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   296篇
  免费   20篇
林业   6篇
农学   1篇
基础科学   2篇
  41篇
综合类   67篇
农作物   36篇
水产渔业   10篇
畜牧兽医   137篇
园艺   5篇
植物保护   11篇
  2022年   5篇
  2019年   6篇
  2018年   6篇
  2017年   6篇
  2016年   8篇
  2015年   4篇
  2014年   3篇
  2013年   13篇
  2012年   12篇
  2011年   3篇
  2010年   3篇
  2009年   5篇
  2008年   15篇
  2007年   7篇
  2006年   12篇
  2005年   5篇
  2004年   13篇
  2002年   10篇
  2001年   10篇
  2000年   8篇
  1999年   5篇
  1998年   3篇
  1996年   4篇
  1995年   11篇
  1993年   3篇
  1992年   10篇
  1991年   4篇
  1990年   5篇
  1989年   6篇
  1988年   5篇
  1987年   9篇
  1986年   3篇
  1985年   6篇
  1984年   4篇
  1983年   5篇
  1979年   5篇
  1973年   2篇
  1972年   5篇
  1971年   3篇
  1968年   5篇
  1967年   3篇
  1965年   2篇
  1964年   4篇
  1963年   3篇
  1962年   3篇
  1959年   2篇
  1948年   2篇
  1947年   2篇
  1922年   2篇
  1891年   2篇
排序方式: 共有316条查询结果,搜索用时 31 毫秒
31.
In an examination of the molecular basis of oral transmission of bunyaviruses by mosquitoes., La Crosse (LAC), snowshoe hare (SSH), and LAC-SSH reassortant viruses were compared in their ability to be transmitted to laboratory mice by the natural mosquito vector of LAC virus, Aedes triseriatus. Both LAC virus and the reassortment viruses containing the middle-sized (M) segment from the LAC parent were efficiently transmitted. In contrast, SSH virus and reassortment viruses containing the M RNA from the SSH parent were inefficiently transmitted. Thus, the M RNA segment, which codes for the virion glycoproteins, may be a major determinant of oral transmission of bunyaviruses by mosquitoes.  相似文献   
32.
33.
34.
35.
36.
Summary Amino acids were extracted from fertility plots of a loamy sand soil with 0.05 M HF-HCl and with a 10% ethanol solution (free amino acids) and analysed by reverse-phase high performance liquid chromatography (HPLC). The total quantities of amino-N compounds analysed were 4.4 g/g soil for the acid treatment and 22.6 g/g soil for the 10% ethanol extract. Glycine and glutamic acid were the most abundant of 15 amino-N compounds in the HF-HCl extracts, whereas glutamic acid and ornithine + NH+ 4 were found in the highest concentration in the 10% ethanol extracts. The HF-HCI pretreatment is used to increase the efficiency of the extraction of soil organic matter. Although this pretreatment removed some amino acids, the acids extract less than 1% of the total amino-N content of the crude soil extracts. The pretreatment, therefore, was not overly destructive. Comparisons between the amino acids extracted from the fertility plots were not conclusive, except for glycine, which was greater in concentration in the higher fertilizer N plots of the same crop rotation.  相似文献   
37.
Cokriging particle size fractions of the soil   总被引:2,自引:0,他引:2  
It is often necessary to predict the distribution of mineral particles in soil between size fractions, given observations at sample sites. Because the contents in each fraction necessarily sum to 100%, these values constitute a composition, which we may assume is drawn from a random compositional variate. Elements of a D‐component composition are subject to non‐stochastic constraints; they are constrained to lie on a D– 1 dimensional simplex. This means we cannot treat them as realizations of unbounded random variables such as the multivariate Gaussian. For this reason, there are theoretical reasons not to use ordinary cokriging (or ordinary kriging) to map particle size distributions. Despite this, the compositional constraints on data on particle size fractions are not always accounted for by soil scientists. The additive log‐ratio (alr) transform can be used to transform data from a compositional variate into a form that can be treated as a realization of an unbounded random variable. Until now, while soil scientists have made use of the alr transform for the spatial prediction of particle size, there has been concern that the simple back‐transform of the optimal estimate of the alr‐transformed variables does not yield the optimal estimate of the composition. A numerical approximation to the conditional expectation of the composition has been proposed, but we are not aware of examples of its application and it has not been used in soil science. In this paper, we report two case studies in which we predicted clay, silt and sand contents of the soil at test sites by ordinary cokriging of the alr‐transformed data followed by both the direct (biased) back‐transform of the estimates and the unbiased back‐transform. We also computed estimates by ordinary cokriging of the untransformed data (which ignores the compositional constraints on the variables) for comparison. In one of our case studies, the benefit of using the alr transform was apparent, although there was no consistent advantage in using the unbiased back‐transform. In the other case study, there was no consistent advantage in using the alr transform, although the bias of the simple back‐transform was apparent. The differences between these case studies could be explained with respect to the distribution on the simplex of the particle size fractions at the two sites.  相似文献   
38.
This study was designed to evaluate the ontogeny of serum IGF-I (SI) concentrations and its relationship to animal performance in a 140-d postweaning feeding trial. Ninety-eight progeny representing six sires (three high and three low feed conversion) and two sexes (43 bulls and 55 heifers) with ad libitum access to feed were allocated by sire and sex to monitor individual weights and pen feed consumption. Blood serum samples were obtained at the beginning of test (average age of 230 d) and every 28 d thereafter until each animal reached a fat thickness (estimated by sonoray) of 8.9 mm. Individual serum samples were acid-ethanol extracted and measured for IGF-I peptide by heterologous RIA. Serum IGF-I concentrations differed (P less than .10) between high (H) and low (L) feed conversion progeny groups at the end of the first 28-d period (125.12 vs 89.52 ng/ml) and tended to differ at the conclusion of the second 28-d period (P less than .15). Weight gains of H and L groups tended to differ in the second and third 28-d periods (P = .11 and .10, respectively). Serum IGF-I concentrations differed (P less than .05) between bulls and heifers for the first through fourth 28-d periods (P less than .01, P less than .05, P less than .10 and P less than .01, respectively). Phenotypic correlations indicated that pens with higher mean SI concentrations at the beginning of the test consumed less feed and had lower cumulative feed:gain ratios.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
39.
A new anthelmintic assay is described which uses immunosuppressed (60 ppm hydrocortisone acetate in diet) rats infected with the nematode Trichostrongylus colubriformis. Immunosuppressed rats were infected with 1500 T. colubriformis larvae, treated either orally or subcutaneously on Day 14 post-infection and necropsied 4 days after treatment. The worm counts in immunosuppressed control animals averaged 775 worms per rat. A range of benzimidazoles, levamisole hydrochloride, morantel tartrate, 22,23-dihydroavermectin B1a and alpha-milbemycin have been evaluated in the assay. The ED95 values obtained indicate that rats infected with T. colubriformis provide a highly predictive model for assaying the activity of experimental drugs in vivo prior to studies in ruminants.  相似文献   
40.
Anthelmintic efficacies of fenbendazole, ivermectin, and levamisole were evaluated against naturally acquired infections of Nematodirus battus in lambs. Four groups of 10 lambs each were used. Oral administration of 8 mg of levamisole/kg of body weight or 5 mg of fenbendazole/kg significantly (P less than 0.01) reduced the degree of infection by N battus (adults) by greater than 99%. An oral formulation of 200 micrograms of ivermectin/kg was 98% effective (P less than 0.01). Numbers of other Nematodirus spp (including N filicollis and N spathiger) were significantly reduced.  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