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   14635篇
  免费   1100篇
  国内免费   71篇
林业   668篇
农学   853篇
基础科学   127篇
  2088篇
综合类   1694篇
农作物   514篇
水产渔业   1115篇
畜牧兽医   7317篇
园艺   235篇
植物保护   1195篇
  2023年   92篇
  2022年   72篇
  2021年   101篇
  2020年   137篇
  2019年   208篇
  2018年   389篇
  2017年   353篇
  2016年   348篇
  2015年   313篇
  2014年   343篇
  2013年   628篇
  2012年   504篇
  2011年   585篇
  2010年   475篇
  2009年   437篇
  2008年   593篇
  2007年   613篇
  2006年   506篇
  2005年   548篇
  2004年   431篇
  2003年   478篇
  2002年   381篇
  2001年   784篇
  2000年   737篇
  1999年   601篇
  1998年   179篇
  1997年   169篇
  1996年   130篇
  1995年   152篇
  1994年   160篇
  1993年   121篇
  1992年   324篇
  1991年   348篇
  1990年   297篇
  1989年   273篇
  1988年   276篇
  1987年   324篇
  1986年   252篇
  1985年   203篇
  1984年   174篇
  1983年   151篇
  1982年   95篇
  1981年   83篇
  1980年   80篇
  1979年   144篇
  1978年   94篇
  1977年   76篇
  1975年   77篇
  1974年   80篇
  1973年   75篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
972.
The digestibility of various diet components as protein sources for common carp, Cyprinus carpio L., at 500–800 g was studied by using an inert marker in the feed and collecting the faeces by stripping. Protein in fish meal (83.8%) and soybean meal (69.8%) were significantly more digestible than that of poultry meal (47.2%). Energy digestibility tests gave the following results: fish meal 93.4%, soybean meal 74.7% and poultry meal 63.9%. It was evident that the ability of carp to digest proteins is very high, and that they can utilize proteins as the main component of their diet.  相似文献   
973.
The gustatory sensitivity of the Nile tilapia, Oreochromis niloticus L., to different amino acids was studied using an electrophysiological approach. The electrical responses were recorded from a branch of the facial nerve innervating the external taste buds of the upper lip. The relative stimulatory effectiveness (RSE) of nine amino acids and betaine were determined at a concentration of 1 mm and all of them elicited neural responses. This species responded well to the neutral, basic and acidic amino acids. The most stimulatory amino acids were L‐histidine, L‐arginine, L‐serine, L‐methionine and L‐glutamine; L‐proline and betaine were the least stimulatory. The results of this study suggest that the Nile tilapia has high external gustatory sensitivity to some amino acids as a physiological adaptation to search effectively for their sources. The effect of the pH, ranging from 4.0 to 9.0, on the RSE of three neutral amino acids and artificial pond water (APW) was also studied. The RSE increased below pH 6.0 and was relatively unaffected from 7.0 to 9.0, indicating that acidified stimulants are highly stimulatory in this species. Nile tilapia did not discriminate the pH of APW as effectively as some of the species studied earlier.  相似文献   
974.
In vitro accumulation of tetrodotoxin (TTX) and paralytic shellfish toxin (PST) in tiger puffer fish Takifugu rubripes was investigated using liver tissue slices. When T. rubripes liver slices were incubated with Leibovitz’s L-15 medium containing 0.13 mM TTX at 20 °C in air with saturated humidity, they accumulated 21.5 ± 7.3 μg TTX g?1 liver after the incubation for 12 h and increased to 55.3 ± 8.2 μg TTX g?1 liver at 48 h. In the incubation of T. rubripes liver slices with 0.13 mM PST-containing medium, PST was detected 6.3 ± 0.9 μg g?1 liver at 12 h and reached a plateau thereafter. These results reveal the difference between TTX and PST in accumulation in T. rubripes liver tissue slices. To examine the variation in PST accumulation among fish species, the liver tissue slices from tiger puffer fish T. rubripes, parrot-bass Oplegnathus fasciatus and green ling Hexagrammos otakii were incubated at a concentration of 0.027 mM PST. The toxin contents of 3.0 μg g?1 liver were observed at 8 h regardless of fish species but were not increased subsequently, showing no variety among these three species as to accumulation patterns of PST. It is noted that the tiger puffer fish T. rubripes liver specifically accumulate TTX in preference to PST.  相似文献   
975.
Along the eastern seaboard of the US, Atlantic menhaden, Brevoortia tyrannus, develop characteristic ulcerative lesions, a condition termed ulcerative mycosis. These lesions are identical to those seen across Asia in fish affected by epizootic ulcerative syndrome, a condition caused by the fungus-like oomycete Aphanomyces invadans. Young-of-the-year menhaden inhabiting estuarine environments are the primary species affected in the USA and little is known about the factors involved in the initiation of the lesions, or why menhaden are predominantly infected. Atlantic menhaden, hogchoker, Trinectus maculatus, striped killifish, Fundulus majalis, and mummichog, Fundulus heteroclitus, were inoculated with A. invadans (80 zoospores per fish) to explore species differences in infection and lesion development. All four species developed lesions. Killifish developed frank lesions similar to those observed in menhaden but the gross lesions occurred later, approximately 5-10 days after those on menhaden. Hogchoker and mummichog did not develop gross skin ulcers; rather, their lesions appeared as reddened areas under the epidermis. Mummichogs also showed evidence of significant healing with a well-developed granuloma and significant myocyte regeneration. These experiments show that species barriers as well as ecological barriers can explain some of the factors involved in the development of lesions in, and specificity of the water mould for, menhaden.  相似文献   
976.
Four diets differing in crude protein/crude lipid concentrations (CP/CL), 570/200, 510/220, 460/240, 430/260 (g kg?1 dry diet) were fed to near‐satiety to rainbow trout (initial body weight, IBW = 268 g, domestic strain) and Atlantic salmon (IBW = 456 g, wild strain) for 308 days to determine the effect of diets, species and fish size on efficiency of feed, nitrogen (N) and energy utilization. Within species, weight gain, feed efficiency (FE), and energy retention efficiency (ERE, E gain/E intake) were not affected by diet (P < 0.05). N retention efficiency (NRE, N gain/N intake) increased linearly (P < 0.05) with decreasing CP/CL for both species. NRE and FE were higher for salmon compared with trout. Whole body CP of salmon was significantly (P < 0.05) higher, and lipid significantly (P < 0.05) lower, than those of trout. There was a significant (P < 0.05) linear decrease in FE as fish grew, regardless of diet and species. NRE linearly decreased (P < 0.0001) and lipid to protein deposition ratio (LD/PD) increased (P < 0.05) as trout grew. NRE linearly increased (P < 0.05) while ERE and LD/PD linearly decreased (P < 0.05) as salmon grew. Salmon appeared to rely to a greater extent on non‐protein energy‐yielding nutrients to support cost of growth than did trout.  相似文献   
977.
Three Brazilian and 9 French agricultural fertilizers were compared with Conway mixture, as nutrient enrichment for supporting the growth of 16 marine algae. Fertilizers containing only nitrogen and phosphorus appeared to be suitable. However, a few algae grew better in all-nutrient-enriched Conway medium. Addition of chelators did not generate better growth. Matrix-free fertilizers were found more convenient in use. A table is given to show the cheapest and/or best nutrient enrichment for each algal strain tested.  相似文献   
978.
This study tested the efficacy of prey extracts to induce food search behavior in juvenile (15–25 cm total length, 0 year group) Atlantic halibut (Hippoglossus hippoglossus). In square culture tanks, halibut responded to shrimp and squid extract by tightly turning towards the source of the stimulus and by swimming in circles for 2–5 min following stimulus delivery. Cod extract, or a synthetic mixture consisting of glycine, proline, betaine, arginine, and alanine, failed to evoke this behavior.  相似文献   
979.
Abstract. Studies were conducted to clarify the relationship between growth rate and disease progression of erythrocytic inclusion body syndrome (EIBS) in artificially infected coho salmon, Oncorhynchus kisutch (Walbaum). In the diseased state, the haematocrit values decreased and the number of erythrocytes with inclusions was higher in faster growing fish. Rapid growth was accompanied by an abundance of immature erythrocytes, which had the greatest incidence of inclusion bodies.  相似文献   
980.
The UV curing reaction of the difference between allyl abietate(AA) and allyl isopimarate(AI) was studied. The conversion rate of the vinyl double bond was characterized and detected by FT-IR analysis. Results showed that conversion of double bonds are firstly increased then decreased with the increases of the dosage of initiators. Conversion of double bonds firstly increased and then leveling off with the increases of the illumination time, and AA is higher before the 200 s. AA is higher then AI under the same illumination intensity. Surface drying time of AA is shorter than AI, and relative curing speed become faster at the same conditions. Under the conditions of 2-hydroxy-2-methylpropiophenone(Darocure-1173)as photo initiator and dosage 6%, illumination time 200 s, illumination distance 4.5 cm and illumination intensity 100%, number average molecular weight of cure products of AA and AI are 4 176 and 9 407 with polymerisation degrees of 12 and 28, respectively. The obtained UV cured coating all have smooth and transparent appearance. It represented some superior properties in flexibility, mechanical impact strength, adhesive power, hardness, acid resistance, alkali resistance, salt resistance, water resistance, etc.. The hardness of AA and AI cured products are H and HB, respectively.  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