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   89756篇
  免费   5529篇
  国内免费   46篇
林业   3739篇
农学   3467篇
基础科学   632篇
  11248篇
综合类   13537篇
农作物   3360篇
水产渔业   4939篇
畜牧兽医   47628篇
园艺   1165篇
植物保护   5616篇
  2020年   732篇
  2019年   953篇
  2018年   1583篇
  2017年   1847篇
  2016年   1660篇
  2015年   1404篇
  2014年   1718篇
  2013年   3717篇
  2012年   2931篇
  2011年   3588篇
  2010年   2339篇
  2009年   2330篇
  2008年   3471篇
  2007年   3208篇
  2006年   3136篇
  2005年   2670篇
  2004年   2576篇
  2003年   2592篇
  2002年   2373篇
  2001年   3390篇
  2000年   3304篇
  1999年   2553篇
  1998年   983篇
  1997年   1051篇
  1996年   883篇
  1995年   1022篇
  1994年   895篇
  1993年   897篇
  1992年   1845篇
  1991年   1919篇
  1990年   1856篇
  1989年   1859篇
  1988年   1608篇
  1987年   1595篇
  1986年   1627篇
  1985年   1526篇
  1984年   1226篇
  1983年   1032篇
  1982年   713篇
  1979年   1060篇
  1978年   816篇
  1975年   816篇
  1974年   898篇
  1973年   864篇
  1972年   806篇
  1971年   795篇
  1970年   801篇
  1969年   780篇
  1968年   708篇
  1967年   734篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
The spontaneous EEG and somatosensory evoked potentials (SEPs) were examined in chickens before and after electrical stunning using a waterbath stunner. Fifty-four per cent of the birds became epileptic and lost their SEPs, and 17% were non-epileptic and appeared to retain their SEPs. It was concluded that there was a reasonably close association between the presence of epileptiform activity in the EEG and the absence of SEPs following electrical stunning, but that the absence of SEPs could be preferred as an indicator of an effective stun on conceptual grounds.  相似文献   
982.
OBJECTIVE: To detect and partially characterize papillomavirus (PV) DNA in squamous cell carcinoma (SCC) tumor specimens from cats. SAMPLE POPULATION: 54 formalin-fixed paraffinembedded skin biopsy specimens were examined. Specimens originated from Bowenoid in situ SCC (BISC; n = 21), invasive SCC (22), and skin affected by miscellaneous nonneoplastic conditions (11). PROCEDURES: Samples from each tissue block underwent DNA extraction after deparaffinization, and PCR assays were performed. Two sets of primers derived from PV E1 were used. The first set of primers was designed for the narrow-range PCR assay and was able to generate amplification products of feline PV (FePV), canine oral PV, or closely related PVs. The second set of primers was selected for the broad-range PCR assay because of its ability to amplify DNA from 64 human PVs. Sequence analysis of each amplified DNA was performed. RESULTS: 1 of the 21 specimens of BISC was positive for PV DNA on the basis of narrow-range PCR assay results, whereas all the other specimens (BISC, invasive SCC, and controls) had negative results for PV DNA. In contrast, 5 of 21 BISC specimens and 4 of 22 invasive SCC specimens were positive for PV DNA on the basis of broad-range PCR assay results. Sequence analysis revealed that only 1 specimen was infected by a virus closely related to classic FePV. In the 8 other specimens positive for PV DNA, DNA of unknown PVs was uncovered. CONCLUSIONS AND CLINICAL RELEVANCE: Bowenoid in situ SCC and invasive SCC of cats may be associated with PVs of genetic diversity.  相似文献   
983.
Multilineage potential of adult human mesenchymal stem cells   总被引:7,自引:0,他引:7  
Human mesenchymal stem cells are thought to be multipotent cells, which are present in adult marrow, that can replicate as undifferentiated cells and that have the potential to differentiate to lineages of mesenchymal tissues, including bone, cartilage, fat, tendon, muscle, and marrow stroma. Cells that have the characteristics of human mesenchymal stem cells were isolated from marrow aspirates of volunteer donors. These cells displayed a stable phenotype and remained as a monolayer in vitro. These adult stem cells could be induced to differentiate exclusively into the adipocytic, chondrocytic, or osteocytic lineages. Individual stem cells were identified that, when expanded to colonies, retained their multilineage potential.  相似文献   
984.
985.
The objectives were (i) to evaluate the effect of temperament, determined by modified 2‐point chute exit and gait score, on artificial insemination (AI) pregnancy rates in beef heifers following fixed time AI and (ii) to determine the effect of temperament on cortisol, substance‐P, prolactin and progesterone at initiation of synchronization and at the time of AI. Angus beef heifers (n = 967) at eight locations were included in this study. At the initiation of synchronization (Day 0 = initiation of synchronization), all heifers received a body condition score (BCS), and temperament score (0 = calm; slow exit and walk or 1 = excitable; fast exit or jump or trot or run). Blood samples were collected from a sub‐population of heifers (n = 86) at both synchronization initiation and the time of AI to determine the differences in serum progesterone, cortisol, prolactin and substance‐P concentrations between temperament groups. Heifers were synchronized with 5‐day CO‐Synch+ controlled internal drug release (CIDR) protocol and were inseminated at 56 h after CIDR removal. Heifers were examined for pregnancy by ultrasound 70 days after AI to determine AI pregnancy. Controlling for synchronization treatment (p = 0.03), facility design (p = 0.05), and cattle handling facility design by temperament score interaction (p = 0.02), the AI pregnancy differed between heifers with excitable and calm temperament (51.9% vs 60.3%; p = 0.01). The alley‐way with acute bends and turns, and long straight alley‐way had lower AI pregnancy rate than did the semicircular alley‐way (53.5%, 56.3% and 67.0% respectively; p = 0.05). The serum hormone concentrations differed significantly between different types of cattle handling facility (p < 0.05). The cattle handling facility design by temperament group interactions significantly influenced progesterone (p = 0.01), cortisol (p = 0.01), prolactin (p = 0.02) and substance‐P (p = 0.04) both at the initiation of synchronization and at the time of AI. Inter‐ and intra‐rater agreement for temperament scoring were moderate and good (Kappa = 0.596 ± 0.07 and 0.797 ± 0.11) respectively. The predictive value for calm and pregnant to AI was 0.87, and excited and non‐pregnant to AI was 0.76. In conclusion, the modified 2‐point temperament scoring method can be used to identify heifers with excitable temperament. Heifers with excitable temperament had lower AI pregnancy. Further, cattle handling facility design influenced the temperament and AI pregnancy.  相似文献   
986.
An intra-abdominal testicular tumour was diagnosed in a 10-year-old, male, cryptorchid golden retriever. Upon surgical removal, torsion of the testis was found. The histopathological diagnosis was seminoma. The ultrasound examination in this case revealed a parenchymal mass lesion. An ultrasound examination is a useful supplemental diagnostic tool for differentiation of abdominal mass lesions.  相似文献   
987.
988.
989.
Trace minerals feeding had significant effects on sperm production and fertility with better absorption and proper utilization within the body for optimum reproductive function. Several studies have shown that more influenced trace elements in the diets of animals are copper (Cu) and zinc (Zn). Bucks showing deficiency of this mineral might affect the quality of semen production which in turn would affect the fertility. This experiment was thus designed to test the effects of organic Cu and Zn supplementation on antioxidants enzyme activities and sperm functional attributes in fresh semen of bucks. Forty bucks (n = 40, Aged 5 months) were assigned to ten groups of four animals in each group, supplemented (for a period of 8 months) with different levels of organic Zn: 20 mg (T2), 40 mg (T3) and 60 mg (T4), organic Cu: 12.5 mg (T5), 25 mg (T6), 37.5 mg (T7) and combined organic Zn and Cu: 20 + 12.5 mg (T8), 40 + 25 mg (T9), 60 + 37.5 mg (T10), respectively, per kg dry matter and no additional mineral diet (control; T1). One hundred and sixty semen samples were collected through electro‐ejaculator and analysed for sperm quantity, quality, acrosome intactness and plasma membrane integrity and correlated with the catalase, superoxide dismutase, glutathione peroxidase and glutathione reductase enzyme activities in seminal plasma. The results indicated organic Cu and zinc supplemented bucks produced more sperm cells, had higher sperm concentrations, maintained higher (< .01) sperm livability, plasma membrane and acrosome integrities, more motility and velocity. The increased antioxidant enzyme activities, reduced oxidative stress and lowered lipid peroxidation were positively correlated (< .05) with the sperm functional attributes. In conclusion, organic Cu and Zn supplement to male goats showed protective roles against oxidative damage and maintained better fresh semen characteristics.  相似文献   
990.
In 1956 a disease of nursing pigs previously unreported in Denmark, was observed in several swine herds. The disease syndrome appeared identical with that later described by Roe and Alexander in 1958 and by S. F. Cartwright and M. Lucas in 1968. Since 1956 there have been no reports of such outbreaks in Danish herds. To investigate whether the virus is still present, a survey was carried out to detect neutralizing antibodies in sera from a number of boars using a micro technique. In 144 sera from Jutland 12% had titers ranging from 10 to 32 ND-50, 22% from 32 to 90, 37% from 90 to 256, 14% from 256 to 724 and 3% above 724 ND-50 with a total of 86% positive. Of 174 sera from the rest of the country 16% had titers ranging from 10 to 32 ND-50, 18% from 32 to 90, 28% from 90 to 256, 13% from 256 to 724 and 3% exceeding 724 ND-50 with a total of 79% positive.  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