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   132167篇
  免费   6732篇
  国内免费   855篇
林业   6239篇
农学   4834篇
基础科学   1250篇
  14759篇
综合类   27409篇
农作物   5281篇
水产渔业   5782篇
畜牧兽医   64161篇
园艺   1787篇
植物保护   8252篇
  2018年   1569篇
  2017年   1825篇
  2016年   1671篇
  2015年   1587篇
  2014年   1828篇
  2013年   4654篇
  2012年   3439篇
  2011年   4016篇
  2010年   2807篇
  2009年   2764篇
  2008年   3920篇
  2007年   3763篇
  2006年   3585篇
  2005年   3461篇
  2004年   3119篇
  2003年   3255篇
  2002年   3063篇
  2001年   3573篇
  2000年   3479篇
  1999年   2828篇
  1998年   1250篇
  1997年   1292篇
  1995年   1419篇
  1994年   1270篇
  1993年   1239篇
  1992年   2547篇
  1991年   2738篇
  1990年   2754篇
  1989年   2800篇
  1988年   2624篇
  1987年   2562篇
  1986年   2645篇
  1985年   2556篇
  1984年   2167篇
  1983年   1871篇
  1982年   1339篇
  1979年   2069篇
  1978年   1636篇
  1977年   1514篇
  1976年   1406篇
  1975年   1550篇
  1974年   1967篇
  1973年   1934篇
  1972年   1943篇
  1971年   1857篇
  1970年   1766篇
  1969年   1616篇
  1968年   1338篇
  1967年   1569篇
  1966年   1268篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Information on 936 birth weights (BW), adjusted weaning weights (AWW) and average daily gains (ADG) from Brahman (B), Indubrazil (I), Gyr (G) and commercial Zebu (C) cattle and their crosses were obtained from the records of a farm in the State of Yucatan, Mexico. Two statistical models were used to analyse the data: the first included the effects of year and season of birth, sex of the calf, cow's parity number and breed group. The other model included the same environmental effects as the previous model and the genetic components of breed group (breed additive, breed maternal and heterosis effects). Additive and maternal effects were expressed as deviations from those for the B breed. The means of BW, AWW and ADG for the B breed were 33.3 ± 0.64 kg, 204.0 ± 4.9 kg and 760.06 ± 19.24 g/day, respectively. Both models gave similar results in terms of the variance explained. Additive effects were significant (p < 0.05) for AWW and ADG but not for BW. The minor additive effect was for the C breed (−10.75 kg and −40.77 g for WW and ADG, respectively). Maternal and heterosis effects were not significant (p > 0.05) for any of the traits. The correlation between the breed group means and the means estimated by the prediction equation was r = 0.88. Finally, there was no significant genetic effect on pre-weaning calf performance among the genotypes evaluated. The genetic model can be used to predict the genotype means with a high degree of accuracy. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
992.
OBJECTIVE: To determine clinical, radiographic, and scintigraphic abnormalities in and treatment and outcome of horses with trauma-induced osteomyelitis of the proximal aspect of the radius. DESIGN: Retrospective study. ANIMALS: 5 horses. PROCEDURE: Data collected from the medical records included signalment; history; horse use; degree of lameness; radiographic, ultrasonographic, and scintigraphic findings; treatment; and outcome. RESULTS: Duration of lameness prior to referral ranged from 14 to 60 days. Mean severity of lameness was grade 3 of 5, and all horses had a single limb affected. All horses had signs of pain during elbow joint manipulation and digital palpation over the lateral aspect of the proximal end of the radius. Radiographic lesions consisted of periosteal proliferation, osteolysis, and subchondral bone lysis. Scintigraphy in 3 horses revealed intense pharmaceutical uptake diffusely involving the proximal end of the radius. Two horses had sepsis of the elbow joint. All horses were treated with antimicrobials long-term; 1 horse was also treated by local perfusion of the radial medullary cavity through an indwelling cannulated screw. At follow-up, all horses had returned to their previous function. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that osteomyelitis of the proximal end of the radius can result from a traumatic injury to the antebrachium. Because lesions may be an extension of septic arthritis, a thorough examination of the wound area and elbow joint is recommended. Prolonged systemic antimicrobial treatment can result in a successful outcome.  相似文献   
993.
AIM: To investigate the effect of suberoylanilide hydroxamic acid (SAHA) on the proliferation and apoptosis of human hepatocellular carcinoma HepG2 cells and to explore its possible mechanism. METHODS: HepG2 cells were treated with SAHA at different concentrations for 48 h. The proliferation of HepG2 cells was detected by real-time cellular analysis. The protein levels of acetylated histones H3K9 and H3K27, glucose-regulated protein 78 (GRP78), protein kinase R-like endoplasmic reticulum kinase (PERK) and p-PERK were determined by Western blot. The cell apoptosis was analyzed by flow cytometry. RESULTS: Compared with control group, treatment with SAHA at 0.1 μmol/L and 1 μmol/L for 48 h showed no significant inhibitory effect on the proliferation of HepG2 cells, while SAHA at 6 μmol/L and 12 μmol/L significantly inhibited the proliferation of HepG2 cells (P<0.05). The results of Western blot showed that the protein levels of acH3K9, acH3K27, GRP78 and p-PERK increased significantly after treated with SAHA at diffe-rent concentrations for 48 h, while the protein level of PERK was decreased significantly (P<0.05). The results of flow cytometry analysis showed that the apoptotic rates of the HepG2 cells increased with the increase in SAHA concentration. CONCLUSION: SAHA up-regulates the acetylation of H3K9 and H3K27 in the HepG2 cells and induces apoptosis of HepG2 cells by activating the endoplasmic reticulum stress-related apoptosis pathway.  相似文献   
994.
995.
以72份蝴蝶兰品种为研究对象,对其叶片、花梗和花器官相关的34个表型性状进行测定与评价,通过表型多样性分析、聚类分析和主成分分析等方法,探讨其种质资源表型性状的遗传多样性。结果表明:72份蝴蝶兰品种的绝大多数性状呈现变异丰富、类型多样的特性,数量性状遗传多样性变异范围为16.39%~157.36%,质量性状Shannon−Wiener多样性指数范围为0.38~1.32,其中叶片的数量性状变异程度较低,但其质量性状的多样性水平较高;R型聚类分析将34个性状分为3个大类,第I类群包含了花部和叶部性状,表明花与叶的表型联系较紧密,第II类群和第III类群包含花序长、最长叶长、植株大小和花序梗长,表明这4个表型性状呈独自进化关系; Q型聚类分析将72份蝴蝶兰种质资源分为4大类,其中第II类可细分为7个亚类群,II−1、II−2亚类群可作为大花和中花育种的亲本,II−3、II−4、II−5和II−7亚类群可作为小花育种的亲本,II−4亚类群可作为香花育种的亲本,同时蝴蝶兰‘JB5342’‘JB5184’‘JB5541’‘JB3697’‘安娜’和‘JB5725’等品种与多数供试蝴蝶兰品种遗传距离较远,可作为重要亲本参考。主成分分析表明,花宽、花瓣长、花瓣宽、萼片长、花长和萼片宽的特征向量绝对值较高,是造成蝴蝶兰表型变异的主要因素。  相似文献   
996.
997.
Rabies diagnosis in western Canada, 1985-1989   总被引:1,自引:0,他引:1       下载免费PDF全文
The results of laboratory examination of 20460 specimens for the diagnosis of rabies by the standard fluorescent antibody and mouse inoculation tests over a five year period are presented. Specimens were received from British Columbia, Alberta, Saskatchewan, Manitoba, and the Yukon and Northwest Territories. Of those examined, 9.96% were positive. The main reservoirs of rabies were skunks, bats, and foxes. During this observation period a rabies epizootic occurred in skunks in Saskatchewan. When both tests were performed, the fluorescent antibody test was found to agree with the mouse inoculation test in over 99% of cases.  相似文献   
998.
Yearling heifers were given SC injections of 10(8) (n = 40), 10(9) (n = 44), or 10(10) (n = 44) colony-forming units of Brucella abortus strain 19 (S19). The proportion of heifers with positive serologic test results at 1 month following vaccination increased as the dose of S19 increased. These proportions decreased with time, and all heifers had negative card, rivanol, and complement fixation test results within 4 months. Positive ELISA results persisted beyond 4 months in all three S19 dose groups; however, all heifers were ELISA-negative within 9 months after vaccination. Comparable lymphocyte transformation activity was stimulated by S19 dose of 10(9) or 10(10) and approximately half of the heifers in both groups had a positive stimulation index at 9 months. Immunity of the pregnant heifers was challenged 9 months after vaccination with 10(7) B abortus strain 2308 as follows: diluent controls (n = 69); 10(8) B abortus S19 (n = 40); 10(9) B abortus S19 (n = 39); and 10(10) B abortus S19 (n = 39). Tissue specimens from heifers were obtained at parturition and necropsy for culturing of B abortus. The proportion of heifers that developed brucellosis, ie, had positive culture results, increased as gestation days at challenge exposure increased. The effect of gestational age was controlled in the analysis using logistic regression. The relative risk of brucellosis was reduced to 0.38, 0.15, and 0.06 for B abortus S19 doses of 10(8), 10(9), and 10(10), respectively, compared with diluent controls at 1.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
999.
非洲猪瘟(African swine fever,ASF)是由非洲猪瘟病毒(African swine fever virus,ASFV)感染家猪和野猪而引起的一种急性烈性、高度接触性传染病,到目前为止仍未研制出有效疫苗,因此病原的早发现对ASF防控尤为重要.ASFV的病原学检测主要包括活病毒检测、核酸检测和抗原检测....  相似文献   
1000.
Heat stress (HS) mitigation strategies are critically needed to combat the substantial economic effects on animal agriculture. The manifestations of seasonal infertility include delayed puberty onset, reduced conception rates, decreased litter size, and increased wean to estrus interval. To assess the effects of HS during early gestation and evaluate the benefit of supplemental altrenogest (ALT) as a mitigation strategy, 30 crossbred postpubertal gilts (157 ± 11 kg body weight) were subjected to estrous synchronization via 14 d oral administration of ALT. Artificial insemination during estrus was performed, and gilts were then placed into one of four treatment groups: HS (35 ± 1 °C for 12 h/31.60 ± 1 °C for 12 h) with (HSALT, n = 7) or without (HSCON, n = 7) 15 mg/d ALT supplementation or thermal neutral (TN; 20 ± 1 °C) conditions with (TNALT, n = 8) or without (TNCON, n = 8) 15 mg/d ALT supplementation until 12 d post-estrus (dpe). Administrating ALT occurred at 0600 hours from 3 to 12 dpe, and rectal temperatures (TR) and respiration rates (RR) were recorded. Blood was collected via jugular venipuncture on 0, 4, 8, and 12 dpe. Gilts were euthanized humanely at 12 dpe followed by the collection of ovarian tissue, and uterine flushing for conceptus collection. In HS compared with TN gilts, RR and TR were increased (P < 0.01) but unaffected by ALT supplementation. Feed intake was reduced (P < 0.01) by HS but unaltered by the ALT treatment. Corpora lutea (CL) weight was reduced (P < 0.01) in HSCON gilts when compared with TNCON and HSALT gilts despite progesterone concentrations in serum and luteal tissue not being affected by treatment (P ≥ 0.10). CL diameter was reduced (P ≤ 0.05) in HSALT gilts compared with other treatments. Interleukin-1β (IL1B) uterine flush concentration was not affected (P > 0.20) by environment or ALT supplementation, although moderate (P = 0.06) interaction between environment and ALT existed, as IL1B concentration in TNALT was increased (P = 0.03) compared with TNCON gilts. While environment did not affect conceptus development (P = 0.90), ALT supplementation advanced conceptus elongation (P < 0.01). Collectively, these data demonstrate that HS may affect luteal development before pregnancy establishment, and ALT increases conceptus elongation by 12 dpe.  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