首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   406篇
  免费   33篇
林业   14篇
农学   10篇
基础科学   2篇
  42篇
综合类   115篇
农作物   7篇
水产渔业   21篇
畜牧兽医   208篇
园艺   4篇
植物保护   16篇
  2021年   8篇
  2019年   5篇
  2018年   4篇
  2017年   7篇
  2016年   5篇
  2015年   4篇
  2014年   6篇
  2013年   11篇
  2012年   14篇
  2011年   20篇
  2010年   8篇
  2009年   10篇
  2008年   13篇
  2007年   12篇
  2006年   14篇
  2005年   11篇
  2004年   17篇
  2003年   14篇
  2002年   10篇
  2001年   10篇
  2000年   10篇
  1999年   10篇
  1998年   9篇
  1997年   7篇
  1995年   11篇
  1994年   3篇
  1992年   10篇
  1991年   14篇
  1990年   6篇
  1989年   12篇
  1988年   12篇
  1987年   9篇
  1986年   9篇
  1985年   10篇
  1984年   4篇
  1983年   5篇
  1980年   4篇
  1977年   3篇
  1976年   3篇
  1974年   5篇
  1973年   7篇
  1972年   9篇
  1971年   5篇
  1970年   8篇
  1969年   3篇
  1968年   6篇
  1967年   5篇
  1966年   5篇
  1959年   2篇
  1941年   2篇
排序方式: 共有439条查询结果,搜索用时 31 毫秒
81.
82.
The accumulation of above-ground biomass and the seasonal patterns of leaf-area development, foliar nutrient concentrations and tree and soil water-status have been measured for fertilised, irrigated, and control stands of Pinus radiata D. Don growing on a low-productivity site, average annual precipitation of 790 mm, near Canberra in southeastern Australia. In the second growing-season after treatments commenced, projected leaf-area index reached peak values of 7 on the irrigated/fertilised stands compared with approximately 5 on the other stands. Average canopy nitrogen concentration (dry-weight basis) varied across the treatments from 9 to 17 mg g−1. Measurements of soil and tree water-status over a 2-year period indicated that stands which were not irrigated experienced summer droughts of up to 4 months duration.

Annual volume production measured over the 2-year period ranged from 17 to 45 m3 ha−1. The extent to which this variation could be attributed to differences in leaf area, rats of photosynthesis, duration of the period of positive net photosynthesis, and hence growth, was analysed in terms of a process-based model of stand growth dependent on climate and soil water-balance.

Annual canopy net photosynthesis simulated by the model ranged from 18 t carbon ha−1 for the control stand to 38 t ha−1 for the irrigated/fertilised stands. Simulations indicated that 67% of this difference could be attributed to the role of irrigation in extending the period of active growth. The additional leaf area carried by the irrigated/fertilised stands contributed a further 23%, while differences in rates of photosynthesis, related to nitrogen nutrition, explained the remaining 10%.  相似文献   

83.
84.
85.
To study the evolution of recombination rates in apes, we developed methodology to construct a fine-scale genetic map from high-throughput sequence data from 10 Western chimpanzees, Pan troglodytes verus. Compared to the human genetic map, broad-scale recombination rates tend to be conserved, but with exceptions, particularly in regions of chromosomal rearrangements and around the site of ancestral fusion in human chromosome 2. At fine scales, chimpanzee recombination is dominated by hotspots, which show no overlap with those of humans even though rates are similarly elevated around CpG islands and decreased within genes. The hotspot-specifying protein PRDM9 shows extensive variation among Western chimpanzees, and there is little evidence that any sequence motifs are enriched in hotspots. The contrasting locations of hotspots provide a natural experiment, which demonstrates the impact of recombination on base composition.  相似文献   
86.
87.
Two 12-year-old cats were diagnosed with chronic kidney disease (CKD) based on physical examination, clinicopathologic data and, in one case, abdominal ultrasound findings. Approximately 1 year after the initial diagnosis of CKD both cats developed renal transitional cell carcinoma (TCC)--bilateral in one cat. Based on post-mortem examination, one cat had no evidence of metastasis and the other had metastasis to the large intestine, heart and lungs. This is the first report of de novo bilateral renal TCC in a cat, as well as the first report of renal TCC developing in cats with previous history of confirmed CKD.  相似文献   
88.
An 8-yr-old, captive, spayed, female maned wolf (Chrysocyon brachyurus) developed progressive lethargy and weakness over a 24-hr period. Clinical signs included vomiting, recumbency, horizontal nystagmus, possible blindness, pale icteric mucus membranes, and port-wine colored urine. A complete blood cell count revealed severe anemia (packed cell volume [PCV], 6%) and intraerythrocytic piroplasms consistent with a Babesia species. Polymerase chain reaction testing later confirmed babesiosis. The wolf was treated with imidocarb dipropionate, antibiotics, and fluid therapy. A whole-blood transfusion from a sibling maned wolf also was performed. Despite aggressive treatment, the wolf failed to improve and was euthanized. To the authors' knowledge, this is the first documented case of babesiosis in a captive maned wolf in North America. Surveillance of infectious diseases in captive and wild maned wolf populations should be expanded to include screening for Babesia species. Tick control also should be implemented to prevent and decrease transmission of the disease to this endangered species.  相似文献   
89.
90.
Rollover protective structures (ROPS) are an effective engineering control known to prevent tractor overturn deaths, the leading cause of occupational fatalities for farmers and farm workers in the U.S. However, the use of ROPS is known to vary greatly from farm to farm. A national sample of 11,458 farm operators from the 2004 Occupational Injury Surveillance of Production Agriculture (OISPA) survey was used to assess the association between the prevalence of ROPS and ten farm operator and farm demographic variables using logistic regression. The variable were: operator's age, operator's sex, operator's education, farm sales, full- or part-time farming, acreage, type of operation, number of hired workers, number of injuries, and region. All ten variables were found to have significant associations with the prevalence of non-ROPS tractors on farms in the univariate logistic regressions. For the multivariate model, all variables except for the sex of the farm operator remained significant. Farms with less than three adult injuries, no hired workers, less than 300 acres in size, a Midwest location, and a primary farm type of tobacco, fruit and nuts, dairy, or poultry and eggs all had adjusted odds ratios of 2 or greater. Increasing the prevalence of ROPS-equipped tractors is essential for reducing the leading cause of death on farms, tractor overturns. Economic factors play a major role in the prevalence and distribution of non-ROPS tractors on farms. The identified associations can be used to effectively target areas of the U.S. for ROPS promotion activities.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号