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   179篇
  免费   8篇
林业   6篇
农学   15篇
基础科学   8篇
  36篇
综合类   15篇
农作物   15篇
水产渔业   7篇
畜牧兽医   64篇
园艺   6篇
植物保护   15篇
  2023年   1篇
  2022年   2篇
  2021年   1篇
  2020年   10篇
  2019年   9篇
  2018年   14篇
  2017年   10篇
  2016年   6篇
  2015年   6篇
  2014年   5篇
  2013年   18篇
  2012年   6篇
  2011年   9篇
  2010年   4篇
  2009年   8篇
  2008年   9篇
  2007年   7篇
  2006年   5篇
  2005年   2篇
  2004年   5篇
  2003年   1篇
  2002年   6篇
  2001年   1篇
  2000年   7篇
  1999年   4篇
  1998年   2篇
  1997年   2篇
  1996年   2篇
  1994年   1篇
  1993年   2篇
  1992年   5篇
  1991年   3篇
  1990年   3篇
  1987年   1篇
  1984年   1篇
  1979年   4篇
  1977年   1篇
  1976年   1篇
  1971年   1篇
  1967年   1篇
  1966年   1篇
排序方式: 共有187条查询结果,搜索用时 281 毫秒
81.
We describe a coupled hydrologic-economic spreadsheet model for the Murray-Darling Basin that allows analyses of water allocation and use by different sectors including agriculture and environment under alternative policy scenarios. The model is a simple, lumped optimisation model which includes partitioning of rainfall into runoff and evapotranspiration, a reach by reach water balance of the river system, irrigation demand and revenue generation. Groundwater is not considered because groundwater use is a small part of the overall water use. The model is used to optimize profit, diversions and flows subject to hydrological and economic constraints determined by the policy scenario.We use the model to examine approaches of acquiring water for reallocation to the environment, and their impacts on irrigation water use and regional income from agriculture. We show that the optimal approach for acquisition depends on: economic factors, including the cost of water and the profits generated by its use; institutional factors, such as restrictions to trade between regions; and, hydrological factors, particularly the connectivity of and losses within the river network.The volume of water to be acquired does not, in general, equal the volume to be allocated. For a downstream site, water must be acquired from upstream, and more water must be acquired than is to be allocated: the volume acquired is that to be allocated plus transmission losses between the locations of acquisition and allocation. For upstream sites, it is optimal to acquire some water from downstream, and less water must be acquired than is to be allocated: the volume acquired is that to be allocated, less the transmission losses no longer incurred between the locations of acquisition and allocation.The volumes of water that must be acquired to satisfy an allocation target and maintain flows in the river system are affected by restrictions on trade between sub-catchments.  相似文献   
82.
In this research we explore the potential of precision surface irrigation to improve irrigation performance under the warabandi system prevalent in the Indus Basin Irrigation System. Data on field dimensions, field slopes along with characteristic soil infiltration properties and outlet discharge were collected through a survey of a sample tertiary unit of Maira Branch Canal, Khyber Pakhtunkhwa Province, Pakistan. The performance of all fields in the tertiary unit was analysed and reported in aggregate, with detailed results of one field presented for illustration. The objective is to determine the optimum field layout, defined as the number of border strips, for the observed field characteristics to maximize performance. The results indicate that performance improvement is relatively easily achievable through changes in field layout within current irrigation services. Estimated application efficiency is sensitive to the selected depth of application, and it is important that a practical depth of application is selected. We recommend a depth of application of 50 mm and show how this is achievable and leads to a low quarter distribution uniformity of 0.750 and an application efficiency of 80 %. We also explore the feasibility of a 10-day warabandi rather than the 7-day warabandi and show that there is no significant change in the performance under a 10-day warabandi.  相似文献   
83.
Tropical Animal Health and Production - The current research study was designed to determine the inclusion of 2% dietary essential coconut oil with and without coccidiosis challenge on...  相似文献   
84.
The objective of the study was to evaluate a pinhole castration technique in male ponies. Adult ponies (n = 12) were randomly allotted to one of the two equal groups. Both of the groups were anaesthetised with xylazine–ketamine–diazepam and had single (SLS, SLC) ligation of the spermatic cord on one side and double (DLS, DLC) ligation on the other side using silk (Group T1) and catgut (Group T2). Single ligation whether using silk or catgut (SLS and SLC) was completed in 3.50 ± 0.34 min. DLS took 6.66 ± 0.49 min and DLC 7.16 ± 0.47 min. Scrotal oedema was noticed in all of the ponies from Day 1 to Day 15. The scrotal circumference and testicular volume in animals of both of the groups showed a significant (P<0.05) increase from Day 1 to Day 4. Orchiectomy to recover testicular remnants was performed in all of the animals on Day 41. Straw-coloured fluid accumulation was noticed in seven testicular remnants (4 T1 and 3 T2). One testicle from Group DLS showed suppuration. Multiple gross and histological abnormalities were detected in all double ligated testes. The changes were more severe in the DLS than the DLC group and their epididymis (n = 3) also showed necrosis, fibroplasia and an obstructed lumen. Severe adhesions had developed in three and one testicle only from the SLS and SLC groups respectively. The remaining testes and epididymis in both of these groups showed only mild-to-moderate adhesions. From this study, it was concluded that castration may not be achieved by percutaneous single ligation of the spermatic cord in ponies. Although double ligation induces marked gross and histopathological changes, assessments of the testosterone levels and sperm analysis are required before recommending this procedure. Use of silk for ligation of the spermatic cord is advantageous over catgut but maintenance of strict asepsis is mandatory.  相似文献   
85.
Twenty single male Assaf lambs were divided into two groups and housed in semi-open barns. Air temperature averaged 35°C. For 9 weeks after parturation, one group was cooled by fan between 1000 and 1600 hours and the other group was not cooled. Rectal and skin (head and testis) temperatures, respiration rate, and pulse rate were recorded twice daily. Lambs were weighed individually every week and levels of T3 and thyroxin were determined. Differences in rectal, head, and testis temperatures (P < 0.05) and respiration rates (P < 0.01) between the two groups were significant. Growth was affected positively by fan treatment, live weight increased by 15%, and cooled lambs had higher thyroxin levels.  相似文献   
86.
Carob (Ceratonia siliqua L.) is natively grown in the coastal areas of the Aegean and Mediterranean region in Turkey. The main carob growing areas (Mediterranean and Aegean) were surveyed and a total 70 promising wild and grafted carob genotypes were selected based on physical and chemical pod properties. Preliminary results showed that Turkey has very rich carob genetic resources containing considerable variations for most of the pod traits. In general, wild carob had smaller pods than the grafted ones. Pod mass ranged from 29.16 g to 120.28 g in the wild genotypes and 71.71 g to 147.91 g in the grafted genotypes with an average 107.61 g for the grafted and 63.72 g for the wild genotypes. The average pod dimensions (width, length and thickness) were 18.16 mm, 16.18 cm and 6.42 mm for the wild genotypes and 21.79 mm, 16.91 cm and 8.79 mm for the grafted genotypes. The average soluble solid content ranged from 59.42% in the wild and 64.06% in the grafted genotypes. The average total acidity of the wild and grafted genotypes was similar (0.78%). The pod traits of the wild genotypes differ significantly from those of the grafted genotypes, and the wild genotypes showed higher variations in these traits compared to the grafted genotypes.  相似文献   
87.
88.
Interesting physical models of threads, rouleaux of red blood cells, and other flexible structures (including macromolecules) have been prepared with chains of spheres and discs. When suspended in a viscous liquid undergoing shear flow, the aggregates rotate as nearly rigid bodies, even though they have no tensile strength or stiffness until shear rates high enough to cause bending and then breaking are reached. By adding a second liquid phase which causes a meniscus to bridge adjacent particles, sufficient tensile strength can be provided to cause bending without breakage.  相似文献   
89.
The purpose of this study was to examine iNOS gene expression and activity in macrophages from different chicken genetic lines against various bacterial LPS. Furthermore, the possible involvement of surface LPS receptors as candidates for differential iNOS gene induction in these genetic lines of chicken was also examined. Sephadex-elicited abdominal macrophages (1 x 10(6)) as well as iNOS hyper-responder macrophages from a transformed chicken macrophage cell line, MQ-NCSU, were exposed to 5 microg/ml LPS from E. coli, Shigella flexneri, Serratia marcensces, and Salmonella typhimurium. Nitrite levels were quantitated in the culture supernatant fractions of macrophages after 24h by the Griess method. The results showed that macrophages from K-strain (B(15)B(15)) (range from two separate trials: 31-89 microM) and MQ-NCSU (22-81 microM) were high responders whereas macrophages from both GB1 (B(13)B(13)) (15-38 microM) and GB2 (B(6)B(6)) (7-15 microM) chickens were low responders against all LPSs used. Northern blot analysis revealed that K-strain macrophages expressed higher intensity of 4.5Kb iNOS mRNA (iNOS/beta-actin ratio) than macrophages from GB2 regardless of the LPS source. To elucidate possible molecular mechanism(s) involved in iNOS gene expression in these two strains of chickens, the constitutive expression of LPS-related macrophage cell surface receptors, CD14, Toll-like receptor-2 (TLR2), and Toll-like receptor-4 (TLR4), was examined via flow cytometry using anti-human CD14, TLR2 and TLR4 antibodies. CD14 surface expression and intensity was not different between macrophages from K-strain or GB2 chickens. In contrast, while the overall percentage of TLR4-positive macrophages was the same (K-strain, trial 1=92%, trial 2=62%; GB2, trial 1=91%, trial 2=64%), the mean fluorescence intensity (MFI), an indicator of receptor number, was significantly higher (P=0.05) in K-strain macrophages (MFI: trial 1=145; trial 2=131) than GB2 macrophages (MFI: trial 1=101; trial 2=98). Furthermore, TLR2 (a previously thought candidate as LPS signaling molecule) positive cell numbers were higher in K-strain than the GB2 macrophages in one of the two trials with no difference in the intensity of TLR2 expression in either trial. These findings suggest that the observed differences in iNOS expression and activity among the K-strain (hyper-responder) and GB2 (hypo-responder) chickens are, at least in part, due to differential expression of TLR4 (an LPS signaling molecule), leading to more intense LPS-mediated activation of K-macrophages.  相似文献   
90.
ABSTRACT

A pot experiment was conducted to examine the uptake of nutrients (K+ (potassium) and Ca2+ (calsium)) and heavy metal (Cu2+ (copper) and Pb2+ (lead)) ions by leaves, seeds, and roots of two black gram [Vigna mungo (L.) Hepper] cultivars, ‘Mash-95018’(V1) and ‘Mash-97’(V2) treated with copper (Cu) and lead (Pb) at 25mg L?1 and 50mg L?1. This study was conducted in a greenhouse in the Botanical Garden, University of Agriculture, Faisalabad, Pakistan, during the spring of 2003. Heavy-metal treatments were applied 30 d after germination, and nutrient and heavy-metal ion uptake data were collected 10 d after treatment application. Both heavy metals in both cultivars substantially reduced nutrient ion accumulation and its translocation to seeds. Leaves had proportionately more K+ and Ca2+ than that recorded for roots and seeds after heavy-metal treatments. Nevertheless, both heavy metal (Cu2+ and Pb2+) ions ware predominantly sequestered in the roots, rather than in leaves and seeds, under their respective treatments. As the external concentrations of heavy metals increased, their uptake by the respective treated plants also increased, but nutrient ion (K+ and Ca2+) uptake was gradually reduced. This result suggests a concentration-dependent phenomenon. Overall, lead (Pb2+) showed more toxic effects on the uptake of essential nutrients compared with Ca2+, while ‘Mash-97’proved more sensitive to heavy metals than ‘Mash-95018.'  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