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   636篇
  免费   19篇
  国内免费   1篇
林业   31篇
农学   23篇
基础科学   5篇
  90篇
综合类   120篇
农作物   32篇
水产渔业   19篇
畜牧兽医   247篇
园艺   18篇
植物保护   71篇
  2021年   4篇
  2020年   12篇
  2019年   5篇
  2018年   6篇
  2017年   5篇
  2016年   12篇
  2015年   9篇
  2014年   8篇
  2013年   28篇
  2012年   25篇
  2011年   36篇
  2010年   18篇
  2009年   15篇
  2008年   35篇
  2007年   36篇
  2006年   27篇
  2005年   31篇
  2004年   32篇
  2003年   27篇
  2002年   37篇
  2001年   10篇
  2000年   9篇
  1998年   4篇
  1997年   5篇
  1996年   5篇
  1995年   5篇
  1992年   10篇
  1991年   5篇
  1990年   4篇
  1988年   8篇
  1986年   8篇
  1985年   7篇
  1984年   8篇
  1983年   4篇
  1982年   5篇
  1981年   6篇
  1979年   10篇
  1978年   5篇
  1977年   6篇
  1975年   4篇
  1974年   8篇
  1973年   6篇
  1972年   4篇
  1971年   4篇
  1970年   5篇
  1968年   5篇
  1967年   4篇
  1965年   9篇
  1962年   5篇
  1937年   3篇
排序方式: 共有656条查询结果,搜索用时 328 毫秒
101.
This study presents four strategies of a novel evolutionary algorithm, multi-objective differential evolution algorithm (MDEA). The four strategies namely, MDEA1, MDEA2, MDEA3 and MDEA4 are adapted to solve the multi-objective crop planning model with multiple constraints in a farmland in the Vaalharts irrigation scheme (VIS) in South Africa. The three objectives of the model are to minimize the total irrigation water (m2) and to maximize both the total net income in South African Rand (ZAR) from farming and the total agricultural output in tons. The total area of the farm is 771,000 m2 and supplied with 704,694 m2 of irrigation water annually. Numerical results produce non-dominated solutions which converge to Pareto optimal fronts. MDEA1 and MDEA2 strategies with binomial crossover method are better for solving the crop planning problem presented than MDEA3 and MDEA4 strategies with exponential crossover method. MDEA1 found a solution with the highest total net income of ZAR 1,304,600 with the corresponding total agricultural output, total irrigation water and total planting areas of 316.26 tons, 702,000 m3 and 725,000 m2, respectively. The planting areas for the crops in the solution are 73,463 m2 for maize, 551,660 m2 for groundnut, 50,000 m2 for Lucerne and 50,000 m2 for Peacan nut. It can be concluded that MDEA is a good algorithm for solving crop planning problem especially in water deficient areas like South Africa.  相似文献   
102.
103.
To facilitate economical culture of black pacu, Colossoma macropomum, and red pacu, Piaractus brachypomus, in the Amazon region of South America, we assessed locally available alternative energy sources for practical diets. We tested the effects of control diets (containing wheat products) versus diets with different Amazonian feedstuffs (yucca, Manihot sculenta, plantain, Musa paradisiaca, or pijuayo, Bactris gasipaes) on the performance of the pacus in three feeding trials. Black pacu (22.5 ± 0.03 g; Trial 1) or red pacu (2.56 ± 0.01 g; Trial 2) were fed diets containing 30% wheat bran (control) or cooked or uncooked yucca, plantain, or pijuayo for 12 wk. In Trial 3, larger black pacu (86.9 ± 6.4 g) were grown to market size in 24 wk on similar diets. Weight gain, feed conversion, survival, alternative complement activity, and lysozyme were similar among diets. Hepatosomatic index, liver glycogen, and dry matter were affected by diet in Trials 1 and 2, but effects were not consistent among trials. In Trial 3, protein efficiency ratio was lower in fish fed the diet containing wheat middlings. However, relative to wheat bran or wheat middlings, all feedstuffs tested were effective energy sources for juvenile black pacu and red pacu.  相似文献   
104.
Journal of Soils and Sediments - Gully and channel erosion are known to export large quantities of soil organic matter (SOM) to stream ecosystems. However, the implications for in-stream processing...  相似文献   
105.
This paper examines the past, present, and future use of the concept of historical range and variability (HRV) in land management. The history, central concepts, benefits, and limitations of HRV are presented along with a discussion on the value of HRV in a changing world with rapid climate warming, exotic species invasions, and increased land development. This paper is meant as a reference on the strengths and limitations of applying HRV in land management. Applications of the HRV concept have specific contexts, constraints, and conditions that are relevant to any application and are influential to the extent to which the concept is applied. These conditions notwithstanding, we suggest that the HRV concept offers an objective reference for many applications, and it still offers a comprehensive reference for the short-term and possible long-term management of our nation's landscapes until advances in technology and ecological research provide more suitable and viable approaches in theory and application.  相似文献   
106.
In Autumn 2009, a faecal egg count reduction test (FERCT) was carried out on three sheep farms. Groups of 8-11 lambs were treated with ivermectin or moxidectin, with a 14-day interval between treatment and sampling. Ivermectin resistance was present on all three farms. Treatment with ivermectin resulted in a reduction in faecal egg numbers of 94.6%, 63%, and 59%. On two farms, 14 days after treatment pooled faecal samples yielded predominantly larvae of Hamonchus contortus (100% and 98%, respectively). On the third farm, H. contortus and (probably) Teladorsagia circumcincta were resistant to ivermectin (64% and 36% of the larvae, respectively). Treatment with moxidectin resulted in a 100% reduction in egg output in sheep on all three farms. More sensitive culture techniques failed to detect any larvae in samples taken from two farms, but a few Ostertagia-type larvae, probably of T. circumcincta, were detected in samples from the third farm. It can be concluded that gastrointestinal nematodes in sheep from these three farms were resistant to ivermectin, whereas resistance to moxidectin was not detected.  相似文献   
107.
108.
Classical studies of mammalian movement control define a prominent role for the primary motor cortex. Investigating the mouse whisker system, we found an additional and equally direct pathway for cortical motor control driven by the primary somatosensory cortex. Whereas activity in primary motor cortex directly evokes exploratory whisker protraction, primary somatosensory cortex directly drives whisker retraction, providing a rapid negative feedback signal for sensorimotor integration. Motor control by sensory cortex suggests the need to reevaluate the functional organization of cortical maps.  相似文献   
109.
Objective To compare the incidence of anesthetic complications in diabetic and nondiabetic dogs undergoing general anesthesia and phacoemulsification cataract surgery. Procedure The medical and anesthetic records of all dogs undergoing phacoemulsification cataract surgery at Davies Veterinary Specialists between 2005 and 2008 were reviewed. Anesthetic records were evaluated by an ECVAA Diplomate. Dogs for which records were incomplete were excluded. The anesthetic technique, including all drugs administered in the perioperative period, was recorded. The anesthetic complications investigated included hypotension (MAP (mmHg): ≥55 none/mild; ≤54 moderate/severe), bradycardia (<60 bpm associated with hypotension) and hypothermia (esophageal temperature <36.7 °C). Where hypotension was present, the method of and response to treatment was recorded. The incidence of severe hyperglycemia (blood glucose >13.75 mmol/L (250 mg/dL)) in the diabetic group was also assessed. Results 66 diabetic and 64 nondiabetic dogs were included in the study. Diabetic dogs were more likely to develop moderate and severe intraoperative hypotension than nondiabetic dogs. Forty‐four percent of diabetic dogs had at least one episode of severe hyperglycemia whilst anesthetized. Conclusions Diabetic dogs undergoing phacoemulsification are more likely to suffer the anesthetic complications of moderate and severe hypotension than nondiabetic dogs. The increased incidence and severity of hypotension in diabetic dogs may be explained by hypovolemia secondary to hyperglycemia and resultant osmotic diuresis.  相似文献   
110.
High herbicide levels can be localized on or near seed of acetolactate synthase (ALS) resistant maize (Zea mays). The magnesium salt of imazapyr was optimal for seed dressings (drenching, priming, and coating), for preventing field damage from parasitic Striga hermonthica (witchweed) in three seasons. Striga emerged on untreated maize from 6 to 12 weeks after planting. There was almost no Striga emergence for 3 months on imazapyr-dressed homozygous ALS-R 3245IR maize seeds. Occasional Striga that emerged and flowered formed no seed by harvest. Dust or polyvinylpyrrolidone adhesive coatings were safer in maize and as effective in Striga control as priming. Seed dressing coupled with pulling Striga escapes reduces infestation and can be used to deplete the Striga seed bank until genetic crop resistance becomes available.  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