Kinetics of hydration and effect of liquid uptake on specific gravity of small hay and silage particles.

Kinetics of hydration of ground hay and silage particles (2-mm screen), determined by a pycnometric technique, was best described by a two- and one-pool exponential model, respectively. Fractional rates of hydration of the large pool, detected in hay particles only, and of the small pool present in both hay and silage particles averaged .135 and .021 min-1, respectively. When hydration was complete, liquid associated with particles averaged 1.16, 1.90, and .83 g/g of insoluble DM for bromegrass hay, alfalfa hay, and alfalfa silage, respectively. Functional specific gravity, which accounts for the effect of associated gas volume, averaged 1.54, 1.46, and 1.54, but unit specific gravity, calculated to include the effect of gases and liquid of hydration, averaged 1.22, 1.14, and 1.26 for bromegrass hay, alfalfa hay, and alfalfa silage, respectively. Preservation of forage as silage not only lowered gas volume, but also reduced water-holding capacity, both of which contribute to greater unit specific gravity and faster rate of escape from the rumen. In addition, estimates of unit specific gravity of approximately 1.2 indicate that even in the absence of associated gas, hydrated forage particles would tend to escape the rumen at a slower rate than that achieved by more dense particles.     

Challenges in measuring insoluble dietary fiber

Objectives of this review are to define criteria for evaluating insoluble dietary fiber (IDF) methods, discuss their relevance in meeting the nutritional needs of ruminants and herbivores, describe problems with empirical IDF methods, and assess their relative merits. The challenge for the researcher, nutritionist, and analyst is to select fiber methods that are relevant and reproducible. Without relevance, there is no reason to measure IDF, and without reproducibility, there is no value in doing so. Insoluble dietary fiber is a complex matrix of chemical components, and there are no primary standards that can be used to establish the validity of methods. Thus, the definition of fiber is crucial in determining method relevance. For ruminants and nonruminant herbivores, the appropriate physiological definition for selecting IDF methods may be as follows: the organic fraction of the diet that is indigestible or slowly digesting and occupies space in the gastrointestinal tract. Crude fiber does not match this definition, and its use should be abandoned. Acid detergent fiber does not measure all IDF but is useful when included with other dietary fiber methods to describe some feeds. Several current methods, including amylase-treated neutral detergent fiber (aNDF) and enzymatic-gravimetric methods, are relevant for measuring IDF. In a collaborative study, aNDF obtained a standard deviation of reproducibility (SD(R)) of 1.3%. Enzymatic-gravimetric methods of measuring IDF have been evaluated using too few feed materials to make statistically valid conclusions, but the SD(R)f IDF, for the few feeds evaluated, were similar to aNDF (0.9 to 2.4%). The enzymatic-chemical method of measuring IDF as the sum of insoluble nonstarch polysaccharides and lignin agrees with NDF, but the SDR of neutral sugar analysis using acid hydrolysis and chromatography is greater (3.2%) than other dietary fiber methods. Empirical methods--such as those used to measure IDF, although based on nutritional concepts--actually define the fraction being measured and must be followed exactly, without modification. The selection of a suitable method for IDF depends on the purpose of analysis. Analysis of sugars in insoluble polysaccharides provides more information but is less reproducible and more expensive to obtain. For routine nutritive evaluation of feeds and formulation of rations, aNDF seems to be a reasonable choice for measuring IDF based on relevance and reproducibility.     

Ganglioradiculitis (sensory neuronopathy) in a dog: clinical,morphologic, and immunohistochemical findings

A 9-year-old Labrador Retriever was diagnosed with ganglioradiculitis (sensory neuronopathy). This idiopathic disease of mature dogs is characterized by a profound loss of sensory nerve function due to mononuclear inflammatory infiltration of peripheral ganglia and spinal nerve roots, with destruction of sensory neurons. Immunohistochemistry demonstrates that the infiltrating cells are primarily T lymphocytes and that immunoglobulins are not present on the cell membranes of affected neurons. The pathogenesis of ganglioradiculitis remains unclear, but the evidence points to a cell-mediated immune mechanism.     

Stimulated activity of the soil nitrifying community accelerates community adaptation to Zn stress

Field data have shown that soil nitrifying communities gradually adapt to zinc (Zn) after a single contamination event with reported adaptation times exceeding 1 year. It was hypothesized that this relatively slow adaptation relates to the restricted microbial diversity and low growth rate of the soil nitrifying community. This hypothesis was tested experimentally by recording adaptation rates under varying nitrification activities (assumed to affect growth rates) and by monitoring shifts in community composition. Soils were spiked at various Zn concentrations (0-4000 mg Zn kg⁻¹) and two NH₄⁺-N doses (N1, N2) were applied to stimulate growth. A control series receiving no extra NH₄⁺-N was also included. Soils were incubated in pots under field conditions with free drainage. The pore water Zn concentration at which nitrification was halved (EC50, mg Zn l⁻¹) did not change significantly during 12 months in the control series (without NH₄⁺-N applications), although nitrification recovered after 12 months at the highest Zn dose only. The EC50 after 12 months incubation increased by more than a factor 10 with increasing NH₄⁺-N dose (p < 0.05) illustrating that increased activity accelerates adaptation to Zn. Zinc tolerance tests confirmed the role of Zn exposure, time and NH₄⁺-N dose on adaptation. Zinc tolerance development was ascribed to the AOB community since the AOB/AOA ratio (AOB = ammonia oxidizing bacteria; AOA = ammonia oxidizing archaea) increased from 0.4 in the control to 1.4 in the most tolerant community. Moreover, the AOB amoA DGGE profile changed during Zn adaptation whereas the AOA amoA DGGE profile remained unaffected. These data confirm the slow but pronounced adaptation of nitrifiers to Zn contamination. We showed that adaptation to Zn was accelerated at higher activity and was associated with a shift in soil AOB community that gradually dominated the nitrifying community.     

Quantitative PCR assays to enumerate Rhizobium leguminosarum strains in soil also target non viable cells and overestimate those detected by the plant infection method

The plant infection method is commonly used to estimate the Most Probable Number (MPN) of soil rhizobia. Here, a qPCR method was set-up and validated with newly developed ANU (strain specific) and RHIZ (more general) primers to quantify the specific Rhizobium leguminosarum bv. trifolii ANU843 strain or general R. leguminosarum strains. Detection limits of qPCR protocols in soil were 1.2 × 10⁴ (ANU) and 4.2 × 10³ (RHIZ) cells per g soil. The qPCR assay appears robust and accurate in freshly inoculated soils but overestimated MPN for indigenous soil rhizobia. An incubation experiment showed that qPCR detected added DNA or non viable cells in soils up to 5 months after addition and incubation at 20 °C in moist conditions.     

A survey of symbiotic nitrogen fixation by white clover grown on metal contaminated soils

There is conflicting evidence about toxic effects of heavy metals in soil on symbiotic nitrogen fixation. This study was set-up to assess the general occurrence of such effects. Soils with metal concentration gradients were sampled from six established field trials, where sewage sludge or metal salts have been applied, or from a transect in a sludge treated soil. Additional contaminated soils were sampled near metal smelters, in floodplains, in sludge amended arable land and in a metalliferous area. Symbiotic nitrogen fixation was measured with ¹⁵N isotope dilution in white clover (Trifolium repens L.) grown in potted soil that was not re-inoculated, and using ryegrass (Lolium perenne L.) as reference crop. The fraction nitrogen in clover derived from fixation (N_dff) varied from 0 to 88% depending on soil. Pronounced metal toxicity on N_dff was only confirmed in a sludge treated soil where nitrogen fixation was halved from the control value at soil total metal concentration of 737 mg Zn kg⁻¹, 428 mg Cu kg⁻¹ and 10 mg Cd kg⁻¹. The N_dff was significantly reduced by increasing metal concentration in soils from two other sites where N_dff was low throughout and where these effects might be attributed to confounding factors. No significant effects of metals on N_dff were identified in all other gradients even up to elevated total metal concentration (e.g. 55 mg Cd kg⁻¹). The variation of N_dff among all soils (n=48), is mainly explained by the number of rhizobia in the soil (log MPN, log (cells g⁻¹ soil)), whereas correlations with total or soil solution metal concentrations were weak (R²<0.25). The is significantly affected by the presence or absence of the host plant at the sampling site. No effects of metals were identified at even at total Zn concentrations of about 2000 mg Zn kg⁻¹, whereas metal toxicity could be identified at lower most probable number (MPN) values. This survey shows that the metal toxicity on symbiotic nitrogen fixation cannot be generalized and that survival of a healthy population of the microsymbiont is probably the critical factor.