AUTOLOGISTIC MODEL OF SPATIAL PATTERN OF PHYTOPHTHORA EPIDEMIC IN BELL PEPPER: EFFECTS OF SOIL VARIABLES ON DISEASE PRESENCE

The pathogen Phytophthora capsici causes lesions on the crown, stem, and leaves of bell pepper, and rapidly causes the plant to die. The spatial patterns of disease in an agricultural field contain information about pathogen dispersal mechanisms and can be useful for developing methods of control of disease. Soil water content, soil pathogen population density, and disease incidence data were collected on a 20 x 20 grid in two naturally infested commercial bell pepper fields. In one field the initial pattern of disease closely matched the soil water content pattern and disease developed in areas where the pathogen population levels were high. In the other field no such correspondence was obvious from maps of disease and soil water content . The auto logistic model is a flexible model for predicting presence or absence of disease based on soil water content and soil pathogen population, while taking spatial correlation into account. In the autologistic model the log odds of disease in a particular quadrat are modeled as a linear combination of disease in neighboring quadrats and the soil variables. Neighboring quadrats can be defined as adjacent quadrats within a row, quadrats in adjacent rows, quadrats two rows away, and so forth. The regression coefficients give estimates of the increase in odds of disease if neighbors within a row or in adjacent rows show disease symptoms; thus we obtain information about the degree of spread in different directions. The coefficients for the soil variables give estimates of the increase in odds of disease as soil water content or pathogen population density increase. In this problem, soil water content is also highly correlated over quadrats. This introduces a kind of collinearity between water content and the disease in neighboring quadrats, making estimation and interpretation of the parameters of the auto logistic model more difficult. We discuss fitting and evaluating the autologistic model when the covariates are themselves spatially correlated

Methane Production by Mixed Ruminal Cultures Incubated in Dual-Flow Fermentors

This study evaluated the effects of dilution rate and forage-to-concentrate ratio on gas production by rumen microbes. Continuous cultures were used to monitor methane production at three liquid dilution rates (3.2, 6.3, or 12.5%/h) and three forage-to-concentrate ratios (70:30, 50:50, or 30:70). Filtered ruminal contents were allowed 6 d of adaptation to diets followed by 7 d of data collection. Forage consisted of pelleted alfalfa and the concentrate mix included ground corn, soybean meal, and a mineral and vitamin premix. The experiment was replicated in a split-plot design. Total volatile fatty acid production averaged 58.0 mmol/d and was not affected by treatment. Molar proportion of acetate increased with increasing forage-to-concentrate ratio. Molar proportion of propionate tended to decrease at dilution rate of 12.5%/h and increased with the medium and low forage-to-concentrate ratio. Culture pH tended to be greater at a dilution rate of 12.5%/h. Methane production that was calculated from stoichiometric equations was not affected by treatments. However, methane production based on methane concentration in fermentor headspace resulted in an interaction effect of treatments. Stoichiometric equations underestimated methane output at higher dilution rates and with high forage diets. Total diet fermentability was lowest at dilution rate of 3.2%/h. Increasing dilution rates increased microbial yield; increasing the proportion of concentrate improved microbial efficiency. Dilution rate and forage-to-concentrate ratio altered the partition of substrate by microbes. Methane production based on actual concentrations differed from values estimated using stoichiometry of end-product appearance

Logistic Regression for Southern Pine Beetle Outbreaks with Spatial and Temporal Autocorrelation

Regional outbreaks of southern pine beetle (Dendroctonus frontalis Zimm.) show marked spatial and temporal patterns. While these patterns are of interest in themselves, we focus on statistical methods for estimating the effects of underlying environmental factors in the presence of spatial and temporal autocorrelation. The most comprehensive available information on outbreaks consist of binary data, annual presence or absence of outbreak for individual counties within the southern United States. We demonstrate a method for modeling spatially correlated proportions, such as the proportion of years that a county experiences outbreak, based on annual outbreak presence or absence data for counties in three states (NC, SC, and GA) over 31 years. In the proposed method the proportion of years in outbreak is predicted using a marginal logistic regression model with spatial autocorrelation among counties, with adjustment of variance terms to account for temporal autocorrelation. This type of m..

Eastern Gamagrass Evaluated as Hay or Silage for Lactating Dairy Cows

Twenty lactating Holstein cows were used to determine the nutritional value of eastern gamagrass. Dietary treatments consisted of 1) gamagrass hay and no corn (HNC), 2) gamagrass silage (S) and no corn (SNC), 3) S and low corn (SLC), 4) S and medium corn (SMC), and 5) S and high corn (SHC). Gamagrass fed as hay or silage did not change (P\u3e0.10) milk yield, but corn inclusion tended to increase (P\u3c0.08) milk yield. Yields of milk protein (0.69 vs 0.80 kg/d), lactose (1.23 vs 1.44 kg/d), and solids-not-fat (2.12 vs 2.48 kg/d) tended to be greater for SNC than for HNC (P\u3c0.08). Corn inclusion increased milk yields compared with SNC. Gamagrass fed as silage resulted in a greater feed conversion efficiency than did gamagrass fed as hay (2.16 vs 1.88; P\u3c0.01). Adding corn to S reduced feed efficiency. Conversion of feed N to milk N was greater (P\u3c0.01) for gamagrass fed as silage than for hay. Milk urea N (MUN) concentration was greater (P\u3c0.01) for cows fed HNC than for cows on all other treatments. Feeding S significantly lessened MUN concentration. Including corn at the medium and high levels further reduced MUN concentration (P\u3c0.05). Increased energy from corn at the high level increased milk yield and tended to increase conversion of feed N into milk protein. Gamagrass fed as silage without or with corn improved the N status of the cows, as indicated by lesser MUN concentrations

Fermentation of Eastern Gamagrass (Tripsacum dactyloides [L.] L.) by Mixed Cultures of Ruminal Microorganisms with or without Supplemental Corn

Five dual-flow fermentors (700 mL) were used to determine the effects of eastern gamagrass (Tripsacum dactyloides [L.] L.) diets on microbial metabolism by mixed rumen cultures. Fermentors were incubated with filtered ruminal contents and allowed to adapt for 4 d to diets followed by 3 d of sample collection. Five dietary treatments were tested: 1) gamagrass hay (GH) + no corn (GHNC), 2) gama grass silage (GS) + no corn (GSNC), 3) GS + low corn (GSLC), 4) GS + medium corn (GSMC); and 5) GS + high corn (GSHC). The experiment was conducted as a randomized complete block design with five treatments and three replications. Total VFA concentrations were not affected by diets. Corn addition linearly decreased (P \u3c 0.001) molar proportion of acetate. In contrast, molar proportion of propionate was reduced in GSLC (cubic effect, P \u3c 0.001) but remained similar across other diets. Corn supplementation linearly increased molar proportion of butyrate (P \u3c 0.001). The acetate + butyrate-to-propionate ratio was highest in cultures offered GSLC (cubic effect, P \u3c 0.001) but similar across other diets. Feeding GSNC resulted in a higher ruminal pH compared with GHNC (P \u3c 0.03). Increasing the level of corn supplementation in GS linearly decreased culture pH (P \u3c 0.001). All diets resulted in similar methane production, with the exception of GSMC, which lowered methane output (quadratic effect, P \u3c 0.004). Total substrate fermented to VFA and gas tended to be greater with GHNC than with GSNC (P \u3c 0.06) and linearly increased with the addition of corn (P \u3c 0.004). Neutral detergent fiber digestibility was similar between GH and GS and was not affected by supplemental corn. Microbial N flow increased in cultures offered GSHC (quadratic effect, P \u3c 0.02). Corn supplementation at the medium and high level linearly decreased C 18:0 (P \u3c 0.02) and increased trans-C18:1 (P \u3c 0.004). Including corn at the high level with GS did not have a detrimental effect on fermentation in dual-flow fermentors