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   3929篇
  免费   125篇
  国内免费   12篇
林业   277篇
农学   165篇
基础科学   27篇
  641篇
综合类   215篇
农作物   252篇
水产渔业   466篇
畜牧兽医   1582篇
园艺   83篇
植物保护   358篇
  2024年   38篇
  2023年   56篇
  2022年   96篇
  2021年   170篇
  2020年   214篇
  2019年   224篇
  2018年   219篇
  2017年   214篇
  2016年   185篇
  2015年   126篇
  2014年   174篇
  2013年   246篇
  2012年   255篇
  2011年   270篇
  2010年   141篇
  2009年   135篇
  2008年   176篇
  2007年   177篇
  2006年   127篇
  2005年   94篇
  2004年   92篇
  2003年   100篇
  2002年   111篇
  2001年   62篇
  2000年   56篇
  1999年   32篇
  1998年   24篇
  1997年   16篇
  1996年   20篇
  1995年   18篇
  1994年   10篇
  1992年   18篇
  1991年   6篇
  1990年   7篇
  1989年   11篇
  1988年   11篇
  1986年   7篇
  1985年   8篇
  1984年   11篇
  1983年   7篇
  1982年   6篇
  1981年   8篇
  1980年   7篇
  1979年   10篇
  1977年   6篇
  1976年   4篇
  1975年   6篇
  1973年   5篇
  1969年   4篇
  1948年   4篇
排序方式: 共有4066条查询结果,搜索用时 15 毫秒
111.
The financial impact and clinical expression of vesicular stomatitis in Colorado dairy herds was studied during the 1982 epidemic. A non-random sample of 15 dairy herds was surveyed, 13 of which had clinical cases in lactating cows. These 13 herds represented 2404 cows among which were 378 clinical cases. The dairymen reported a distribution of lesions among the cases as follows: 262 (69.3%) oral only; 87 (23.0%) teat only; 22 (5.8%) both oral and teat and 7 (1.9%) with foot lesions. Herds experiencing primarily oral lesions had an attack rate of 19.8% with an average clinical course of 23.8 days. The attack rate in 2 of the 4 herds with teat lesions was 55.8% and 1.6% in the other 2 herds. Mastitis complicated 72% of the cases with teat lesions.The direct costs resulting from 378 cases of vesticular stomatitis reported by 13 dairymen totaled $95 752, which is an average cost of $253.31 per clinical case. The average cost per case with oral lesions is estimated to be $174.06 in contrast to $568.22 average for cases with teat lesions. The greatest loss was due to cows culled, 36.6% of the total direct costs. Decreased milk production was second with 30.3% of the total.The average decrease in milk production per clinical case was 261.3 kg (576 lbs). Deaths accounted for 11.3% of the loss and the combined costs of drugs, extra labour, veterinary service, weight loss and extra ectoparasite control accounted for 11.9% of the $95 752 total.  相似文献   
112.
113.
Beetles secrete an array of chemicals generally believed to mitigate attack by predators. Methylcyclopentanoid monoterpenes secreted by larvae of the willow leaf beetle, Plagiodera versicolora, deter feeding by conspecific adults. Furthermore, the secretion elicits a strong repugnancy response in larvae of another willow herbivore, Nymphalis antiopa. Leaves bearing beetle larvae are less likely than leaves not bearing beetles to be frequented and consumed by Nymphalis larvae. Predator defense may not be the sole function of glandular secretions produced by herbivorous insects; secretions may also mediate interactions among herbivores that use a common resource.  相似文献   
114.
Sixty-three Escherichia coli strains isolated from broilers with respiratory problems were examined for virulence factors, hemolysin synthesis ability, motility, hemagglutination capacity, operon pap presence, colicin production, and serum resistance. The capacity to hemagglutinate guinea pig erythrocytes was found in 53 (84.1%) of the samples, but only 30 (47.6%) agglutinated chicken erythrocytes. D-mannose-sensitive hemagglutination against guinea pig erythrocytes was found in 19 (30.2%) samples and against chicken erythrocytes, in 15 (23.8%) samples, whereas the D-mannose-resistant hemagglutination with guinea pig erythrocytes was found in 34 (54%) samples, and 13 of these (20.6%) showed this characteristic against chicken erythrocytes. Operon pap, P fimbria codifier, was detected in 26 samples in a total of 34 D-mannose-resistant samples. Colicin production was observed in 55 (87.3%) of the strains, and 41.8% presented V colicin production. Of the samples analyzed, 56 (88.9%) presented serum resistance, six (9.5%) were intermediate, and only one (1.6%) was sensitive to the action of the complement. The diversity of virulence profiles detected in the samples in this study explains in part the multifactorial characteristics of avian colibacillosis.  相似文献   
115.
Fruit-set of ‘Golden Delicious’ apple trees was found to be largely independent of seasonal differences and much more closely related to the initial number of blossom clusters, when compared with many other apple cultivars under English conditions. In one young orchard, yield became increasingly a function of trunk girth during the first 3 cropping-years, but the trees then abruptly developed a biennial rhythm. Thereafter, blossom production was negatively correlated with the number of clusters in the previous year rather than with the number of fruits.Applications of daminozide or GA3 appeared to increase or decrease, respectively, the amount of return bloom without altering the negative correlation with the number of clusters in the previous year. Chemical fruit-thinning with a mixture of carbaryl and NAA also appeared to be effective in increasing return bloom.  相似文献   
116.
The natural lipophilic amides piperine and piperiline were isolated from Piper nigrum L (Piperaceae). Piperine was hydrolysed into piperic acid (85% yield) which was converted into 16 amides (28–89% yield). The contact toxicity of all synthetic amides, and also that of piperine and piperiline, at the dose 10 µg per insect, was evaluated for the Brazilian economically important insects Ascia monuste orseis Latr, Acanthoscelides obtectus Say, Brevicoryne brassicae L, Protopolybia exigua DeSaus and Cornitermes cumulans Kollar. The results demontrated that the insects have different sensivities to the various amides, with mortality ranging from 0 to 97.5% according to the compound and insect species. © 2000 Society of Chemical Industry  相似文献   
117.
This paper reports on two cases of laparoscopic nephrotomy employed in the treatment of canine dioctophymosis, which is considered a unusual procedure and a new treatment proposal heretofore not performed in veterinary medicine. Two patients were treated, one with a history of hematuria and the other with incidental finding of the parasite in the abdominal cavity during elective ovariohysterectomy. Both dogs were subjected to abdominal ultrasound, which produced images indicating the presence of the parasite in the right side kidney, but with partial parenchymal preservation. The patients were therefore subjected to laparoscopic nephrotomy. The surgical procedure was effective in treating dioctophymosis and enabled minimum tissue invasion during surgery, in addition to preservation of the kidney.  相似文献   
118.
This study was performed to evaluate the structural preservation of antral follicles after bovine ovarian tissue vitrification using histological analysis. Ovaries (n = 30) of slaughtered cows were cut into small fragments using a scalpel blade, and the ovarian tissues were randomly assigned to vitrification using 15% dimethyl sulphoxide (DMSO) and 15% ethylene glycol (EG) and fresh tissues (control) groups. For histological evaluations, fresh and post‐thawing ovarian tissues were immediately fixed, serially sectioned into 5‐μm sections and stained with haematoxylin and eosin (H&E). Nine serial sections per fragment were subjected for morphological assessment. The diameter of the antral follicles was determined and classified into four groups: 1 (≤1 mm), 2 (>1–2 mm), 3 (>2–3 mm) and 4 (>3–4 mm). Then, follicular morphology was evaluated in relation to atresia and categorized into seven grades: Grade A (healthy follicle); Grades B, C and D (early atresia); Grades E and F (moderate atresia); and Grade G (advanced atresia). The results revealed that small diameters of antral follicles (1 and 2 mm) were more susceptible for cryoinjury. The normal follicular morphology (Grade A) was not affected by vitrification throughout follicle diameters. Nevertheless, some damage features were monitored after vitrification. In conclusion, the morphological structure of bovine antral follicles could be successfully preserved by ovarian tissue vitrification.  相似文献   
119.
In this study, we evaluate the use of macroalgae as vectors of probiotics bacteria into the digestive tract of abalone to improve their survival and growth. It is shown that when abalone Haliotis rufescens of different sizes were fed with a natural diet composed of fronds of the macroalga Macrocystis integrifolia supplemented with a mixture of Vibrio sp. C21‐UMA, Agarivorans albus F1‐UMA and Vibrio sp. F15‐UMA bacteria, there was a significant increase (P<0.05) in the average monthly growth rate and survival (%)in a period of 210 days, compared with the control without a probiotic supplement. The permanence of the probiotics in the digestive tract of the animals was monitored, and it was found that the number of culturable C21‐UMA and F1‐UMA bacteria decreased significantly (P<0.05) in recently weaned and adult abalone, and they almost disappeared completely on day 19 of the bioassay. However, the culturable Vibrio sp. F15‐UMA disappeared completely from the digestive tract on day 22 of the bioassay, and these were the bacteria that remained at the highest concentration compared with the other two bacterial strains over the experimental period. It is therefore shown that it is feasible to use a probiotic mixture to improve the profitability of the H. rufescens culture.  相似文献   
120.
This work studied the potential biotechnological applications of a naviculoid diatom (IMA053) and a green microalga (Tetraselmis marina IMA043) isolated from the North Adriatic Sea. Water, methanol, and dichloromethane (DCM) extracts were prepared from microalgae biomass and evaluated for total phenolic content (TPC) and in vitro antioxidant properties. Biomass was profiled for fatty acid methyl esters (FAME) composition. The DCM extracts had the highest levels of total phenolics, with values of 40.58 and 86.14 mg GAE/g dry weight (DW in IMA053 and IMA043, respectively). The DCM extracts had a higher radical scavenging activity (RSA) than the water and methanol ones, especially those from IMA043, with RSAs of 99.65% toward 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)diammonium salt (ABTS) at 10 mg/mL, and of 103.43% against 2,2-diphenyl-1-picrylhydrazyl (DPPH) at 5 mg/mL. The DCM extract of IMA053 displayed relevant copper chelating properties (67.48% at 10 mg/mL), while the highest iron chelating activity was observed in the water extract of the same species (92.05% at 10 mg/mL). Both strains presented a high proportion of saturated (SFA) and monounsaturated (MUFA) fatty acids. The results suggested that these microalgae could be further explored as sources of natural antioxidants for the pharmaceutical and food industry and as feedstock for biofuel production.  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