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   49479篇
  免费   2724篇
  国内免费   33篇
林业   1745篇
农学   1671篇
基础科学   251篇
  5896篇
综合类   8242篇
农作物   1552篇
水产渔业   2733篇
畜牧兽医   26636篇
园艺   585篇
植物保护   2925篇
  2019年   418篇
  2018年   793篇
  2017年   837篇
  2016年   753篇
  2015年   620篇
  2014年   764篇
  2013年   1749篇
  2012年   1368篇
  2011年   1719篇
  2010年   1165篇
  2009年   1103篇
  2008年   1695篇
  2007年   1605篇
  2006年   1522篇
  2005年   1401篇
  2004年   1404篇
  2003年   1396篇
  2002年   1198篇
  2001年   1926篇
  2000年   1995篇
  1999年   1507篇
  1998年   571篇
  1997年   502篇
  1996年   451篇
  1995年   615篇
  1994年   527篇
  1993年   461篇
  1992年   1156篇
  1991年   1218篇
  1990年   1193篇
  1989年   1109篇
  1988年   1064篇
  1987年   1089篇
  1986年   1044篇
  1985年   976篇
  1984年   810篇
  1983年   686篇
  1982年   422篇
  1979年   645篇
  1978年   505篇
  1977年   445篇
  1976年   413篇
  1975年   454篇
  1974年   527篇
  1973年   518篇
  1972年   533篇
  1971年   454篇
  1970年   427篇
  1969年   471篇
  1967年   390篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
A prospective study of vesicular stomatitis was conducted in two bovine herds in southeastern Mexico. In July 1987, an initial serological screening showed that 64% and 87% of the 654 cattle tested negatively to vesicular stomatitis New Jersey and Indiana antibodies, respectively, using the plaque-reduction serum neutralization test. Most seropositive animals were at least 24 months of age. Based on the initial serological screening, cohorts of seronegative and seropositive cattle were monitered (January–December 1988) for the prevalence of vesicular stomatitis virus (VSV) antibodies using an enzyme-linked immunosorbent assay (ELISA). The serological results, using ELISA, indicated that no VSV activity occured in the two study herds. The seronegative cohort of cattle did not yield a positive seroconversion pattern to either VSV Indiana or New Jersey. The seropositive cohort showed a variable antibody response pattern against the VSV. There were no clinical cases of vesicular stomatitis (VS) in the two herds. The data from the national surveillance program for vesicular diseases suggested that 1988 was a year of low VSV infection incidence in southeastern Mexico.  相似文献   
992.
Eleven feline immunodeficiency virus (FIV) infected asymptomatic carrier (AC) cats were observed for 2 years for development of acquired immunodeficiency syndrome (AIDS). Four of the 11 (36.4%) showed progression of the clinical stage. Persistent generalized lymphadenopathy was noted in three cats as the first sign of illness after the AC phase, while the other showed lymphadenopathy with signs of AIDS-related complex. In all four cats the AIDS-related complex stage lasted for 10 months or longer, and two showed progression of the disease into AIDS. The two cats showing AIDS illnesses died within approximately 1 year after they had developed persistent generalized lymphadenopathy. Pathology confirmed the diagnosis of AIDS characterized by the presence of depletion lesions in the lymphoid organs, and of severe infections of an opportunistic nature. The overall mortality of FIV infected AC cats during a 2 year period was two out of 11 (18.2%). These cats showed decreased concanavalin A mitogen response of peripheral blood mononuclear cells as the disease progressed.  相似文献   
993.
994.
Macrophages function as phagocytes and antigen-presenting cells in the body. As has been demonstrated in mammals, administration of clodronate [dichloromethylene bisphosphonate (Cl2MBP)] encapsulated liposomes results in depletion of macrophages. Although this compound has been used in chickens, its effectiveness in depleting macrophages has yet to be fully determined. Here, we show that a single administration of clodronate liposomes to chickens results in a significant depletion of macrophages within the spleen and lungs of chickens up to 4 d post-treatment. This finding suggests that, in order to obtain depletion of macrophages in chickens for greater than 5 d, it is necessary to administer clodronate liposomes 4 d apart. The study also showed that 2 treatments of clodronate liposomes at 4-day intervals resulted in the depletion of macrophages for up to 10 d. The findings of the present study will encourage more precise studies to be done on the potential roles of macrophages in immune responses and in the pathogenesis of microbial infections in chickens.  相似文献   
995.
996.
Investigation of historic population processes using molecular data has been facilitated by the use of approximate Bayesian computation (ABC), which enables the consideration of multiple alternative demographic scenarios. The Lincoln Red cattle breed provides a relatively simple example of two well‐documented admixture events. Using molecular data for this breed, we found that structure did not resolve very low (<5% levels) of introgression, possibly due to sampling limitations. We evaluated the performance of two ABC approaches (2BAD and DIYABC) against those of two earlier methodologies, ADMIX and LEADMIX, by comparing their interpretations with the conclusions drawn from herdbook analysis. The ABC methods gave credible values for the proportions of the Lincoln Red genotype that are attributable to Aberdeen Angus and Limousin, although estimates of effective population size and event timing were not realistic. We suggest ABC methods are a valuable supplement to pedigree‐based studies but that the accuracy of admixture determination is likely to diminish with increasing complexity of the admixture scenario.  相似文献   
997.
The effects of natural dietary carotenoid supplementation of the maternal diet (tomato powder and marigold extract) on transfer to the egg yolk and on the development of the antioxidant system of the young quail liver in early postnatal life were investigated. Sixty Japanese quail (Coturnix coturnix japonica) were allocated to four treatment groups, each with three replicates consisting of four females and one male each. The quail were fed on one of four different diets for 23 d, each of them based on a low carotenoid, wheat/barley-based control diet. Tomato powder and marigold extract were added at rates of 20 and 2 g/kg to treatments 2 and 3, respectively. Marigold extract and tomato powder were also used in combination in treatment 4 at 2 g marigold + 20 g tomato powder/kg of diet. At 20 weeks of age, 60 eggs from each treatment were collected and placed in an incubator. After hatching, d-old quail from each group were reared (under standard commercial conditions) up to 14 d of age. They were fed on a low-carotenoid commercial diet. After hatch, at 1, 7 and 14 d, the livers of five young quail from each treatment were assessed for total carotenoid concentration and carotenoid profile. Results indicated that lycopene is transferred from the feed to the egg yolk and further to the liver of the developing embryo. Elevated carotenoid concentration in the egg yolk and correspondingly in the liver of newly hatched quail remains significant during first week posthatch. Lutein and lycopene did not affect vitamin E concentration in the egg yolk or liver of the newly hatched quail. A combination of increased concentrations of lycopene and lutein in the egg yolk results in elevated concentrations of coenzyme Q in the liver of the newly hatched quail.  相似文献   
998.
REASON FOR PERFORMING STUDY: Trimetoquinol (TMQ) is a potent beta-adrenoceptor agonist bronchodilator used in human medicine but has not been evaluated for potential use as a therapeutic agent for horses with 'heaves'. OBJECTIVES: To assess the pharmacodynamics of TMQ in horses with 'heaves' to determine potential therapeutic effects. METHODS: Increasing doses of TMQ were administered to horses with 'heaves' by i.v. and intratracheal (i.t.) routes. Doses ranged 0.001-0.2 microg/kg bwt i.v. and 0.01-2 microg/kg bwt i.t. Cardiac and airways effects were assessed by measurement of heart rate (HR) and maximal change in pleural pressure (deltaPplmax), respectively. Side effects of sweating, agitation and muscle trembling were scored subjectively. Duration of action to i.v. (0.2 microg/kg bwt) and i.t. (2 microg/kg bwt) TMQ was evaluated over 6 h. RESULTS: Intravenous TMQ was an exceptionally potent cardiac stimulant. Heart rate increased at 0.01 microg/kg bwt, and was still increasing after administration of highest dose, 0.2 microg/kg bwt. Airway bronchodilation, measured as a decrease in deltaPplmax, also commenced at 0.01 microg/kg bwt. By the i.t. route, TMQ was 50-100-fold less potent than by i.v. Side effects included sweating, agitation and muscle trembling. Overall, the onset of HR and bronchodilator effects was rapid, within about 3 min, but effects were over at 2 h. CONCLUSION: When administered i.v. and i.t., TMQ is a highly potent cardiac stimulant and a modest bronchodilator. It may not be an appropriate pharmacological agent by i.v. and i.t. routes for the alleviation of signs in horses with 'heaves'. Further studies of TMQ by oral and aerosol routes are necessary. POTENTIAL RELEVANCE: In horses, TMQ is a fast-acting bronchodilator with a short duration of action. It could be used as a rescue agent during an episode of 'heaves'. The i.v. and i.t. administration of TMQ is associated with side effects, similar to those reported for all other beta-agonists. However, other routes, such as aerosol and oral, may prove useful and safe for the alleviation of bronchoconstriction typical of 'heaves'.  相似文献   
999.
A 3.5-yr-old, 2.5-kg female African penguin (Spheniscus demersus) was diagnosed with a choanal squamous cell carcinoma on the basis of biopsy after a history of choanal ulceration and anorexia with weight loss. Therapeutic modalities included the use of intralesional cisplatin followed by localized, topical cryotherapy with liquid nitrogen. The penguin remained free of clinical disease for a period of 9 mo, at which time a second cryotreatment was performed. The patient has remained free of all signs of choanal carcinoma for a period of 13 mo since the second cryotreatment.  相似文献   
1000.
In this study, the cross‐amplification of a commercial multiplex set of 11 cattle (Bos taurus) microsatellites was tested on a panel of 35 European bison (Bison bonasus) individuals. After polymerase chain reaction optimization, all loci cross‐amplified successfully in investigated bisons. Number of alleles and observed and expected heterozygosity per locus are in the range of 2–4, 0.086–0.629 and 0.288–0.621 respectively. The availability of a heterologous set of multiplexed microsatellite markers derived from cattle opens an avenue for collecting profound genetic data for efficient conservation management strategies of the European bison.  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