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   16377篇
  免费   5340篇
  国内免费   464篇
林业   1153篇
农学   3263篇
基础科学   458篇
  1416篇
综合类   6502篇
农作物   1826篇
水产渔业   402篇
畜牧兽医   2068篇
园艺   808篇
植物保护   4285篇
  2024年   32篇
  2023年   387篇
  2022年   615篇
  2021年   641篇
  2020年   776篇
  2019年   632篇
  2018年   532篇
  2017年   806篇
  2016年   874篇
  2015年   833篇
  2014年   983篇
  2013年   988篇
  2012年   1185篇
  2011年   1266篇
  2010年   1069篇
  2009年   1024篇
  2008年   981篇
  2007年   985篇
  2006年   911篇
  2005年   790篇
  2004年   674篇
  2003年   604篇
  2002年   494篇
  2001年   514篇
  2000年   414篇
  1999年   360篇
  1998年   282篇
  1997年   287篇
  1996年   306篇
  1995年   295篇
  1994年   240篇
  1993年   230篇
  1992年   245篇
  1991年   173篇
  1990年   151篇
  1989年   143篇
  1988年   93篇
  1987年   77篇
  1986年   39篇
  1985年   35篇
  1984年   27篇
  1982年   35篇
  1981年   19篇
  1980年   27篇
  1979年   20篇
  1978年   20篇
  1977年   21篇
  1976年   12篇
  1962年   10篇
  1955年   12篇
排序方式: 共有10000条查询结果,搜索用时 21 毫秒
1.
2.
The effects of six new synthetic carbamates on fully engorged females of four Rhipicephalus microplus strains (one reference strain susceptible to conventional ixodicides, two strains multiresistant to ixodicides and one tick field isolate) were compared. In addition, the effect of two other new synthetic carbamates was tested on larvae from the same strains. The first six tested carbamates reduced egg laying and inhibited egg hatching in the four studied strains (P < 0.05). Compared with untreated females, the eggs produced by the treated engorged female ticks of all strains had a dark, dry, opaque appearance and were less adherent. The remaining two tested carbamates induced larval mortality in all of the evaluated strains. The three studied R. microplus strains displayed 50% resistance ratios (RR50) of less than 2 when compared to the susceptible reference strain. These results demonstrate that both carbamates with a larvicidal effect and carbamates that inhibit egg laying and embryo development are efficacious against tick strains that are resistant to commercial ixodicides, no cross resistance was observed.  相似文献   
3.
The in vivo faecal egg count reduction test (FECRT) is the most commonly used test to detect anthelmintic resistance (AR) in gastrointestinal nematodes (GIN) of ruminants in pasture based systems. However, there are several variations on the method, some more appropriate than others in specific circumstances. While in some cases labour and time can be saved by just collecting post-drench faecal worm egg counts (FEC) of treatment groups with controls, or pre- and post-drench FEC of a treatment group with no controls, there are circumstances when pre- and post-drench FEC of an untreated control group as well as from the treatment groups are necessary. Computer simulation techniques were used to determine the most appropriate of several methods for calculating AR when there is continuing larval development during the testing period, as often occurs when anthelmintic treatments against genera of GIN with high biotic potential or high re-infection rates, such as Haemonchus contortus of sheep and Cooperia punctata of cattle, are less than 100% efficacious. Three field FECRT experimental designs were investigated: (I) post-drench FEC of treatment and controls groups, (II) pre- and post-drench FEC of a treatment group only and (III) pre- and post-drench FEC of treatment and control groups.To investigate the performance of methods of indicating AR for each of these designs, simulated animal FEC were generated from negative binominal distributions with subsequent sampling from the binomial distributions to account for drench effect, with varying parameters for worm burden, larval development and drench resistance. Calculations of percent reductions and confidence limits were based on those of the Standing Committee for Agriculture (SCA) guidelines. For the two field methods with pre-drench FEC, confidence limits were also determined from cumulative inverse Beta distributions of FEC, for eggs per gram (epg) and the number of eggs counted at detection levels of 50 and 25. Two rules for determining AR: (1) %reduction (%R) < 95% and lower confidence limit <90%; and (2) upper confidence limit <95%, were also assessed. For each combination of worm burden, larval development and drench resistance parameters, 1000 simulations were run to determine the number of times the theoretical percent reduction fell within the estimated confidence limits and the number of times resistance would have been declared.When continuing larval development occurs during the testing period of the FECRT, the simulations showed AR should be calculated from pre- and post-drench worm egg counts of an untreated control group as well as from the treatment group. If the widely used resistance rule 1 is used to assess resistance, rule 2 should also be applied, especially when %R is in the range 90 to 95% and resistance is suspected.  相似文献   
4.
5.

Background

In EMS‐associated laminitis, laminar failure may occur in response to energy failure related to insulin resistance (IR) or to the effect of hyperinsulinemia on laminar tissue. 5′‐Adenosine‐monophosphate‐activated protein kinase (AMPK) is a marker of tissue energy deprivation, which may occur in IR.

Hypothesis/Objectives

To characterize tissue AMPK regulation in ponies subjected to a dietary carbohydrate (CHO) challenge.

Animals

Twenty‐two mixed‐breed ponies.

Methods

Immunohistochemistry and immunoblotting for total AMPK and phospho(P)‐AMPK and RT‐qPCR for AMPK‐responsive genes were performed on laminar, liver, and skeletal muscle samples collected after a 7‐day feeding protocol in which ponies stratified on body condition score (BCS; obese or lean) were fed either a low‐CHO diet (ESC + starch, approximately 7% DM; n = 5 obese, 5 lean) or a high‐CHO diet (ESC + starch, approximately 42% DM; n = 6 obese, 6 lean).

Results

5′‐Adenosine‐monophosphate‐activated protein kinase was immunolocalized to laminar keratinocytes, dermal constituents, and hepatocytes. A high‐CHO diet resulted in significantly decreased laminar [P‐AMPK] in lean ponies (P = .03), but no changes in skeletal muscle (lean, P = .33; obese, P = .43) or liver (lean, P = .84; obese, P = .13) [P‐AMPK]. An inverse correlation existed between [blood glucose] and laminar [P‐AMPK] in obese ponies on a high‐CHO diet.

Conclusions and Clinical Importance

Laminar tissue exhibited a normal response to a high‐CHO diet (decreased [P‐AMPK]), whereas this response was not observed in liver and skeletal muscle in both lean (skeletal muscle, P = .33; liver, P = .84) and obese (skeletal muscle, P = .43; liver, P = .13) ponies.  相似文献   
6.
大菱鲆选育家系抗鳗弧菌性能   总被引:5,自引:2,他引:3  

基于家系选育技术开展了大菱鲆(Scophthalmus maximus)抗鳗弧菌病选育研究, 2010年构建的37个选育二代家系中选择成活率高的30个家系进行鳗弧菌(Vibrio anguillarum)感染实验, 开展对鳗弧菌的抗病性研究。结果表明, 不同家系间抗鳗弧菌感染能力存在着显著差异(P0.05); 30个家系中, 鳗弧菌感染后, 12个家系的存活率达到65% 以上, 其余家系的存活率则低于65%; 通过bCOX回归分析, 计算各家系的优势比, 优势比最高的5个家系的存活率达到65% 以上; 各家系的成活率、优势比和死亡历时差4项指标进行聚类分析, 优势比最高且存活率达到65% 以上的5个家系聚为一类。综合感染家系的高成活率、高优势比以及聚类分析的结果, 选育出5个抗病力较强的优良家系。选育出的抗病力较强的家系可做为抗鳗弧菌选育的核心育种群体, 为抗鳗弧菌病的传代选育奠定良好的基础。

  相似文献   
7.
Two trials, including firstly, diets incorporating individual or combined species of probiotics, Bacillus subtilis E20 and Lactobacillus plantarum 7‐40, were fed to the mud crab, Scylla paramamosian, for 28 days to evaluate the growth, immunity and disease resistance, and secondly, the optimal level of B. subtilis E20 in the diet by tracking the growth, immunity and disease resistance after 28 days of feeding were analysed. No significant differences in growth, total haemocyte counts, respiratory bursts, superoxide dismutase, or glutathione peroxidase were found in the two trials. Crabs fed the B. subtilis E20‐containing diet (D20) had significantly increased phenoloxidase (PO) activity, phagocytic activity (PA) and disease resistance compared with crabs fed the control and the mixed probiotics diet (MD). The mortality of crabs fed the D20 was significantly lower than that of crabs fed the Lplantarum 7‐40‐containing diet. L. plantarum 7‐40 had a great inhibitory effect on the growth of B. subtilis E20, which may have led to the decreased probiotic effect of the MD. An analysis of the optimal level of B. subtilis E20 in the diet showed that crabs fed the B. subtilis E20‐containing diet at a level of 109–10 cfu kg?1 had better immune response of PO and PA, and disease resistance in the second trial.  相似文献   
8.
The effect of fish protein hydrolysate (FPH) on the growth, feed utilization, digestive enzymes, gut microflora and resistance to bacterial challenge in Persian sturgeon (Acipenser persicus) larvae during a 45‐day feeding trial has been investigated. Artificial diets containing different levels of tuna visceral protein hydrolysate (PH0, PH10, PH25 and PH50) and live foods (Artemia: 5 days, Daphnia: 40 days) were fed to Persian sturgeon larvae. Larvae fed on the PH10 and PH25 had growth, feed utilization, protein content and trypsin activity significantly better than PH0 and PH50 groups (< 0.05). Condition factors, survival rate and pepsin activity were not significantly different (> 0.05). Brush border membrane enzymes to cytosolic enzyme ratio revealed that maturation had been occurred in 41 days post hatch (dph) at moderate FPH levels. In 54 dph larvae, this ratio was not affected by the level of FPH in the diet. No significant differences were observed in the total number of gut microflora or in the resistance to Aeromonas hydrophila (> 0.05), whereas lactic acid bacteria was significantly higher in PH50 (< 0.05). The results of this study show that application of moderate levels of FPH in Persian sturgeon diets could enhance larvae performance due to the early maturation in intestinal.  相似文献   
9.
Use of immunostimulants as a prophylactic measure against diseases in fish is considered as an effective alternative to antibiotic use. Plant‐derived immunostimulants have recently received more attention, as most of them are cost‐effective and eco‐friendly. This study was carried out with the objective of evaluating the possible immunostimulatory activity of intraperitoneally injected water soluble fraction of Nyctanthes arbortristis (an Indian medicinal plant) leaves on (i) specific immunity (antibody response), (ii) nonspecific immunity (Lysozyme activity, ROS production) and (iii) functional immunity in terms of disease resistance against Aeromonas hydrophila in Oreochromis mossambicus. The results of the study showed that all the tested doses (i.e. 3.2, 16, 80 and 400 mg kg?1) of water soluble fraction of Nyctanthes arbortristis leaves significantly enhanced both primary and secondary antibody responses to heat killed A. hydrophila. Significant enhancement in serum lysozyme activity by all the doses of water soluble fraction was observed on day 10. The lower doses of 3.2 and 16 mg kg?1 of water soluble fraction alone enhanced intracellular ROS production. Fish treated with both single and double dose of water soluble fraction showed maximum disease resistance. The highest dose (400 mg kg?1) of water soluble fraction exhibited highest relative per cent survival in both single dose and double dose groups.  相似文献   
10.
Field trials with perennial crops give rise to repeated measurements taken on the same plot on several occasions. It is important to account for serial correlation among repeated measurements in such trials. This study illustrates the use of mixed models for this purpose. We consider the analysis of trials conducted at several locations and the combination of trials with different starting years. A key issue in the analysis is the distinction between effects of calendar years, which are associated with external environmental variation, and harvest years, which represent internal yield formation processes of the perennial crop. Two methods of two‐stage analysis are compared with single‐stage analysis. It is shown that results of two‐stage analysis are very similar to those of single‐stage analysis, if serial correlation is properly taken into account. Program code for the MIXED procedure of the SAS system is given in an Appendix S1 for all considered cases.  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