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   149781篇
  免费   8297篇
  国内免费   90篇
林业   6808篇
农学   5187篇
基础科学   933篇
  17589篇
综合类   26427篇
农作物   5806篇
水产渔业   7410篇
畜牧兽医   76875篇
园艺   1892篇
植物保护   9241篇
  2018年   2148篇
  2017年   2413篇
  2016年   2231篇
  2015年   1906篇
  2014年   2363篇
  2013年   5686篇
  2012年   4057篇
  2011年   5002篇
  2010年   3297篇
  2009年   3230篇
  2008年   4889篇
  2007年   4695篇
  2006年   4499篇
  2005年   4004篇
  2004年   3845篇
  2003年   3917篇
  2002年   3643篇
  2001年   4857篇
  2000年   4808篇
  1999年   3832篇
  1998年   1523篇
  1997年   1579篇
  1995年   1658篇
  1994年   1399篇
  1993年   1437篇
  1992年   2975篇
  1991年   3090篇
  1990年   3137篇
  1989年   3099篇
  1988年   2830篇
  1987年   2773篇
  1986年   2847篇
  1985年   2747篇
  1984年   2248篇
  1983年   1985篇
  1982年   1365篇
  1979年   2133篇
  1978年   1671篇
  1977年   1394篇
  1976年   1398篇
  1975年   1586篇
  1974年   1930篇
  1973年   1950篇
  1972年   1904篇
  1971年   1832篇
  1970年   1750篇
  1969年   1670篇
  1968年   1480篇
  1967年   1547篇
  1966年   1386篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
To determine whether turkey herpesvirus (HVT) impairs the aspecific and specific defense against an avian pneumovirus (APV) infection, specific-pathogen-free turkeys were inoculated at 7 days of age with HVT and 1, 5, or 7 wk later with APV. Clinical signs, APV replication, and development of antibodies against APV were evaluated. No differences were found between the birds that received both HVT and APV and those that received only APV.  相似文献   
43.
44.
45.
46.
Twelve (54.5%) of 22 free-roaming dogs in Ishigaki Island had tick infestation identified as Rhipicephalus sanguineus. There were 121 ticks recovered and consisted of 28 females, 58 males, 22 nymphs and 3 larvae. Infection of dogs possibly with canine ehrlichial pathogens was examined by both indirect immunofluorescence assay and polymerase chain reaction (PCR). Two dogs of the 13 examined were sero-positive for the human granulocytic ehrlichia agent, and one of two dogs was PCR positive for Ehrlichia platys. This dog had platelet numbers slightly lower than normal value, however, no morulae were found within platelet on peripheral blood smear stained with Giemsa.  相似文献   
47.
OBJECTIVE: To evaluate trends in feedlot cattle mortality ratios over time, by primary body system affected, and by type of animal. DESIGN: Retrospective cohort study. ANIMALS: Approximately 21.8 million cattle entering 121 feedlots in the United States during 1994 through 1999. PROCEDURES: Yearly and monthly mortality ratios were calculated. Numbers of deaths were modeled by use of Poisson regression methods for repeated measures. Relative risks of death over time and by animal type were estimated. RESULTS: Averaged over time, the mortality ratio was 12.6 deaths/1,000 cattle entering the feedlots. The mortality ratio increased from 10.3 deaths/1,000 cattle in 1994 to 14.2 deaths/1,000 cattle in 1999, but this difference was not statistically significant (P = 0.09). Cattle entering the feedlots during 1999 had a significantly increased risk (relative risk, 1.46) of dying of respiratory tract disorders, compared with cattle that entered during 1994, and respiratory tract disorders accounted for 57.1% of all deaths. Dairy cattle had a significantly increased risk of death of any cause, compared with beef steers. Beef heifers had a significantly increased risk of dying of respiratory tract disorders, compared with beef steers. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggested that although overall yearly mortality ratio did not significantly increase during the study, the risk of death attributable to respiratory tract disorders was increased during most years, compared with risk of death during 1994. The increased rates of fatal respiratory tract disorders may also reflect increased rates of non-fatal respiratory tract disorders, which would be expected to have adverse production effects in surviving animals.  相似文献   
48.
The isolation of (+)-totarol as active compound against Mycobacterium tuberculosis is reported from Chamaecyparis nootkatensis outerbark.  相似文献   
49.
The morphology of 16–17 days old embryos from virgin heifers (VH) and repeat breeder heifers (RBH) was compared using light and electron microscopy. In addition some embryos transferred from one heifer category to the other were studied. Embryos from VH were elongated blastocysts and the oval embryonic disc had three germ layers. The ectoderm was stratified and many mitoses were seen. The endoderm lining the blastocoelic cavity consisted of almost squamous cells conjoined by tight junctions. Between the ectoderm and the endoderm the mesoderm had developed and expanded laterally and the coelom had formed. The trophoblastic cells adjacent do the embryonic disc were cylindrical, whereas those more peripheral located were cuboidal. The trophoblastic cells were conjoined by tight junctions and they had numerous long microvilli on their peripheral surface. Except in the embryonic disc region, the endodermal cells had filopodial processes towards the trophoblast. The embryos from RBH varied in appearance. One was similar to those from VH whereas the others were, more or less retarded, without formation of mesoderm. The smaller one consisted ot trophoblastic cells only. The transferred embryos (representing surviving embryos: 2 out of 9 in VH-RBH and 5 of 6 in RBH-VH) had a morphology similar to that of VH blastocytes two though, appeared somewhat retarded. It is suggested that the retarded embryos lack the ability to complete embryonic development and that the uterine environment of RBH is not favourable to sustain normal embryonic development.  相似文献   
50.
Abstract. Anecdotal and circumstantial evidence have suggested that the Olsen test underestimates plant-available phosphorus (P) in basaltic soils in Northern Ireland. Therefore, the ability of this test to predict plant-available P in basaltic (and non-basaltic) soils was investigated by regressing Olsen-P data against herbage P indices calculated from plant tissue test data using the diagnosis and recommendation integrated system. The average Olsen-P concentration for a range of fields situated on basaltic soils was considerably lower than the average Olsen-P concentration for a range of fields situated on non-basaltic soils, and yet mean sward P status, as given by the herbage P indices, was similar for both groups of fields. Herbage P indices were also much better correlated with Olsen-P measurements in non-basaltic soils than in basaltic soils. Furthermore, at low Olsen-P values (≶9mgPL−1) some swards on basaltic soils were genuinely deficient in P, while others were sufficient or even in surplus for this nutrient. The results confirm that Olsen-P is inadequate as a predictor of plant-available P in basaltic soils. It is concluded that an alternative soil test is needed to provide a reliable assessment of plant-available P in basaltic soils, to prevent overuse of fertilizer and manure P and to minimize the amounts of P entering local watercourses.  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