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   5690篇
  免费   376篇
  国内免费   2篇
林业   420篇
农学   234篇
基础科学   18篇
  1437篇
综合类   237篇
农作物   420篇
水产渔业   615篇
畜牧兽医   2093篇
园艺   120篇
植物保护   474篇
  2023年   63篇
  2022年   104篇
  2021年   258篇
  2020年   252篇
  2019年   289篇
  2018年   257篇
  2017年   258篇
  2016年   241篇
  2015年   183篇
  2014年   259篇
  2013年   376篇
  2012年   454篇
  2011年   468篇
  2010年   251篇
  2009年   222篇
  2008年   345篇
  2007年   391篇
  2006年   260篇
  2005年   209篇
  2004年   191篇
  2003年   168篇
  2002年   172篇
  2001年   51篇
  2000年   37篇
  1999年   34篇
  1998年   20篇
  1997年   24篇
  1996年   13篇
  1995年   15篇
  1994年   17篇
  1993年   11篇
  1992年   13篇
  1991年   13篇
  1990年   11篇
  1989年   15篇
  1988年   11篇
  1987年   9篇
  1986年   10篇
  1985年   11篇
  1984年   10篇
  1983年   8篇
  1982年   4篇
  1980年   9篇
  1978年   3篇
  1977年   3篇
  1976年   3篇
  1973年   5篇
  1972年   4篇
  1969年   6篇
  1966年   5篇
排序方式: 共有6068条查询结果,搜索用时 468 毫秒
91.
A gas chromatographic method using a capillary column is described for the determination of total amitraz residue in honey samples. The method consists of a preliminary extraction of the sample, an alkaline hydrolysis, an extraction of the 2, 4-dimethylaniline formed, without the aid of steam distillation/continuous extraction, its derivatisation with heptafluorobutyric anhydride and a gas chromatographic determination using an electron capture detector. The method is very rapid and can be carried out in a test tube. The detection limit is 0.020 mg kg?1 and recoveries are of the order of 85%.  相似文献   
92.
A new class of compounds derived from hydroquinone ethers showed high and predominantly ovicidal activity against the two-spotted spider mite, Tetranychus urticae Koch. The sequence of compounds synthesised and the factors which appeared to have contributed most to the discovery of these promising acaricides are presented.  相似文献   
93.
94.
95.
The aim of the present study was to investigate genetic parameters for racing time in Thoroughbred horses racing at distances between 1000 and 1600 m subdivided into 100‐m intervals. The data provided by TURFETOTAL Ltda comprised races that occurred in the Gávea and Cidade Jardim race tracks over a period of 11 years (1992–2002) and consisted of 32 145 races and 238 890 time records. The variance components necessary to obtain the heritability and repeatability estimates of the traits studied were estimated with the MTDFREML program, and animal age at race (3 years old or younger, 4, 5 and older than 5 years), sex (male and female), number of races (1–32 145), and postposition at start (1–11) as fixed effects, and animal and permanent environmental random effects were included in a one‐trait animal model. Males were significantly superior to females at all distances. Excluding the 1100 m distance, animals 4 years of age were significantly faster than the mean of the other ages for all distances analysed. Horses older than 5 years showed a significantly lower performance than the mean of the other ages for all distances analysed, except for the 1100 m. Postpositions one and two did not differ significantly from one another for any of the distances analysed. These two inner positions both together varied from the other positions depending on race length. The components of additive genetic and permanent environmental variance varied in a similar way, tending to decrease with increasing racing distance, and the other temporary environmental variance almost doubled from 1000 to 1600 m. As was the case for the additive genetic and environmental variances, heritability and repeatability estimates tended to decrease with increasing distance, indicating that selection based on racing time will be less successful when the racing distance increases.  相似文献   
96.
Preference for goat characteristics was determined from 42 traders in six markets in Pangasinan, an important trading center for goats in Luzon, the Philippines, and analyzed using a hedonic price model to determine whether these characteristics are important price determinants of goats. This information is important to goat raisers to respond to the needs of the local market, and thus to achieve a better price and higher income in goat raising. Meatiness, size, breed, sex and age (in descending order) were the dominant goat characteristics preferred by traders. Meatiness had the largest positive influence on price, with a price premium of US$6.24 (US$1.00 = PhP 53), whereas the age of the animal had the smallest influence at US$3.72. However, size and meatiness are manifestations of good breeds. Therefore, continued efforts to improve breeds in order to produce good quality goats are imperative to cater to market needs. Different characteristics implied different price premiums. Traders offered a higher price premium for better goat characteristics; thus, the improvement of these characteristics could result in higher returns for goat raisers.  相似文献   
97.
The macroscopic and microscopic features of a retroperitoneal perirenal pseudocyst in a 12-month-old ram without impairment of renal function are described. In humans and animals, uriniferous pseudocysts may be of traumatic origin, resulting from rupture of kidney, renal pelvis, or ureter, or congenital. Lymphatic pseudocysts may develop secondary to inflammatory obstruction of the hilar lymphatics after perinephritis or renal transplantation. In this case, histologic characteristics of the pseudocyst wall were suggestive of development from the parietal peritoneal layer encapsulating the kidney. This is the first case of retroperitoneal perirenal pseudocyst in a sheep.  相似文献   
98.
Calcidiol (25-hydroxyvitamin D) values are reported for 22 wild Ricord's iguanas (Cyclura ricordii) and seven wild rhinoceros iguanas (Cyclura cornuta cornuta). Calcitriol (1,25-hydroxyvitamin D) values are reported for 12 wild Ricord's iguanas and seven wild rhinoceros iguanas. These animals were captured as part of a larger health assessment study being conducted on Ricord's iguanas in Isla Cabritos National Park, Dominican Republic. A total of 13 captive rhinoceros iguanas held outdoors at Parque Zoológico Nacional were also sampled for comparison. Mean concentrations of 25-hydroxyvitamin D were 554 nmol/L (222 ng/ml) with a range of 250-1,118 nmol/L (100-448 ng/ml) for wild Ricord's iguanas, 332 nmol/L (133 ng/ml) with a range of 260-369 nmol/L (104-148 ng/ml) for wild rhinoceros iguanas, and 317 nmol/L (127 ng/ml) with a range of 220-519 nmol/L (88-208 ng/ml) for captive rhinoceros iguanas. On the basis of these results, serum concentrations of at least 325 nmol/L (130 ng/ml) for 25-hydroxyvitamin D should be considered normal for healthy Ricord's and rhinoceros iguanas.  相似文献   
99.
The effects of four pesticides (three herbicides, atrazine, paraquat, glyphosate, and an insecticide, carbaryl) on the activity and kinetics of an invertase (from yeast), a urease (from jack bean) and an acid phosphatase (from potato) were investigated. Glyphosate and paraquat showed a marked activation effect on invertase activity. From the dependence of Vmax and Km parameters on pesticide concentrations, a mixed-type activation mechanism was suggested. The kinetic behaviour of urease and acid phosphatase appeared to be uninfluenced by both pesticides. Methanol, used as solvent for both carbaryl and atrazine, inhibited invertase, urease and acid phosphatase with, respectively, competitive, non-competitive and uncompetitive mechanisms. The extent of the inhibition was different, as estimated by the values of the inhibition constant K1. An additional inhibition effect of urease and invertase activities was achieved in the presence of carbaryl, whereas no influence was detected on the activity and kinetics of acid phosphatase. Finally, atrazine was shown to alter the kinetics of the enzymes only at higher concentrations.  相似文献   
100.
An understanding of seasonal dynamics in the free-living phase of the tropical horse tick, Anocentor nitens, is vital for effective control measures. This study examines seasonal changes in the oviposition and incubation periods, eclosion rates, reproductive success, and larval survival in the free-living phase during the period June 1995-July 1997. Ten engorged female ticks, placed individually in wire-gauze tubes, were exposed in a field of Brachiaria decumbens fortnightly each month, and monitored to measure the duration of the various reproductive parameters, and success. Larval abundance was estimated indirectly, through the percentage of females with viable progeny, and directly by using a flagging method. Duration of the free-living cycle ranged from 12.3 weeks (October and December) to 23.5 weeks (March). The cycle was prolonged in cold and dry months. Larval survival was longer from February to July, and reduced from August to February. Oviposition was recorded in all months, and the egg production index ranged from 26.7 to 47.1%. The eclosion rate ranged from 0.1 to 76.6% in different months. Monthly estimates of larval abundance in the pasture were similar for the two methods used, being higher in months with higher temperatures, relative humidity and rainfall.  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