaming reduced P, WSC, ESC, mold concentrations, and TSP but did not affect DMI. Steaming represents a strategy for reducing TSP and mold concentrations and increasing DMI in some hays but can result in leaching of essential nutrients.

Plant-parasitic Nematode Communities and Their Associations with Soil Factors in Organically Farmed Fields in Minnesota.

A survey was conducted to determine the assemblage and abundance of plant-parasitic nematodes and their associations with soil factors in organically farmed fields in Minnesota. A total of 31 soil samples were collected from southeast (SE), 26 samples from southwest (SW), 28 from west-central (WC), and 23 from northwest (NW) Minnesota. The assemblage and abundance of plant-parasitic nematodes varied among the four regions. The soybean cyst nematode, Heterodera glycines, the most destructive pathogen of soybean, was detected in 45.2, 88.5, 10.7, and 0% of organically farmed fields with relative prominence (RP) values of 10.3, 26.5, 0.6, and 0 in the SE, SW, WC, and NW regions, respectively. Across the four regions, other common genera of plant-parasitic nematodes were Helicotylenchus (42.6, RP value, same below), Pratylenchus (26.9), Tylenchorhynchus and related genera (9.4), Xiphinema (5.6), and Paratylenchus (5.3). Aphelenchoides, Meloidogyne, Hoplolaimus, Mesocriconema, and Trichodorus were also detected at low frequencies and/or low population densities. The similarity index of plant-parasitic nematodes between two regions ranged from 0.44 to 0.71 and the similarity increased with decreasing distance between regions. The densities of most plant-parasitic nematodes did not correlate with measured soil factors (organic matter, pH, texture). However, the densities of Pratylenchus correlated negatively with % sand, and Xiphinema was correlated negatively with soil pH.

Genetic diversity of rhizobia nodulating Trifolium ambiguum in North America.

Kura clover (Trifolium ambiguum M.B.) is a persistent rhizomatous forage legume, whose use in the U.S.A. is limited by establishment difficulties in part attributable to nodulation problems. In this study, soil was collected from established stands of Kura clover growing in 9 diverse North American environments. Rhizobia were plant-trapped using Kura clover cv. Endura as host, then rhizobia from nodules fingerprinted using BOX-PCR. The diversity of isolates from North America was then contrasted to that of rhizobia from a single Caucasian environment (Russia), the center of origin for this species. Populations were characterized using clustering methods, and genetic diversity estimated using the Shannon-Weaver diversity index. The genetic diversity of the North American populations was extremely limited, all isolates being closely related to two of the strains found in a locally available commercial inoculant. In contrast, Russian isolates formed a distinct cluster with significant internal genetic diversity. Genetic diversity indices for the North American and Russian populations were 3.5 and 10.76, respectively. The implication of this and other studies is that Kura clover is highly specific in Rhizobium requirement. If the performance of this legume in the U.S.A. is to be improved, either by modifying current establishment practices or plant breeding, it is essential that these studies be paralleled by more collections and evaluation of rhizobia from its center of origin, given the extremely limited diversity of rhizobia found in North America.