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   154篇
  免费   8篇
林业   1篇
农学   6篇
  5篇
综合类   24篇
农作物   3篇
水产渔业   14篇
畜牧兽医   87篇
园艺   1篇
植物保护   21篇
  2021年   2篇
  2020年   2篇
  2019年   2篇
  2018年   1篇
  2017年   1篇
  2016年   3篇
  2015年   3篇
  2014年   1篇
  2013年   3篇
  2012年   3篇
  2011年   10篇
  2010年   11篇
  2009年   6篇
  2008年   4篇
  2007年   9篇
  2006年   4篇
  2005年   1篇
  2004年   2篇
  2003年   2篇
  2002年   6篇
  2001年   2篇
  2000年   2篇
  1999年   3篇
  1998年   4篇
  1997年   2篇
  1996年   5篇
  1995年   2篇
  1994年   5篇
  1993年   3篇
  1992年   3篇
  1991年   3篇
  1990年   4篇
  1989年   3篇
  1988年   2篇
  1987年   4篇
  1986年   7篇
  1985年   1篇
  1984年   6篇
  1983年   2篇
  1981年   2篇
  1975年   1篇
  1974年   3篇
  1972年   3篇
  1971年   3篇
  1970年   1篇
  1969年   5篇
  1968年   2篇
  1967年   2篇
  1966年   1篇
排序方式: 共有162条查询结果,搜索用时 312 毫秒
131.
A common argument advanced in the non-metropolitan counties of the US West is that the Endangered Species Act (ESA) can have, and has had, a devastating effect on local economies. However, to date, there has not been a systematic empirical analysis of the ESA's effect on local economies. This paper reports on such an analysis. Based on a sample of all 333 non-metropolitan counties in the eleven-state West, the statistical effect of the listing of threatened and endangered species on county employment growth between 1980 and 1990 is estimated. The paper's primary finding is that the hypothesis that endangered species listing has had a negative effect on the non-metropolitan county economies of the US West is not supported by the data.  相似文献   
132.
133.
The discovery of sulfoxaflor [N-[methyloxido[1-[6-(trifluoromethyl)-3-pyridinyl]ethyl]-λ(4)-sulfanylidene] cyanamide] resulted from an investigation of the sulfoximine functional group as a novel bioactive scaffold for insecticidal activity and a subsequent extensive structure-activity relationship study. Sulfoxaflor, the first product from this new class (the sulfoximines) of insect control agents, exhibits broad-spectrum efficacy against many sap-feeding insect pests, including aphids, whiteflies, hoppers, and Lygus, with levels of activity that are comparable to those of other classes of insecticides targeting sap-feeding insects, including the neonicotinoids. However, no cross-resistance has been observed between sulfoxaflor and neonicotinoids such as imidacloprid, apparently the result of differences in susceptibility to oxidative metabolism. Available data are consistent with sulfoxaflor acting via the insect nicotinic receptor in a complex manner. These observations reflect the unique structure of the sulfoximines compared with neonicotinoids.  相似文献   
134.
135.
136.
A microencapsulated formulation that gives a burst release of piperonyl butoxide (PBO) several hours before a burst release of a conventional pyrethroid can effectively overcome metabolic resistance in Bemisia tabaci Gennadius, Helicoverpa armigera (Hübner), Aphis gossypii Glover and Myzus persicae Sulzer. This increase in efficacy against resistant pests was reflected in a field trial against B. tabaci on cotton, eliminating the need for two treatments. The ratio between the active insecticide and the synergist was found to be crucial in reducing resistance factors.  相似文献   
137.
Historically, cisco Coregonus artedi Lesueur was the predominant prey fish and target of commercial fisheries throughout Lake Superior, but most spawning stocks collapsed by the mid-1900s. Stocks partially recovered by the early 1990s, but contemporary abundance is considered to be below historical levels and driven by intermittent recruitment. Stochastic, age-structured simulation models were used to determine whether historical (pre-1955) cisco yield in Lake Superior was consistent with contemporary (1992–2015) abundance, life-history characteristics and recruitment dynamics. When compared to contemporary stocks, the findings suggest historical stocks had: (1) similar recruits per spawner at low spawning stock sizes; (2) lower rates of compensatory density dependence; (3) similar or lower recruitment variation depending on the area and (4) higher median adult and age-1 density. These findings are consistent with the hypothesis that eutrophication during the historical period supported greater recruitment and adult abundance and that re-oligotrophication during the contemporary period may be limiting full recovery.  相似文献   
138.
ABSTRACT Proponents of petroleum industry subsidies often assert that such policies will have positive economic implications for rural communities. This paper examines the economic impacts of such a policy in Utah. Specifically, this paper quantifies the direct and indirect economic and fiscal impacts of a tax credit granted for oil and gas well workovers in Utah's Uintah Basin. The analysis is made possible by an input-output model constructed specifically for Utah's oil producing economy. The tax credit policy was found to generate a net fiscal loss for the state. However, it does generate employment in the Uintah Basin. The total per job cost to the state of generating an average of one job per year for 5 years through the tax credit policy is $24,056 (1991 dollars). However, if the public expenditure impacts are taken into account, then the cost per job could be as high as $48,423 (1991 dollars). Whether there are other ways to generate the same employment gains at a lower cost was lost in the political debate surrounding this petroleum industry tax credit.  相似文献   
139.
The clinicopathological features of 90 cases of canine multicentric lymphoma are described. The majority of cases occurred in middle-aged dogs (five to eight years of age) with a mean age at presentation of 6–7 years. A marked sex predisposition was noted with male dogs out numbering females by 2:1. No significant breed predispositions were found. The most common presentation was a generalised lymphadenopathy but in approximately one-third of the cases this was accompanied by other signs including polyuria and polydipsia, facial oedema and upper respiratory tract obstruction. Approximately two-thirds of all the cases showed non-specific haematological abnormalities including thrombocytopenia, anaemia and white blood cell anomalies. All the cases were clinically staged according to the anatomical extent of the disease. The majority of the dogs were classified as stage 3 or above, denoting that the disease is invariably widespread or systemic upon initial presentation.  相似文献   
140.
Genetic reassortment studies of bluetongue virus (BTV) Types 20 and 21 have revealed a reassortant genotype that was not neutralized serotype-specifically. In reciprocal neutralization tests, BTV 20 and 21 were neutralized specifically by homologous antiserum. Similarly, reassortants that possessed both outer capsid proteins (i.e., VP2 and VP5) from the same parent virus reacted with that antiserum specifically. However, two reassortants, 16(9) and 19(1), with VP2 of BTV 20 and VP5 of BTV 21 had intermediate neutralization characteristics. These reassortants were neutralized to high titres by antiserum to BTV 20 and to lower, but significant titres by antiserum to BTV 21. In addition, antiserum to BTV 20 induced 10-16-fold higher titres in plaque reduction neutralization (PRN) tests with these two reassortants compared with BTV 20 itself. Evidence of the serological cross-reactivity of Reassortants 16(9) and 19(1) was also found with respect to reductions in plaque sizes observed in the PRN tests. The average plaque sizes of these reassortants were reduced to differing extents by antiserum to BTV 20 and 21, while those formed by the parent viruses were reduced in size by homologous antiserum only. Immunoblotting analysis of the structural proteins of BTV 20 and 21 demonstrated that VP2 alone was antigenically distinct, therefore confirming its role in determining serotype specificity in virus-neutralization tests. Electrophoretic analysis revealed considerable migrational differences between VP2 and VP5 of the parent viruses, suggesting that there was some divergence in their molecular weights, intrinsic charges or structural compositions. Taken together, the data suggest that the intermediate neutralization characteristics of the reassortants that contain VP2 and VP5 from different parent viruses are due to conformational alterations in their outer capsid structure which allow antibody recognition of common neutralizing epitopes that are not exposed on BTV 20 or BTV 21.  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