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   17037篇
  免费   898篇
  国内免费   17篇
林业   828篇
农学   558篇
基础科学   122篇
  2505篇
综合类   2664篇
农作物   911篇
水产渔业   950篇
畜牧兽医   8309篇
园艺   197篇
植物保护   908篇
  2020年   161篇
  2019年   182篇
  2018年   310篇
  2017年   365篇
  2016年   261篇
  2015年   198篇
  2014年   316篇
  2013年   665篇
  2012年   496篇
  2011年   620篇
  2010年   428篇
  2009年   410篇
  2008年   628篇
  2007年   552篇
  2006年   513篇
  2005年   453篇
  2004年   420篇
  2003年   430篇
  2002年   369篇
  2001年   486篇
  2000年   475篇
  1999年   373篇
  1998年   180篇
  1997年   201篇
  1996年   156篇
  1995年   192篇
  1993年   164篇
  1992年   280篇
  1991年   313篇
  1990年   307篇
  1989年   312篇
  1988年   289篇
  1987年   246篇
  1986年   295篇
  1985年   253篇
  1984年   209篇
  1983年   207篇
  1979年   250篇
  1978年   165篇
  1977年   155篇
  1975年   171篇
  1974年   234篇
  1973年   215篇
  1972年   251篇
  1971年   230篇
  1970年   203篇
  1969年   192篇
  1968年   158篇
  1967年   160篇
  1966年   161篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
952.
Eight purified diets were fed to juvenile white sturgeon, Acipenser transmontanus Rick, for 9 weeks to investigate the effect of dietary lipids on the fatty acid composition of phospholipids and triglycerides from muscle, liver and brain. The diets contained 150 g kg?1 of oils from canola, corn, cod liver, lard, linseed, soybean, safflower, or a control mixture (corn oil/cod liver oil/lard, 1:1:1, by wt). Dietary lipids significantly (P≤ 05) affected the composition of tissue triglycerides and phospholipids. Tissue triglyceride fatty acid composition ranged widely, in parallel with the dietary lipids, while phospholipids changes were more conservative. Brain phospholipid fatty acid composition was less responsive to diet compared with that in muscle and liver. Considerable amounts of n-6 and n-3 long chain polyun-saturated fatty acids (> C20) were found in triglycerides and phospholipids with all diets, demonstrating that white sturgeon can desaturate and elongate linoleic acid (18:2n–6) and linolenic acid (18:3n–3). Further, the products of the Δ6 desaturase, i.e. 18:3 n–6 and 18:4n–3, were relatively abundant in triglyceride, suggesting that the Δ6 desaturase might not be a limiting step in the process in white sturgeon. Nevertheless, accumulation of both EPA and DHA was greater in the sturgeon fed fish oil than those fed linseed oil, indicating that muscle triglyceride EPA and DHA levels are best enhanced by diets rich in preformed EPA and DHA.  相似文献   
953.
Abstract. The effects of temperature on the activity of antisera from coho salmon ( oncorhynchus kisutch ) and from rabbit were compared. Agglutination activity of salmon anti-Aeromonas salmonicida serum declined following incubation at 55°C whereas rabbit antiserum lost agglutination activity after incubation at 65°C.  相似文献   
954.
955.
Susceptibility of crops to drought may change under atmospheric CO2 enrichment. We tested the effects of CO2 enrichment and drought on the older malting barley cultivar Golden Promise (GP) and the recent variety Bambina (BA). Hypothesizing that CO2 enrichment mitigates the adverse effects of drought and that GP shows a stronger response to CO2 enrichment than BA, plants of both cultivars were grown in climate chambers. Optimal and reduced watering levels and two CO2 concentrations (380 and 550 ppm) were used to investigate photosynthetic parameters, growth and yield. In contrast to expectations, CO2 increased total plant biomass by 34 % in the modern cultivar while the growth stimulation was not significant in GP. As a reaction to drought, BA showed reduced biomass under elevated CO2, which was not seen in GP. Grain yield and harvest index (HI) were negatively influenced by drought and increased by CO2 enrichment. BA formed higher grain yield and had higher water‐use efficiency of grain yield and HI compared to GP. CO2 fertilization compensated for the negative effect of drought on grain yield and HI, especially in GP. Stomatal conductance proved to be the gas exchange parameter most sensitive to drought. Photosynthetic rate of BA showed more pronounced reaction to drought compared to GP. Overall, BA turned out to respond more intense to changes in water supply and CO2 enrichment than the older GP.  相似文献   
956.
The European Union (EU) countries are searching for new ways to certify meat free of Trichinella; however, with the expansion of the EU, the acceptance of a unilateral method is complicated by the variability of pig and human trichinellosis among EU countries, where significantly higher prevalence rates have been observed in the newly added eastern countries. Several attempts have been made to define Trichinella-free areas, but certification of Trichinella-free pig production farms appears to be the only feasible approach. The increasing prevalence of the non-encapsulating species, Trichinella pseudospiralis, in game, domestic pigs and humans has eliminated the compression technique from the new EU legislation to be enacted in 2006. Also, the observation that several species of Trichinella tolerate freezing in horse meat for up to 4 weeks has forced a change in legislation as well where freezing is no longer an option for certifying horse meat. Because current serological detection methods are not suited for meat inspection, classical direct detection methods and inactivation by freezing remain the methods of choice for pork. It has been proposed, therefore, to automate direct inspection methods as a cost effective alternative to certify pig farms free of Trichinella.  相似文献   
957.
The purpose of this study was to determine whether intravaginal prostaglandin F2 alpha (PGF2 alpha) would be effective for the treatment of metritis or pyometra in the bitch. Seventeen bitches with metritis or pyometra were treated with PGF2 alpha. Prostaglandin F2 alpha (150 micrograms/kg body weight) was administered once or twice daily by infusing 0.3 ml per 10 kg body wt into the vaginal lumen. Bitches were also treated with amoxicillin (15 mg/kg body wt/48 h) and/or gentamicin (4 mg/kg body wt/day) administered as intramuscular (i.m.) injections. Fifteen bitches were treated successfully with intravaginally administered PGF2 alpha for 3 to 12 days and with intramuscularly administered antibiotics for 4 to 12 days. Success of treatment was judged by cessation of vaginal discharge, the absence of fluid in the uterus as determined by ultrasonography, and the overall health status of the animal. As two bitches with pyometra showed clinical deterioration in spite of medical treatment, ovariohysterectomy was performed after the first and the second treatment, respectively. No side effects (salivation, vomiting, diarrhoea, hyperpnoea, ataxia, urination, anxiety, pupillary dilatation followed by contraction) were observed after PGF2 alpha treatment. The disease did not recur during the subsequent oestrous cycles within 12 months after the initial treatment. The results demonstrate that intravaginal administration of PGF2 alpha was effective in 13 dogs (86.6%) with metritis or pyometra, and caused no side effects. Although the study was based on a relatively small number of cases, it is concluded that prostaglandin F2 alpha can be a useful means of treating bitches with metritis or pyometra. However, in severe cases of pyometra ovariohysterectomy is needed.  相似文献   
958.
Quality effects of rye (Secale cerealeL.) chromosome arm 1RL transferred to wheat (Triticum aestivumL.) were characterised by comparison of a group 1R(1B) substitution lines and 1BL.1RS translocation lines. The experimental materials were sister lines derived from the mating Mironovskaya10/NE7060//NE80413. 1R(1B) substitution lines were identified by the presence of ryeomega- andgamma-secalins among 70% ethanol soluble proteins, combined with the presence of high molecular weight (HMW) secalin proteins in total grain protein extracts. Genes on 1RL reduced grain weight, grain hardness, Mixograph time, Mixograph tolerance and SDS sedimentation volumes. 1RL had no effect on flour yield or grain and flour protein concentrations. The HMW secalin proteins encoded by genes on 1RL most probably caused the decline in dough strength seen in 1R(1B) lines relative to that of 1BL.1RS sister lines. Reduced grain hardness might also be related to the presence of HMW secalins, although a role for additional, unidentified genes on 1RL could not be discounted.  相似文献   
959.
1. This study was conducted to determine the effects of heat stress on fearfulness, leucocyte components, oxidative stress and lipid peroxidation in two commercial broiler strains, Cobb (C) and Ross (R). 2. At 36 and 37 d of age birds were exposed to 38 +/- 1 degree C for 3 h. Rectal temperatures, duration of tonic immobility (TI), haematocrit values, proportions of leucocyte components (heterophil, lymphocyte, basophil, eosinophil, monocyte), malondialdehyde (MDA) concentrations and antioxidant enzyme activities (CAT, SOD, GPx) of all the birds were determined, before and after heat treatment. 3. Rectal temperatures increased and haematocrit values decreased in birds exposed to heat stress. Heat stress caused a significant increase in heterophil/lymphocyte and in basophil ratios. 4. Exposing birds to heat stress increased duration of TI, suggesting heat-stressed birds tended to be more fearful. 5. Heat stress resulted in a significant Genotype x Treatment interaction for MDA concentration. CAT, SOD and GPx activities; MDA concentrations in heat-stressed R strain birds were greater than in heat-stressed C strain birds.  相似文献   
960.
We develop a method for multiscale estimation of pollutant concentrations, based on a nonparametric spatial statistical model. We apply this method to estimate nitrate concentrations in groundwater over the mid-Atlantic states, using measurements gathered during a period of 10 years. A map of the fine-scale estimated nitrate concentration is obtained, as well as maps of the estimated county-level average nitrate concentration and similar maps at the level of watersheds and other geographic regions. The fine-scale and coarse-scale estimates arise naturally from a single model, without refitting or ad hoc aggregation. As a result, the uncertainty associated with each estimate is available, without approximations relying on high spatial density of measurements or parametric distributional assumptions.  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