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   83704篇
  免费   4138篇
  国内免费   61篇
林业   4143篇
农学   2606篇
基础科学   447篇
  9138篇
综合类   16580篇
农作物   3495篇
水产渔业   3984篇
畜牧兽医   41251篇
园艺   1104篇
植物保护   5155篇
  2018年   997篇
  2017年   1064篇
  2016年   1015篇
  2015年   900篇
  2014年   1118篇
  2013年   2923篇
  2012年   2045篇
  2011年   2577篇
  2010年   1653篇
  2009年   1560篇
  2008年   2527篇
  2007年   2514篇
  2006年   2344篇
  2005年   2306篇
  2004年   2155篇
  2003年   2208篇
  2002年   2054篇
  2001年   2299篇
  2000年   2299篇
  1999年   1933篇
  1998年   772篇
  1997年   769篇
  1995年   894篇
  1993年   763篇
  1992年   1540篇
  1991年   1598篇
  1990年   1724篇
  1989年   1679篇
  1988年   1561篇
  1987年   1508篇
  1986年   1570篇
  1985年   1558篇
  1984年   1293篇
  1983年   1188篇
  1982年   797篇
  1981年   739篇
  1979年   1274篇
  1978年   1010篇
  1977年   884篇
  1976年   833篇
  1975年   949篇
  1974年   1226篇
  1973年   1256篇
  1972年   1265篇
  1971年   1188篇
  1970年   1109篇
  1969年   1012篇
  1968年   871篇
  1967年   912篇
  1966年   842篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
942.
943.
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   
944.
Sclerotinia sclerotiorum, causal agent of white mould, is the most destructive and widely distributed soilborne pathogen of common bean during the autumn–winter season in Brazil. Nevertheless, little is known about the genetic structure of the pathogen population. Microsatellite (SSR) markers and mycelial compatibility groups (MCGs) were used to characterize 118 isolates collected from 20 bean fields located in the most important growing regions of Minas Gerais State (MG). Additionally, the genetic variability among 10 isolates obtained from a single sclerotium was investigated in 10 different sclerotia. Seventy SSR haplotypes and 14 MCGs were identified among the 118 isolates. The genetic differences within bean growing areas accounted for most of the genetic variation (72%). Despite the relatively high genotypic diversity, the SSR loci were at linkage disequilibrium. Moreover, 70% of the isolates were assigned to only two MCGs, and haplotypes of a given MCG were closely related. The discriminant analysis of principal components revealed five groups. There was strong genetic differentiation between isolates collected in one municipality in southern MG when compared to other regions. Common bean resistance to white mould should be assessed with representative isolates of the five genetic groups and, if possible, of the different MCGs detected in the present study. One to five haplotypes were detected among the 10 isolates obtained from a single sclerotium. Therefore, in order to ensure genetic identity of an isolate, hyphal tip or monoascosporic isolates should be used.  相似文献   
945.
946.
THOMAS  R. C.; MILLER  H. G 《Forestry》1994,67(4):329-341
In an 11-year-old stand of Sitka spruce (Picea sitchensis [Bong.]Carr.) application of nitrogen fertilizers, at a rate of 10kgN ha–1 month–1, increased mean diameter incrementby 12 per cent, while the further addition of phosphorus, at5 kg ha–1 month–1, resulted in a 23 per cent increase.An attack by the green spruce aphid (Elatobium albietinum Walker)occurred during the period of fertilizer addition. The mostseverely affected trees showed a reduction in diameter growthof 50 to 56 per cent but the severity of the attack betweentrees was unrelated to the treatments applied. However, fertilizerapplication did hasten the recovery of diameter growth afterdefoliation.  相似文献   
947.
OBJECTIVE: To explore the effect of BRAC (formerly Bangladesh Rural Advancement Committee) initiatives towards promoting gender and social equity in health among children of poor mothers who are BRAC members. DESIGN: A cohort of 576 children from the prospective study of a BRAC- International Centre for Diarrhoeal Disease Research, Bangladesh joint research project was analysed. Data were collected three times during 1995-1996 with approximately 4-month intervals. Stunting, defined as height-for-age below minus two standard deviations from the reference median, was the outcome health measure. The study children were stratified into three groups according to their mother's social and BRAC membership status: poor and BRAC member (BM), poor non-member (TG) and non-poor non-member (NTG). SETTING: Matlab, rural area of Bangladesh. SUBJECTS: Children aged 6-72 months. RESULTS: The overall prevalence of stunting was 76%; the highest prevalence was found among TG (84.6%) children and no significant difference was observed between BM and NTG children (67.3% and 69.4%, respectively). In all groups, a significantly larger proportion of girls was stunted compared with boys in the first round. Group-level analysis showed that stunting decreased among all children except BM boys at the end of third round, with the largest decline among BM girls. In contrast, stunting prevalence increased among BM boys. A similar trend was found in the individual-level analysis, where a larger proportion of BM girls recovered from stunting compared with other groups and no recovery was observed among BM boys. At the end of the third round, the nutritional status of BM girls was almost equal to that of the BM boys, while gender inequity remained large among TG and NTG children. CONCLUSION: The BRAC initiative appeared to contribute to a significant equity gain in health for girls, as well as to decreased differences in ill health between the poor and the non-poor.  相似文献   
948.
Variations in the seroprevalence of antibody to Babesia bigemina infection by farm and animal level risk factors were investigated for 2 contrasting regions of Tanga and Iringa in Tanzania. Tanga is situated in the eastern part of the country and has typical tropical coast climate while Iringa is situated in the Southern Highlands and has a tropical highland climate. Two hundred farms from each region were selected using simple random sampling procedure and visited once between January 1999 and April 1999. Blood samples were collected from 1329 smallholder dairy animals on selected farms for harvesting serum which was subsequently used for serodiagnosis of B. bigemina using an indirect enzyme linked immuno-sorbent assay (ELISA). Of the 1329 sera samples screened, 34.9 % were positive for B. bigemina. The prevalence was higher in Iringa Region [43%, 95% confidence intervals (CI) = 39.5-47.3] than in Tanga Region (27%, CI = 23.6-30.5). Using a logistic binomial regression model as an analytical method for predicting the likelihood of animal seropositivity, we found (in both regions) that the risk of positive reaction varied with the animal's age, history of grazing and geographical location. Seroprevalence increased with age (beta = 0.01 and 0.01 per year of age, P < 0.005 in Tanga and Iringa, respectively). Animals located in Lushoto and Iringa urban district were associated with increased risk of seropositivity [Odds ratio (OR) = 4.24, P = 0.001, for Lushoto, and OR = 1.81, P = 0.040, for Iringa Urban, respectively). Animals grazed 3 months prior to sampling had higher odds for seropositivity than zero/semi-grazed, despite farmer-reported high frequency of tick control (OR = 2.71, P = 0.0087, for Tanga, and OR = 4.53, P = 0.001, for Iringa). Our study suggests that even though herd sizes are small, B. bigemina infection is widespread in many smallholder dairy farms and endemic stability with respect to this disease has not yet been attained, but the observed levels are sufficiently high to ensure that clinical disease would be a risk.  相似文献   
949.
950.
Including Baytril, in various parts of the world many commercial preparations of enrofloxacin for parenteral administration are being employed for the treatment of bacterial diseases in cows. To optimize clinical responses and to minimize development of bacterial resistance to this agent, the copied pharmaceutical preparations must comply with some key pharmacokinetic features when bioequivalence studies are performed. To assess whether or not there was bioequivalence among nine commercial preparations of enrofloxacin and the original one, a controlled pharmacokinetic study was carried out. These was done utilizing the microbiological agar-diffusion test as quantitative/qualitative analytical method. A non-compartmental model defined kinetic variables. Results for Baytril revealed that maximal serum concentration (Csmax) was only matched by one preparation while area under the curve (AUC) of the serum concentration/activity of enrofloxacin and metabolites in time was not matched by any preparation. Time to Csmax (Tmax), elimination half-life, and shape of the time-serum concentrations of enrofloxacin profiles obtained for the nine generic preparations differ significantly somehow from the corresponding data obtained for the reference enrofloxacin. The need for studies to demonstrate bioequivalence becomes mandatory if similar preparations of enrofloxacin become commercially available. Enrofloxacin should be used selectively and cautiously to limit development of bacterial resistance. Non-bioequivalence of relevant pharmacokinetic values, such as Csmax and bioavailability (AUC) would facilitate development of bacterial resistance and limit the useful life span of this antibacterial agent.  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