不同基因型玉米生育前期磷锌的吸收

研究了４个对锌缺乏敏感程度不同的玉米基因型磷锌的相关性。结果表明，锌胁迫条件下各玉米基因型敏感程度不同，不同磷水平对各基因型锌吸收产生不同的影响。与之相同，锌的用量也对不同基因型的吸收产生不同影响。不同玉米基因型在苗期和大喇叭口期植株体内磷和锌的浓度变化揭示了基因型间对锌、磷吸收、运输及再利用程度的差异。     

Associations between total sulfation, hydrogen sulfide deposition, and beef-cattle breeding outcomes in western Canada.

Beef cows sometimes are pastured close to oil- and gas-production and processing facilities. We measured the health and productivity of cow-calf herds with differing exposure to sour-natural-gas processing facilities in western Canada. In seven cow-calf herds, the numbers of eligible mature females included in the study for the breeding seasons beginning in 1993, 1994, and 1995 were 1177, 1251, and 1236, respectively. Outcomes included pregnancy status, calving interval, and the occurrence of twins, abortions, stillbirths, and neonatal mortality. Information also was collected on other risk factors known to influence beef-herd health and productivity.Monthly measurements from a network of passive air-monitoring devices were used to estimate exposure. Total sulfation and H2S deposition were used as markers for the complex mixture of compounds found in emissions from sour-gas processing plants and sour flares. Most herds were managed in multiple pasture groups. Cumulative exposure assessments were made from records of individual-animal movements between pastures. Generalized estimating equations were used to evaluate the association between exposure and outcome and to adjust for potential confounders and clustering of binomial outcomes within herd. No consistent associations were found between either total sulfation or H2S deposition and productivity parameters across the cow-calf production cycles. There were, however, five examples of significant associations between increasing cumulative exposure to total sulfation and decreased productivity in the 18 models examined.     

The predictive value of pelvimetry in beef cattle.

To elucidate reasons for failure of pelvimetry to predict dystocia, we collected data from 1146 heifers and 210 cows in five beef cow herds in Saskatchewan. We assessed the reliability of pelvic area measurements, the generalizability of findings, various modifications of the technique, and the statistical association between pelvic area measurements and dystocia. The repeatability (kappa) of pelvic area measurements between and within veterinarians for the Rice and Krautmann pelvimeters were low to moderate, indicating pelvic area measurements were imprecise. The positive predictive values and sensitivities of pelvic area measurements were consistently poor across herds, years of study, breeds of heifers, times of measurement, various pelvic area cut-off points, and sires. Various modifications of the technique, including pelvic area/calf birth weight ratios, pelvic area/heifer weight ratios, and Ko's calving prediction equation were also poor on-farm tests for predicting dystocia. Although the mean pelvic area in heifers with dystocia was smaller than those without dystocia, there was a large overlap in the distribution of their measurements. Far too many heifers with a small pelvic area had no dystocia (false positives) and far too many heifers with a large pelvic area had dystocia (false negatives) for pelvimetry to be useful. We conclude there is little evidence to justify the continued use of pelvimetry as an on-farm test to reduce dystocia in beef cattle.     

Coinfection with bovine viral diarrhea virus and Mycoplasma bovis in feedlot cattle with chronic pneumonia

Chronic, antibiotic-resistant pneumonia, sometimes with concurrent polyarthritis, occurs in feedlot cattle in western Canada. The prevalence of Mycoplasma bovis, bovine viral diarrhea virus, and Haemophilus somnus was determined by using immunohistochemical staining of lung and heart tissue from 2 groups of animals with this history. Mycoplasma bovis antigen was present in 44/48 cases submitted between 1995 and 1998 (retrospective group) and 15/16 of cases from 1999 (prospective group), and was associated with pulmonary necrosis. Bovine viral diarrhea virus antigen was present in association with microscopic vascular lesions in 31/48 retrospective and 9/16 of prospective cases. Types Ib and II bovine viral diarrhea virus were isolated from 4/16 prospective cases. Haemophilus somnus antigen was present in heart, lung, or both of 15/48 retrospective and 8/16 prospective cases. The results suggest that there may be synergism between bovine viral diarrhea virus and M. bovis in this pneumonia with arthritis syndrome.     

Survey of Mycobacterium avium subspecies paratuberculosis serological status in beef herds on community pastures in Saskatchewan

Johne's disease is a well recognized problem in dairy herds. Relatively little information is available on either the prevalence or the control of Johne's disease in commercial cow-calf operations. In the fall of 1999, blood samples were collected during pregnancy testing from cows on community pastures in Saskatchewan. Sera from these cows were analyzed using a commercial ELISA for antibodies to Mycoplasma avium subspecies paratuberculosis. All cows from each herd examined at the community pastures were sampled. Of the 1799 samples tested, 15 had sample to positive (S/P) ratios greater than 0.25 and were considered positive (apparent sample prevalence, 0.8%; 95% CI, 0.4% to 1.5%). If we assume test sensitivity of 25% and specificity of 98% as recommended by the National Johne's Working Group, the true sample prevalence is not significantly different from 0.0%. The ELISA S/P results for the antibody test-positive animals ranged from 0.27 to 2.5. If a herd was classified as positive based on one test-positive animal, the average herd apparent prevalence was 15.2% (95% CI, 7.1% to 28.6%). If the potential for false-positive results was considered with 2 or more positive animals being required for positive herd status, the herd prevalence was 3.0% (95% CI, 0.4% to 13.4%). Because of the very low prevalence in cow-calf herds, future research to identify risk factors and control points should target problem herds and utilize a case-control study design.     

Constant Light Exposure Causes Dissociation in Gonadotrophin Secretion and Inhibits Partially Neuroendocrine Differentiation of Leydig Cells in Adult Rats

The objective of this work was to study the changes that occur in the Leydig cells of rats exposed to continuous light. The laboratory rat is considered a non-photoperiodic species because exposure to short photoperiod has little or no effect on the reproductive status. However, exposure of adult female rats to constant light induces polycystic ovaries, indicating that extreme changes in the photoperiod affect the reproductive function seriously. Adult male rats were placed under continuous light conditions for a duration of 15 weeks. After this period, the animals were killed and testicles were dissected and processed by routine histologic protocols. Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) serum levels were determined by radioimmunoassay (RIA). The visualization of antigens was achieved by the streptavidin-peroxidase immunohistochemical method. Antibodies against chromogranin A, S-100 protein, P substance, synaptofisin, neurofilament protein-200, gliofibrillary acidic protein and neurone-specific enolase were used. The mean LH serum concentration was significantly lower, while the mean FSH level was significantly higher in treated animals. The expression of S-100, NSE, CrA, SP and SYN was significantly lower in treated animals. In conclusion, the constant light exposure acting directly at the pituitary level decreases LH secretion. The increased FSH secretion may be due to a partial reduction of the negative androgen feedback in the pituitary gland. Moreover, the constant light exposure affects the expression of some immunomarkers in Leydig cells, possibly because of the changes found in the gonadotrophin level and feedback mechanism.     

Species composition, similarity and diversity in three successional stages of a seasonally dry tropical forest

The objectives of this study were to describe the floristic composition, species diversity, similarity and richness among three stages of forest regeneration, and to investigate the influence of the previous land use on species composition in a seasonally dry tropical forest in northwestern Costa Rica. The species diversity and richness of woody stems with diameter greater or equal to 5 cm in 26 0.1 ha plots was found to be the greatest in the intermediate stage followed by the late and early stages. The structural changes of this chronosequence of successional stages were quantified with a modification of the Holdridge complexity index. Using satellite imagery for site selection and historical ground truth analysis the influence of past land use and the frequency of anthropogenic disturbances on species composition was illustrated.     

Long-term agroecosystem experiments: assessing agricultural sustainability and global change

Long-term agroecosystem experiments can be defined as large-scale field experiments more than 20 years old that study crop production, nutrient cycling, and environmental impacts of agriculture. They provide a resource for evaluating biological, biogeochemical, and environmental dimensions of agricultural sustainability; for predicting future global changes; and for validating model competence and performance. A systematic assessment is needed to determine the merits of all known experiments and to identify any that may exist in tropical and subtropical environments. The establishment of an international network to coordinate data collection and link sites would facilitate more precise prediction of agroecosystem sustainability and future global change.