Denitrification potential in a grassland subsoil: effect of carbon substrates

We examined the potential of a subsoil to denitrify nitrate under optimal anaerobic conditions in a laboratory-based incubation when supplied with a range of C substrates of increasing recalcitrance. Both topsoil and its associated subsoil were supplied with nitrate and either glucose, starch or cellulose. Microbial respiration and the evolution of N₂O and N₂ were measured. The subsoil supported low amounts of microbial activity and responded only to the glucose treatment; with less than one-fifth of the N₂O production measured in the top soil. Overall, our findings demonstrated that the denitrification potential of this particular subsoil is relatively low and that only simple carbohydrates could be utilised readily by the resident microorganisms.     

A preliminary investigation of area‐selective grazing in the Southern Tall Grassveld of Natal

The consequences of area‐selective grazing by cattle under three stocking rates and at two sites in the Southern Tall Grassveld of Natal were investigated. Soil compaction was found to significantly distinguish area‐selected patches from non‐selected veld (P≤0.01), while non‐significant differences in soil nutrient and moisture status were recorded. Area‐selected and non‐selected patches may be distinguished on the basis of species composition. Area‐selected patches are characterized by Aristida congesta subsp. barbicollis, Microchloa caffra and Eragrostis racemosa. Non‐selected veld is characterized by Themeda iriandra, Tristachya leucothrix and Hyparrhenia hirta.     

Nitrogen mineralization and microbial activity in permanent pastures amended with nitrogen fertilizer or dung

 Gross rates of soil processes and microbial activity were measured in two grazed permanent pasture soils which had recently been amended with N fertilizer or dung. ¹⁵N studies of rates of soil organic matter turnover showed gross N mineralization was higher, and gross N immobilization was lower, in a long-term fertilized soil than in a soil which had never received fertilizer N. Net mineralization was also found to be higher in the fertilized soil: a consequence of the difference between the opposing N turnover processes of N mineralization and immobilization. In both soils without amendments the soil microbial biomass contents were similar, but biomass activity (specific respiration) was higher in the fertilized soil. Short-term manipulation of fertilizer N input, i.e. adding N to unfertilized soil, or witholding N from previously fertilized soil, for one growing season, did not affect gross mineralization, immobilization or biomass size and activity. Amendments of dung had little effect on gross mineralization, but there was an increase in immobilization in both soils. Total biomass also increased under dung in the unfertilized soil, but specific respiration was reduced, suggesting changes in the composition of the biomass. Dung had a direct effect on the microbial biomass by temporarily increasing available soil C. Prolonged input of fertilizer N increases soil C indirectly as a result of enhanced plant growth, the effect of which may not become evident within one seasonal cycle. Received: 18 December 1998     

Demonstrating the Use of Risk Programming for Aquacultural Farm Management:

A risk programming method that evaluates production plans based on expected income and risk was demonstrated using representative data from Panama. Results illustrate the usefulness of the program in presenting production choices to decision makers and evaluating those choices under changing conditions.
Less experienced or less financially secure farmers may select farm plans that differ quite importantly from those chosen by more experienced or more financially secure farmers. If farm survival is in doubt, more conservative farm plans will generally be adopted. Without consideration of the potential losses, it will often appear that all farmers should pursue the most intensive farm plan within their farming capabilities. However, many farmers will employ less intensive management strategies if the potential for losses associated with more intensive strategies is perceived to be excessive.
The model indicated that management strategies were sensitive to changes in shrimp price and feed cost, but less sensitive to the costs of post larvae or energy. Also, shrimp farm managers should give serious consideration to not producing in the dry season due to relatively high risk and low expected returns. Farmers should be most concerned with quality and availability of post larvae, not price. If hatcheries can address these concerns, the higher price of hatchery post larvae will be justified.     

Effect of some enzyme inducers, fluids, and methionine-thiosulfate on induced acute aflatoxicosis in goats

Groups of four 6- to 12-month-old male goats were injected intraruminally with a lethal dose (3 mg/kg of body weight) of aflatoxin B1 (AFB1). Drugs were administered parenterally before (pretreatment) or beginning 8 hours after goats were doses with AFB1. These drugs were phenobarbital (PB), phenylbutazone (PBZ), piperonyl butoxide (PRO), benzoflavones, water, and 5% glucose solution (D5W). Most groups given the drugs after AFB1 was administered also were given intraperitoneal injections of methionine-sodium thiosulfate (MET-TS) solution. Clinical signs of toxicosis, serum aspartate aminotransferase activities, serum bilirubin concentrations, duration of illness, mortality, and gross and microscopic pathologic findings taken together indicated that toxicosis was increased with MET-TS + PB therapy, PBZ pretreatment, PBZ therapy, benzoflavone pretreatment, benzoflavone therapy, MET-TS + benzoflavone therapy, and MET-tS + water therapy. Toxicosis was not altered appreciably by MET-TS + PBO therapy. Beneficial effects (less severe toxicosis) were produced by PB pretreatment; these effects were prolonged maintenance of strength, vigor, and appetite and (in 1 goat that recovered) absence of pathologic changes or serum bilirubin increase. Therapy with MET-TS + D5W (but not MET-TS alone) also lengthened maintenance of strength, vigor, and appetite, but did not prevent pathologic changes. The beneficial effect of MET-TS therapy reported in a previous study (AFB2 dosage of 4 mg/kg) was not observed with the 3 mg/kg lethal dose. In conclusion, therapy for acute aflatoxicosis with inducers of hepatic microsomal enzymes is ineffective (PBO) or contraindicated (PB, PBZ, benzoflavones). Therapy with D5W may be a useful adjunct to other therapeutic drugs, but multiple intraperitoneal injections of D5W may decrease survival time because of stress.     

Pharmacokinetics of tobramycin in cats

Tobramycin was administered to cats and its serum concentration vs time data were analyzed by use of a noncompartmental model. In the first experiment, 5 mg of tobramycin/kg of body weight was administered IV, IM, and then SC to 6 cats, 3 weeks apart. After IV administration, the mean +/- SD total body clearance of tobramycin was 2.21 +/- 0.59 ml/min/kg, and the apparent volume of distribution at steady state was 0.19 +/- 0.03 L/kg. The mean residence time was 90.5 +/- 16.2 minutes, with a harmonic mean serum half-life of 68.9 +/- 9.7 minutes. Blood urea nitrogen and serum creatinine concentrations were increased 3 weeks after the IV injection and also 3 weeks after the IM injection, which suggested possible renal damage. Moreover, large area under the curve values developed after IM and SC administrations, resulting in bioavailabilities of 159.5% and 189.9%, respectively, with no change in elimination rate. These results suggested a change in distribution, possibly caused by saturation of renal binding sites by residual tobramycin from the previous injection of 5 mg/kg. In experiment 2, 6 other cats were given 3 mg of tobramycin/kg by the same routes as before, but using a crossover design. Bioavailability after IM and SC administrations was 102.5% and 99.2%, respectively, indicating complete absorption of tobramycin. The BUN concentration increased in 3 cats, and serum creatinine concentration increased in 1 of these 3 cats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prompt arousal from fentanyl-droperidol-pentobarbital anesthesia in dogs: a preliminary study.

Groups of fentanyl-droperidol-pentobarbital-anesthetized dogs (n = 6 dogs/group) were given IV saline solution (control group), graded doses of naloxone (0.01, 0.1, 1.0, 10.0 mg/kg) or fixed doses of 4-aminopyridine (0.5 mg/kg), yohimbine (0.4 mg/kg), or doxapram (5.0 mg/kg) alone or in combination with a fixed dose of naloxone (1.0 mg/kg). The purpose was to determine which drug or drug combination would produce arousal most quickly without producing obvious undesirable side effects. Control group mean arousal time, mean walk time and mean duration of postarousal sedation were 66.1 minutes, 112.4 minutes and 5.6 hours, respectively. Naloxone (1.0 mg/kg) decreased mean arousal time to 10.8 minutes without significantly decreasing mean walk time or mean duration of postarousal sedation. The combination of naloxone + doxapram decreased mean arousal time and mean walk time to 1.0 minute and 57.1 minutes, respectively, without decreasing mean duration of postarousal sedation. In all groups, emergence from anesthesia was smooth. Relapses or undesirable side effects were not observed. Naloxone + doxapram is superior to naloxone alone for arousal of fentanyl-droperidol-pentobarbital-anesthetized dogs.