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   60929篇
  免费   3462篇
  国内免费   36篇
林业   2467篇
农学   1861篇
基础科学   301篇
  6721篇
综合类   10319篇
农作物   2108篇
水产渔业   2887篇
畜牧兽医   33243篇
园艺   737篇
植物保护   3783篇
  2018年   911篇
  2017年   935篇
  2016年   828篇
  2015年   791篇
  2014年   869篇
  2013年   2607篇
  2012年   1712篇
  2011年   2074篇
  2010年   1311篇
  2009年   1274篇
  2008年   1919篇
  2007年   1893篇
  2006年   1775篇
  2005年   1623篇
  2004年   1531篇
  2003年   1568篇
  2002年   1492篇
  2001年   1809篇
  2000年   1787篇
  1999年   1463篇
  1998年   621篇
  1997年   606篇
  1996年   547篇
  1995年   641篇
  1994年   600篇
  1993年   572篇
  1992年   1221篇
  1991年   1369篇
  1990年   1258篇
  1989年   1265篇
  1988年   1163篇
  1987年   1136篇
  1986年   1194篇
  1985年   1194篇
  1984年   960篇
  1983年   900篇
  1982年   582篇
  1979年   891篇
  1978年   701篇
  1977年   694篇
  1976年   711篇
  1975年   714篇
  1974年   866篇
  1973年   842篇
  1972年   812篇
  1971年   727篇
  1970年   769篇
  1969年   734篇
  1968年   650篇
  1967年   656篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
Ohne Zusammenfassung  相似文献   
83.
Ohne Zusammenfassung  相似文献   
84.
85.
Seven cultivars of red raspberry ( Rubus idaeus ) showed different susceptibility to infection by yellow rust ( Phragmidium rubi-idaei ) at two commercial plantations in Scotland and North Wales, The cultivars Latham, Chief and Boyne were completely resistant. Mailing Delight and Glen Clova supported more telial more than Mailing Jewel. In contrast the last named was the most susceptible cultivar at the aecial stage in Scotland but developed similar numbers of aecia to Glen Clova and Mailing Delight in Wales.
Meeker had an extended latent period, fewer and smaller pustules and a very small yield of urediniospores compared to the other susceptible cultivars, thus showing the characteristics of slow-rusting genotypes. At the Scottish site in 1982, where rust was less than in Wales, Meeker and Mailing Jewel performed differently to the other cultivars at the telial stage. Inoculation studies under controlled environmental conditions with urediniospores from each site revealed an interaction between cultivars and isolates suggesting that variation in pathogenicity is present in populations of P. rubi-idaei in the UK.  相似文献   
86.
The COVID-19 pandemic presents a continued public health challenge. Veterinary diagnostic laboratories in the United States use RT-rtPCR for animal testing, and many laboratories are certified for testing human samples; hence, ensuring that laboratories have sensitive and specific SARS-CoV2 testing methods is a critical component of the pandemic response. In 2020, the FDA Veterinary Laboratory Investigation and Response Network (Vet-LIRN) led an interlaboratory comparison (ILC1) to help laboratories evaluate their existing RT-rtPCR methods for detecting SARS-CoV2. All participating laboratories were able to detect the viral RNA spiked in buffer and PrimeStore molecular transport medium (MTM). With ILC2, Vet-LIRN extended ILC1 by evaluating analytical sensitivity and specificity of the methods used by participating laboratories to detect 3 SARS-CoV2 variants (B.1; B.1.1.7 [Alpha]; B.1.351 [Beta]) at various copy levels. We analyzed 57 sets of results from 45 laboratories qualitatively and quantitatively according to the principles of ISO 16140-2:2016. More than 95% of analysts detected the SARS-CoV2 RNA in MTM at ≥500 copies for all 3 variants. In addition, for nucleocapsid markers N1 and N2, 81% and 92% of the analysts detected ≤20 copies in the assays, respectively. The analytical specificity of the evaluated methods was >99%. Participating laboratories were able to assess their current method performance, identify possible limitations, and recognize method strengths as part of a continuous learning environment to support the critical need for the reliable diagnosis of COVID-19 in potentially infected animals and humans.  相似文献   
87.
OBJECTIVE: To characterize the normal ultrasonographic appearance of the podotrochlear apparatus in horses by use of standardized measurements and identify soft tissue changes associated with navicular syndrome. DESIGN: Prospective study. ANIMALS: 7 clinically normal horses and 28 horses with navicular syndrome. PROCEDURE: The feasibility of identifying and measuring the soft tissue structures of the podotrochlear apparatus ultrasonographically via the transcuneal approach was assessed in 2 additional horses without navicular syndrome; both horses were euthanatized, and the structures identified ultrasonographically were confirmed at necropsy. Ultrasonographs were obtained in the study horses. Objective and subjective data were obtained to characterize ultrasonographic changes associated with navicular syndrome. RESULTS: Abnormalities of the flexor surface of the distal sesamoid (navicular) bone, the impar ligament, the distal digital annular ligament, deep digital flexor tendon (DDFT), and the podotrochlear (navicular) bursa were assessed via the transcuneal ultrasonographic approach. No significant differences were found between the measurements of the podotrochlear apparatus in normal horses and those with navicular syndrome; however, important subjective differences were detected ultrasonographically in horses with navicular syndrome. In horses with navicular syndrome, ultrasonographic findings were indicative of navicular bursitis, dystrophic mineralization of the DDFT and impar ligament, tendonitis and insertional tenopathy of the DDFT, desmitis of the impar ligament, and cortical changes in the flexor surface of the navicular bone. CONCLUSIONS AND CLINICAL RELEVANCE: Findings of ultrasonographic evaluation of the hoof appear to be useful in determining the cause of caudal heel pain and characterizing the components of navicular syndrome in horses.  相似文献   
88.
89.
We developed a method to derive optimum composite genotypes when the proportion of a specific breed (or combination of breeds) in the composite is constrained to a desired level. The method allows development of composites that balance net merit according to available parameters and reflects the production system and(or) risk preference, particularly when traits in the breeding objective do not represent all traits important for production. The method can also be used to evaluate the cost of constraining breed use. For example, when the true optimal composite is difficult to obtain in the time frame available to the producer, the predicted merit of the composite with breed proportion restricted to those that are obtainable in practice can be compared with the predicted merit of the true optimum composite. In a case study the method is used to optimize a tropical composite using Brahman, Charolais, and Belmont Red breeds. The proportion of Charolais is to be constrained to 50%. Using growth parameters from a tropical crossbreeding experiment, the unconstrained optimum composite was 32.5% Brahman, 11.1% Belmont Red, and 56.3% Charolais. When the proportion of Charolais in the composite was constrained to 50%, optimum proportions of Brahman and Belmont Red were 33.9% and 16.1% respectively. In the case study net merit (annualized profit per cow exposed, $), as defined by the growth parameters, was .2% less than that of the unconstrained optimum composite.  相似文献   
90.
The effect of a novel lairage environment on the ability of sheep to recover from 16 h of transport was investigated. Sheep were transported from grass paddocks to either novel outside paddocks or inside pens, and housed groups were transported to either familiar or novel inside pens. During transport, sheep from outside paddocks lay down less than those from inside pens. In sheep transported to inside pens, those from outside paddocks spent more time lying and spent less time eating; hay and water intakes during the first 12 h post-transport were lower than those previously kept inside. There was no obvious effect of a novel environment post-transport on blood biochemistry, suggesting that the lower post-transport feed and water intakes in a novel environment did not have a significant effect on the ability of the sheep to recover from the feed and water deprivation associated with transport.  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