全文获取类型
收费全文 | 5649篇 |
免费 | 314篇 |
国内免费 | 13篇 |
专业分类
林业 | 324篇 |
农学 | 168篇 |
基础科学 | 50篇 |
980篇 | |
综合类 | 964篇 |
农作物 | 277篇 |
水产渔业 | 450篇 |
畜牧兽医 | 2206篇 |
园艺 | 169篇 |
植物保护 | 388篇 |
出版年
2023年 | 44篇 |
2022年 | 33篇 |
2021年 | 102篇 |
2020年 | 119篇 |
2019年 | 118篇 |
2018年 | 111篇 |
2017年 | 112篇 |
2016年 | 129篇 |
2015年 | 81篇 |
2014年 | 123篇 |
2013年 | 237篇 |
2012年 | 294篇 |
2011年 | 346篇 |
2010年 | 217篇 |
2009年 | 174篇 |
2008年 | 296篇 |
2007年 | 305篇 |
2006年 | 273篇 |
2005年 | 278篇 |
2004年 | 275篇 |
2003年 | 263篇 |
2002年 | 239篇 |
2001年 | 104篇 |
2000年 | 96篇 |
1999年 | 84篇 |
1998年 | 57篇 |
1997年 | 55篇 |
1996年 | 57篇 |
1995年 | 42篇 |
1994年 | 44篇 |
1993年 | 61篇 |
1992年 | 51篇 |
1991年 | 64篇 |
1990年 | 55篇 |
1989年 | 86篇 |
1988年 | 68篇 |
1987年 | 58篇 |
1986年 | 66篇 |
1985年 | 46篇 |
1984年 | 54篇 |
1983年 | 45篇 |
1982年 | 28篇 |
1981年 | 25篇 |
1979年 | 33篇 |
1978年 | 29篇 |
1977年 | 26篇 |
1976年 | 29篇 |
1974年 | 33篇 |
1973年 | 35篇 |
1972年 | 30篇 |
排序方式: 共有5976条查询结果,搜索用时 15 毫秒
91.
Some Limiting Factors in Superintensive Production of Juvenile Pacific White Shrimp,Litopenaeus vannamei,in No‐water‐exchange,Biofloc‐dominated Systems
下载免费PDF全文
![点击此处可从《Journal of the World Aquaculture Society》网站下载免费的PDF全文](/ch/ext_images/free.gif)
David I. Prangnell Leandro F. Castro Abdulmehdi S. Ali Craig L. Browdy Paul V. Zimba Susan E. Laramore Tzachi M. Samocha 《Journal of the World Aquaculture Society》2016,47(3):396-413
Superintensive shrimp culture in zero‐exchange, biofloc‐dominated production systems is more biosecure and sustainable than traditional shrimp farming practices. However, successful application of this technology depends upon optimizing dietary formulations, controlling Vibrio outbreaks, and managing accumulative changes in water quality and composition. A 49‐d study investigated the effect of two commercial feeds of differing protein content and an indoor limited‐exchange, biofloc‐dominated culture environment on Litopenaeus vannamei performance and tissue composition, water quality and ionic composition, and Vibrio dynamics. Juveniles (5.3 g) were stocked at 457/m3 into four 40 m3 shallow raceways containing biofloc‐dominated water and fed one of two commercial feeds with differing protein content, 35 or 40%. Shrimp performance, Vibrio populations, and changes in shrimp and culture water composition were monitored. There were no significant differences (P > 0.05) in shrimp performance (survival, weight, growth, specific growth rate, total biomass, yield, feed conversion ratio, and protein efficiency ratio) or proximate composition between feed types. The 40% protein feed resulted in higher culture water nitrate and phosphate concentrations, alkalinity consumption and bicarbonate use, and higher phytoplankton density. The presence of Vibrio, specifically Vibrio parahaemolyticus, reduced shrimp survival. This survival decrease corresponded with increased culture water Vibrio concentrations. Culture water K+ and Mg2+ increased significantly (P < 0.05), and Sr2+, Br?, and Cl? decreased significantly (P < 0.05) over time. While Cu2+ and Zn2+ did increase in shrimp tissue, no heavy metals accumulated to problematic levels in culture water or shrimp tissue. These results demonstrate the importance of monitoring Vibrio populations and ionic composition in limited‐exchange shrimp culture systems. 相似文献
92.
The dispersal ecology of most stream fishes is poorly characterised, complicating conservation efforts for these species. We used microsatellite DNA marker data to characterise dispersal patterns and effective population size (Ne) for a population of Roanoke logperch Percina rex, an endangered darter (Percidae). Juveniles and candidate parents were sampled for 2 years at sites throughout the Roanoke River watershed. Dispersal was inferred via genetic assignment tests (ATs), pedigree reconstruction (PR) and estimation of lifetime dispersal distance under a genetic isolation‐by‐distance model. Estimates of Ne varied from 105 to 1218 individuals, depending on the estimation method. Based on PR, polygamy was frequent in parents of both sexes, with individuals spawning with an average of 2.4 mates. The sample contained 61 half‐sibling pairs, but only one parent–offspring pair and no full‐sib pairs, which limited our ability to discriminate natal dispersal of juveniles from breeding dispersal of their parents between spawning events. Nonetheless, all methods indicated extensive dispersal. The AT indicated unrestricted dispersal among sites ≤15 km apart, while siblings inferred by the PR were captured an average of 14 km and up to 55 km apart. Model‐based estimates of median lifetime dispersal distance (6–24 km, depending on assumptions) bracketed AT and PR estimates, indicating that widely dispersed individuals do, on average, contribute to gene flow. Extensive dispersal of P. rex suggests that darters and other small benthic stream fishes may be unexpectedly mobile. Monitoring and management activities for such populations should encompass entire watersheds to fully capture population dynamics. 相似文献
93.
Movement and habitat use of the freshwater catfish (Tandanus tandanus) in a remnant floodplain wetland
下载免费PDF全文
![点击此处可从《Ecology of Freshwater Fish》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Wayne M. Koster David R. Dawson Pam Clunie Fern Hames John McKenzie Paul D. Moloney David A. Crook 《Ecology of Freshwater Fish》2015,24(3):443-455
Knowledge of the movement and habitat use of fishes is important in identifying and understanding the causes of population declines and predicting how populations are likely to respond to management interventions. In this study, radiotelemetry was used to examine the spring and summer movement and habitat use patterns of the freshwater catfish (Tandanus tandanus) in a remnant wetland to inform the development of recovery actions for this threatened species. Twenty‐one adult fish were tagged and released within Tahbilk Lagoon, Victoria, Australia, in September 2009. Fish were located every 1–2 weeks between September 2009 and February 2010, a period which coincides with the spawning period for the species. Eleven of the fish were also tracked every 2 h for 68 consecutive hours in December 2009 to examine diel movements. The study revealed that freshwater catfish make extensive use of cover (e.g. wood and macrophytes) and typically have limited ranges (median total linear range and 90% linear range 599 and 173 m respectively), although they occasionally moved more extensively (up to 1.5 km) between floodplain and riverine habitats. Fish moved over much greater areas at night compared with during the day. There was also evidence of sex‐specific variation in movement, with a trend for greater movement of female fish at night compared with males. The results of the study suggest that strategies that protect macrophyte and wood habitats and improve connectivity between riverine and floodplain habitats are likely to be important in maintaining and restoring remnant populations of this species. 相似文献
94.
Joseph H. Anderson George R. Pess Peter M. Kiffney Todd R. Bennett Paul L. Faulds William I. Atlas Thomas P. Quinn 《Ecology of Freshwater Fish》2013,22(1):30-42
Anadromous fishes are frequently restricted by artificial barriers to movement such as dams and culverts, so measuring dispersal helps identify sites where improved connectivity could promote range expansion and population viability. We used a combination of DNA‐based parentage analysis and mark–recapture techniques to evaluate dispersal by juvenile coho salmon (Oncorhynchus kisutch) in a population in the initial stages of colonisation following installation of fish passage structures at a previously impassable dam on the Cedar River, WA, USA. The spatial distribution of individuals within maternal families revealed that dispersal was common. Among the offspring of radio‐tagged mothers, 28% were collected outside the spawning reach and dispersed up to 6.3 km (median = 1.5 km). Most juveniles captured in a tributary (Rock Creek, where few adults spawned) had immigrated from the Cedar River and represented many different families. Juvenile dispersal therefore provided a secondary phase of spatial expansion following initial colonisation by adults. Consistent with the condition‐dependent dispersal hypothesis, juveniles that dispersed farther upstream in the tributary were larger than fish collected near the tributary mouth. Overall, the results demonstrated widespread dispersal in a system with low coho salmon densities, and this might increase the rate of population growth if it reduces the effects of local density dependence. By implication, juveniles can take advantage of rearing habitats reconnected through barrier removal, even when such areas are located several kilometres from adult breeding grounds. 相似文献
95.
96.
Flowing solution culture enables the researcher to study effects of root environment variables independently. This flowing solution culture system utilizes air pressure and syphoning or a peristaltic pump to circulate the nutrient solution. Because this system is inexpensive, simple in design, and compact, several factors can be studied concurrently, treatments can be replicated to test for system effects, nutrient deviations, and block for growth effects due to environment. These systems have been used successfully with experiments in N form, plant growth regulators, salt stress, and mechanical stress. Non‐destructive measurements of root growth and nutrient uptake are possible. 相似文献
97.
Comparisons were made between the amounts of abamectin and total radioactivity recovered from Spodoptera littoralis and Heliothis armigera larvae after topical application of [3H]abamectin. Penetration (as shown by wash-off experiments) did not differ significantly between the Spodoptera instars. Significantly more abamectin was recovered from ventral nerve cord samples of larvae showing symptoms of poisoning than from larvae not showing these symptoms. Fifth-instar S. littoralis larvae had a significantly lower proportion of radioactivity as abamectin in the ventral nerve cord than in sixth-instar S. littoralis or fifth- and sixth-instar H. armigera. The proportion of radioactivity present as abamectin (but not total radioactivity) was significantly increased when the fifth-instar S. littoralis larvae were pre-treated with piperonyl butoxide (PB) suggesting that the relative insensitivity of fifth-instar S. littoralis larvae to abamectin is due at least in part to greater metabolism, particularly by microsomal oxidases. Fat body samples consistently had a greater proportion of radioactivity as abamectin than the nerve cord and the former may act as storage sites for abamectin. 相似文献
98.
Many insecticides (e.g. DDT, lindane, pyrethroids) are hydrophobic molecules which bind extensively to biological membranes. Binding of insecticides to phospholipid bilayers frequently shows saturation, with binding increasing with increasing insecticide concentration up to some limit, beyond which no further insecticide is incorporated into the bilayer. For lindane, this limit has been shown to correspond to the concentration at which the aqueous phase becomes saturated. Simple alkanes show similar saturation phenomena, and binding of halogenated alkanes can be followed by fluorescence quenching methods. It is shown that effects of hydrophobic molecules on bilayer fluidity are small. Effects of alkanes and insecticides on the activity of the (Ca2+-Mg2+)-ATPase purified from sarcoplasmic reticulum follow from direct binding to the ATPase, at sites which are not at the lipid-protein interface of the ATPase. 相似文献
99.
B. Terence Grayson Karen S. Williams Paul A. Freehauf Rodney R. Pease William T. Ziesel Richard L. Sereno Ronald E. Reinsfelder 《Pest management science》1987,21(2):143-153
Cinmethylin (SD 95481), is a novel herbicide developed for the selective pre-emergence control of many annual grass weeds in a wide range of temperate and tropical crops. Representing new herbicide chemistry, cinmethylin is in the cineole family. Cinmethylin is a mobile colourless liquid with a boiling point of 313°C under an inert atmosphere at atmospheric pressure. It has a density of 1015 kg m?3 and a viscosity of 70–90 mPa s, both at 20°C. It is miscible in all proportions with most organic solvents but has a low solubility, 63 mg litre?1, in water. It has a vapour pressure of 10.2 mPa (20°C) and the vapour pressure/temperature relationship is given by loge P(Pa)=28.9–9816/T (K). The n-octanol/water partition coefficient is 6850 and soil organic matter/water sorption coefficient (Kom) ranges between 165 and 235 over the three types of soil used in these studies. Cinmethylin is stable in water over the pH range 3–11. Solutions of cinmethylin in water or solvents are reasonably stable to sunlight, though thin films on a quartz surface photooxidise mainly to an ester within 24 h. This rate can be reduced by the addition of photostabilisers or by sorption onto soil surfaces. In an inert atmosphere cinmethylin is stable to high temperatures, though, in air, oxidation occurs at temperatures above 100°C to give the same product as by photodecay. 相似文献
100.
The efficacies of nine structural analogues of the herbicide antidote naphthalene-1,8-dicarboxylic acid anhydride (naphthalic anhydride, NA) for the protection of maize (Zea mays L. cv. DeKalb XL72AA and DeKalb XL67) against injury by the herbicide S-ethyl dipropyl(thiocarbamate) (EPTC) were elevated under greenhouse conditions. The chemical analogues of NA tested were: acenaphthenequinone (ACQ); 4-aminonaphthalene-1,8-dicarboxylic acid anhydride (NH2NA); 1,8:4,5-naphthalenetetracarboxylic acid dianhydride (NDiA); naphthalene- 1,8-carboximide (NHNA); 4-chloronaphthalene-1,8-dicarboxylic acid anhydride (C1NA); biphenyl-2,2′-dicarboxylic acid anhydride (diphenic anhydride; DA); 2-phenylglutaric anhydride (PGA); phthalic anhydride (PHA); phenalen-1-one (PA). Pre-plant incorporated applications of EPTC at 2.2, 4.5, 6.7, and 9.0 kg ha?1 were highly toxic to XL67 maize. Appreciable injury to XL72AA maize by EPTC was observed only with the high rates of EPTC (6.7 and 9.0 kg ha?1). Of the analogues tested PGA and PA were very toxic and inhibited germination of both maize hybrids. NA, ACQ, NH2NA, NDiA, NHNA, C1NA, DA, and PHA applied as seed dressings at 5.0 and 10 g per kg of seed offered satisfactory protection to XL72AA maize against EPTC rates higher than 6.7 kg ha?1. The same antidotes significantly antagonised the EPTC activity against XL67 maize but the overall protection obtained was partial and not agronomically important. The presence of the dicarboxylic anhydride group and of at least one aromatic ring attached directly to the anhydride appeared to be essential for the exhibition of protective activity by the structural analogues of NA. NA was slightly toxic to both hybrids of maize and chlorination of NA increased the phytotoxicity of this molecule. A genetic component that is present in the thiocarbamate-tolerant XL72AA hybrid but absent from the thiocarbamate-susceptible XL67 hybrid of maize appeared to be important for the phytotoxic activity of EPTC and may be involved in the protective activity of NA and its structural analogues. 相似文献