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   687篇
  免费   24篇
  国内免费   2篇
林业   56篇
农学   21篇
基础科学   3篇
  136篇
综合类   93篇
农作物   75篇
水产渔业   37篇
畜牧兽医   236篇
园艺   14篇
植物保护   42篇
  2022年   7篇
  2021年   14篇
  2020年   20篇
  2019年   9篇
  2018年   14篇
  2017年   7篇
  2016年   8篇
  2015年   14篇
  2014年   20篇
  2013年   29篇
  2012年   29篇
  2011年   45篇
  2010年   19篇
  2009年   20篇
  2008年   39篇
  2007年   39篇
  2006年   20篇
  2005年   41篇
  2004年   29篇
  2003年   37篇
  2002年   28篇
  2001年   7篇
  2000年   8篇
  1999年   12篇
  1998年   4篇
  1995年   6篇
  1994年   6篇
  1993年   5篇
  1992年   6篇
  1991年   7篇
  1990年   9篇
  1989年   9篇
  1988年   6篇
  1987年   6篇
  1986年   13篇
  1985年   6篇
  1984年   9篇
  1980年   4篇
  1979年   8篇
  1978年   5篇
  1975年   4篇
  1974年   4篇
  1971年   3篇
  1969年   4篇
  1966年   4篇
  1962年   3篇
  1961年   4篇
  1960年   3篇
  1955年   4篇
  1913年   3篇
排序方式: 共有713条查询结果,搜索用时 15 毫秒
101.
Acute and long-term effects of a single, relatively high oral dose (0.25 and 0.30 mg/kg) of sodium monofluoroacetate (1080) on the survival and productivity of sheep were evaluated to establish a better understanding of 1080 poisoning and identify more specific changes diagnostic of toxicosis. In survivors, clinical signs of acute 1080 toxicosis such as salivation and lethargy were generally very mild. Fasted animals were more prone to 1080 toxicity. In animals that died, more severe signs, including tachypnoea, dyspnoea, and tremors occurred for 15-20 min prior to death. 1080 concentrations were highest in the blood > heart > skeletal muscle > liver. 1080 could not be detected in any of these organs of the animals that survived. Serum citrate concentrations were elevated for 4 days after dosing. No clinical or biochemical abnormalities were found in any animal after 4 days. Histopathological lesions were most marked in the heart and lung with inflammation, necrosis, and scattered foci of fibrous tissue in the myocardium, pulmonary oedema and inflammation of the lung. No adverse long-term effects on general health or reproductive performance were observed in any sheep that survived the first 4 days following exposure to 1080. The most reliable diagnostic indicators of 1080 exposure in sheep were measurement of its residues in blood, skeletal muscle and ruminal contents, increased serum citrate concentration, elevated heart rate, and characteristic electrocardiograph changes (up to 4 days after exposure). Death from 1080 is most likely to occur within 96 h, and animals that survived this period appeared normal.  相似文献   
102.
The optimum duration of test for the measurement of ADG, ADFI, feed:gain ratio [which is the reciprocal of the efficiency of gain (G:F) and therefore increases as the efficiency of gain decrease and vice versa], and residual feed intake was examined in growing pigs. Data from 144 hybrid (mainly Large White x Landrace) pigs involved in a longitudinal (n = 54) and serial slaughter (n = 90) experiment were used. The pigs were housed in individual pens from 70 +/- 1 d of age (mean +/- SD) and fed ad libitum a pelleted commercial diet. Feed intake and BW data on pigs that had a minimum of 10-wk records were partitioned into a 14-d adjustment and a 56-d test period. Phenotypic correlations among weekly measurements were used to examine the repeatability of each trait. Changes in phenotypic residual variance and correlation using shortened (7-, 14-, 21-, 28-, 35-, 42-, and 49-d) tests compared with the full-length 56-d test were used as criteria to assess the optimum test duration. The results of the phenotypic correlations among weekly measurements indicated that ADFI, which was characterized by moderate to high correlations (0.41 to 0.81), was more repeatable than ADG, which was characterized by low correlations (0.00 to 0.43). Mean gut fill (n = 107) was 4.2% of BW but was characterized by large variation among the pigs (SD = 1.8; CV = 42.2%). This variation in gut fill was a major contributor to the low repeatability of the measurement of ADG. These repeatability results indicated that ADG, rather than ADFI, will determine the optimum duration of test for the feed efficiency traits. The results of the shortened relative to the full-length test indicate that for growing pigs under good nutrition and ad libitum feeding, a 28-d test was adequate for the measurement of feed intake, whereas a 35-d test was required to measure ADG, feed:gain ratio, and residual feed intake without compromising the accuracy of measurement.  相似文献   
103.
Variance components (VC) were estimated for the semen production trait ejaculate volume, sperm concentration and sperm motility in the Swiss cattle breeds Brown Swiss (BS), Original Braunvieh (OB), Holstein (HO), Red‐Factor‐Carrier (RF), Red Holstein (RH), Swiss Fleckvieh (SF) and Simmental (SI). For this purpose, semen production traits from 2,617 bulls with 124,492 records were used. The data were collected in the years 2000–2012. The model for genetic parameter estimation across all breeds included the fixed effects age of bull at collection, year of collection, month of collection, number of collection per bull and day, interval between consecutive collections, semen collector, bull breed as well as a random additive genetic component and a permanent environmental effect. The same model without a fixed breed effect was used to estimate VC and repeatabilities separately for each of the breeds BS, HO, RH, SF and SI. Estimated heritabilities across all breeds were 0.42, 0.25 and 0.09 for ejaculate volume, sperm concentration and sperm motility, respectively. Different heritabilities were estimated for ejaculate volume (0.42; 0.45; 0.49; 0.40; 0.10), sperm concentration (0.34; 0.30; 0.20; 0.07; 0.23) and number of semen portions (0.18; 0.30; 0.04; 0.14; 0.04) in BS, HO, RH, SF and SI breed, respectively. The phenotypic and genetic correlations across all breeds between ejaculate volume and sperm concentration were negative (?0.28; ?0.56). The other correlations across all breeds were positive. The phenotypic and genetic correlations were 0.01 and 0.19 between sperm motility and ejaculate volume, respectively. Between sperm motility and sperm concentration, the phenotypic and genetic correlations were 0.20 and 0.36, respectively. The results are consistent with other analyses and show that genetic improvement through selection is possible in bull semen production traits.  相似文献   
104.
105.
106.
Soluble solids comprise most of onion bulb dry mass, and dehydrator onion cultivars are developed from breeding populations that have high dry mass content. Realized and narrow-sense heritability estimates were obtained for the soluble solids content (SSC) trait in two open-pollinated dehydrator onion breeding populations (BP) using response to selection and half-sib family analysis. Parental populations, designated as BP9335-U and BP9243-U, were derived from two-way crosses of lines advanced as open- pollinated (OP) populations to the F7 or F_6 generation, respectively. BP9335-U had one previous selection cycle for increased SSC and BP9243-U had three SSC selection cycles. In these experiments, parental populations were screened again for high SSC, and selected bulbs were intermated to form half-sib progeny groups, designated as BP9335-S and BP9243-S. Mean SSC was increased by 6.6% and a realized heritability estimate of 0.64 was obtained for BP9335-S. Mean SSC was increased by 6.3% and a realized heritability estimate of 0.36 was obtained for BP9243-S. Narrow-sense heritability estimates of 0.58 ±0.05 and 0.30 ±0.03 were obtained for parental populations BP9335-U and BP9243-U, respectively. Narrow-sense heritability estimates of 0.40 ± 0.03 and 0.63 ± 0.23 were obtained for progeny populations derived from selected high-SSC bulbs of these lines (BP9335-S and BP9243-S), respectively, indicating that there is significant additive genetic control of the SSC trait in these populations. Significant differences in half-sib family performance in the advanced groups BP9335-S and BP9243-S demonstrate that progeny testing was effective for evaluating phenotypic selections. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
107.
Sugar accumulation is a very important physiological process that determines dessert-melon fruit quality. Considerable variation in the sugar content and composition in the mature flesh of Cucumis melo L. fruits was observed among 56 genotypes which represent the wide range of morphological and horticultural types found in this species. Sucrose accumulation was observed not only among 'dessert melons' of the inodorus and cantalupensis types, but also in representatives of other subgroups of C. melo, including some of the accessions from agrestis and conomon groups. Among the genotypes that accumulate sucrose, the levels of this sugar, and not of the hexoses, were correlated with the total sugar concentration. Hexose levels were correlated with total sugar levels only among low-sugar genotypes that did not accumulate sucrose. Activities of the sucrose hydrolyzing enzymes acid invertase (EC 3.2.1.26) and alkaline invertase (EC 3.2.1.27) were measured in the mature fruit flesh and the results of this survey support the hypothesis that low acid invertase activity is a prerequisite for sucrose accumulation. The sucrose accumulating, high sugar genotypes had less-acidic flesh pH values (pH> 6) than the low-sugar genotypes. The latter were characterized by a broader range of pH values, including acidic (pH< 5.5) fruit flesh.  相似文献   
108.
109.
A field trial was conducted in 1994 to determine the foliar deposit of tebufenozide (RH5992), applied aerially, and its efficacy against spruce budworm, Choristoneura fumiferana (Clem.). A commercial 240 g litre-1 formulation of the insecticide (Mimic 240LV) was mixed with water, dyed with a tracer dye (Rhodamine WT) and sprayed with a light fixed-wing aircraft. Six application strategies were tested. Five used 70 g AI ha-1 in a spray volume of 1 or 2 litre-1 ha-1 with single or double applications; the sixth was an unsprayed control. Results show that the spectra of the spray applications were, with one exception, fairly uniform. Volume and number median diameters ranged from 100 to 130 μm and 27 to 72 μm, respectively. Mean number of drops cm-2 on Kromekote cards were <2·0 for strategies where either 1 or 2 litre ha-1 were sprayed. Nevertheless no one strategy produced droplet densities that were significantly different (P<0·05) from the other strategies. Tebufenozide recovered from foliage averaged 2·5 to 5·9 μg g foliage-1 when 1 litre ha-1 was sprayed and 5·8 to 6·8 μg g foliage-1 after 2 litre ha-1 were sprayed. When a single application was the strategy used, the mean number of droplets cm-2 and μg tebufenozide g foliage-1 ranged from 1·2 to 1·4 and 2·5 to 5·9, respectively. With double applications, the same response parameters ranged from 0·3 to 1·9 and 2·5 to 6·8, respectively. Budworm population reductions (%) and the number of larvae that survived tebufenozide treatments were significantly different (P<0·05) from the controls. After strategies that used 1 litre spray ha-1, mean percentage population reductions ranged from 61·4 to 93·6 whereas populations were reduced by 85·6 to 98·3% when 2 litre ha-1 were sprayed. After double applications the mean percentage population reductions ranged from 93·6 to 98·3, but single application strategies resulted in mean reductions of 61 to 86%. Mean population reductions in the controls were 61%. The mean number of larvae per branch that survived spray strategies of 1 litre ha-1 ranged from 1·3 to 7·4, and from 0·4 to 1·3 when 2 litre ha-1 was the spray volume. In the controls an average of 10·2 larvae survived. With one exception, mean percentage defoliation in the treated areas was also significantly less (P<0·05) than that in the control. Mean defoliation in trees sprayed at 1 litre spray ha-1 ranged from 40 to 62·8% whereas those treated at 2 litre ha-1 had mean defoliation levels from 31·5 to 62·8%. In contrast, average defoliation in the controls was 92·1%. When a single application was the spray strategy, mean defoliation ranged from 31·5 to 62·8%. These data imply that a double application of tebufenozide at 70 g in 2 litre ha-1 was the most efficacious strategy. However, analyses of the data also show that the primary influence on deposits and defoliation was interactions between number of applications and spray. Nevertheless the two independent variables acted without significant interactions when influencing percentage reductions of spruce budworm populations. © 1998 SCI  相似文献   
110.
Extreme environments are hostile for most organisms, but such habitats represent suitable settings to be inhabited by specialized microorganisms. A marine shallow-water hydrothermal vent field is located offshore in northeast Taiwan, near the shallow shore of the southeast of Kueishantao Island (121°55′ E, 24°50′ N). Research on extremophilic microorganisms makes use of the biotechnological potential associated with such microorganisms and their cellular products. With the notion that extremophiles are capable of surviving in extreme environments, it is assumed that their metabolites are adapted to function optimally under such conditions. As extremophiles, they need specific culture conditions, and only a fraction of species from the original samples are recovered in culture. We used different non-selective and selective media to isolate bacterial species associated with the hydrothermal vent crab Xenograpsus testudinatus and the sediments of its habitat. The highest number of colonies was obtained from Zobell marine agar plates with an overall number of 29 genetically distinct isolates. 16S RNA gene sequencing using the Sanger sequencing method revealed that most of the bacterial species belonged to the phylum Firmicutes and the class Bacilli. The present study indicates that hydrothermal vent bacteria and their secondary metabolites may play an important role for the reconstruction of the evolutionary history of the phylum Procaryota.  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