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   556篇
  免费   33篇
  国内免费   2篇
林业   33篇
农学   16篇
基础科学   3篇
  170篇
综合类   36篇
农作物   37篇
水产渔业   40篇
畜牧兽医   199篇
园艺   18篇
植物保护   39篇
  2024年   1篇
  2023年   7篇
  2022年   10篇
  2021年   13篇
  2020年   17篇
  2019年   13篇
  2018年   24篇
  2017年   21篇
  2016年   23篇
  2015年   11篇
  2014年   22篇
  2013年   39篇
  2012年   37篇
  2011年   63篇
  2010年   42篇
  2009年   24篇
  2008年   37篇
  2007年   32篇
  2006年   35篇
  2005年   28篇
  2004年   26篇
  2003年   17篇
  2002年   20篇
  2001年   6篇
  2000年   3篇
  1999年   3篇
  1997年   5篇
  1996年   3篇
  1995年   2篇
  1994年   1篇
  1993年   1篇
  1990年   1篇
  1988年   1篇
  1980年   1篇
  1979年   1篇
  1978年   1篇
排序方式: 共有591条查询结果,搜索用时 13 毫秒
551.
Housing conditions and human presence have been shown to have an influence on daily amount of activity in herbivores. Only few studies on the behavior of cats and interactions between cat and human were conducted, and these few works are referred to cats housed in laboratories. At the same way, only few and conflicting reports about the presence of circadian rhythms in cats are present. In view of this incomplete literature, the main aim of our study was to investigate the influence of different housing conditions on daily rhythm of total locomotor activity (TLA) in cats.For this purpose, 10 clinically healthy adult domestic cats were used. They were divided into 2 equal groups living with their owner in their home. Group A lived in 80-100 m2 house and had free daily access to 20-40 m2 garden from 08:00 to 9:00. Group B lived in 200-250 m2 house, had free access to 2000-2500 m2 garden all day, and from 21:00 to 08:00 they were kept outside. Our results showed that in group A, that live in strong symbiosis with owners respect to group B, the amount of activity was higher during the photophase. This supporting the view that domestic cats adapt their lifestyle with their owners. Group B exhibited the highest level of locomotor activity during the scotophase. Moreover, a robust daily rhythmicity of TLA was observed in group B; on the contrary, in group A, TLA showed no daily rhythmicity. Our findings underline the high influence of human presence and care on the amount of activity and daily rhythm in cats.  相似文献   
552.
To determine lignin content in triticale and wheat straws, calibration models were built using Fourier transform mid-infrared spectroscopy combined with partial least-squares regression. The best model for triticale and wheat straws was built using averaged spectra with raw spectrum in spectrum format and constant in path length as spectral pretreatments. The values of r(2), root-mean-square error of prediction (RMSEP), and residual predictive deviation (RPD) for the triticale straw model were 0.935, 0.305, and 3.89, respectively. The r(2), RMSEP, and RPD values for the wheat straw model were 0.985, 0.163, and 8.50, respectively. Both models showed good predictive ability. A model built using both triticale and wheat straws indicated that the values of r(2), RMSEP, and RPD were 0.952, 0.27, and 4.63, respectively. This model also showed good predictive ability and could predict lignin contents in triticale and wheat straws with the same high accuracy.  相似文献   
553.
A total of 1,000 rectal samples were collected from rabbits coming from 25 rabbit farms in southern Italy. All samples were processed for isolation of Salmonella spp. by standard culture method based on the ISO 6579:2002 method. Salmonella spp. was isolated from 1/25 rabbit farms analyzed. In particular, four out of 1,000 rectal swab samples, taken from young rabbits, were serotyped as S. Typhimurium and phage typed as S. Typhimurium DT104. All the isolates were resistant to ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline (ACSSuT pentaresistance type). The findings of the present study suggest the rabbit as potential carrier of S. Typhimurium DT104.  相似文献   
554.
Size selectivity of basket traps for changeable nassa (Nassarius mutabilis) was estimated through comparative fishing trials with traps varying in mesh size and net colour. Selectivity curves of two commercial (19 mm white twine and 19 mm black twine) and three experimental (23, 26 and 28 mm white twine) traps were estimated using SELECT method. In addition, theoretical size-selectivity curves were calculated from morphometric data using a simplified version of Sechin method.Differing from the opinion of fishermen, N. mutabilis did not show any preference for dark coloured twine. Retention curves estimated with SELECT method had a very sharp selectivity and well fitted catch data in all cases. Theoretical size-selectivity curves were very close to size-selectivity curves obtained from experimental data. The two types of curve were very similar in shape, although the theoretical ones were shifted towards larger size classes and showed a slightly steeper slope than SELECT curves. The mesh size commonly used by fishermen is not adequate for rationally exploiting the resource, as catches mainly included undersized specimens, whilst the 23 and 26 mm mesh sizes would represent a good compromise between reduced occurrence of undersized individuals and satisfactory yields of commercial product.  相似文献   
555.
  • 1. Nine cetacean species have been reliably reported to occur in the shallow northern Adriatic Sea since the 17th century. However, only two species were considered regular there until the 1970s: the short‐beaked common dolphin (Delphinus delphis) and the common bottlenose dolphin (Tursiops truncatus).
  • 2. Short‐beaked common dolphins have progressively disappeared from the northern Adriatic and are now rare in the region. The systematic culling campaigns and other takes that occurred between the second half of the 18th century and the 1960s, and habitat degradation in subsequent years are the most likely causes of their decline.
  • 3. Today, common bottlenose dolphins are the only regular component of the northern Adriatic cetacean fauna; however, they now occur at low densities, and their fragmented groups are facing significant anthropogenic threats.
  • 4. The future of northern Adriatic dolphins will depend on precautionary action to prevent further decline and on intensified research effort aimed at identifying the most effective mitigation strategies.
Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
556.
557.
Soy isoflavones (the phytoestrogens genistein, daidzein and glycitein) may act as estrogen receptor agonists or antagonists. The aim of this study was to examine the effect of dietary isoflavones on growth, reproduction and health in rainbow trout (Oncorhynchus mykiss). Rainbow trout fed three experimental diets containing different concentrations of isoflavones (0, 500 and 1,500 ppm). Growth, estrogenic activity, plasma estradiol levels, gonadal development, state of stress and histological changes in selected tissues were evaluated at the end of 70 days. Neither growth performance nor the relative mRNA levels of Insulin Growth Factor I (igf‐I) in the liver were influenced by different levels of dietary isoflavones. Plasma and liver vitellogenin (VTG) protein levels and plasma 17‐β‐estradiol (E2) were unaffected by treatments, although the correlation between plasma levels of E2 and VTG densitometry values was significant (< .05). The fish gonadosomatic index (GSI) did not significantly differ among the three experimental groups but correlated with plasma VTG densitometry values (< .05). Plasma, muscle and fin cortisol concentrations fell within the normal welfare range and were not correlated with isoflavone levels. Histologically, the distal intestine showed a normal morphology with well‐differentiated enterocytes and in the liver hepatocytes were also normal. A supranuclear accumulation of lipid droplets in enterocytes and some lipid droplets in hepatocytes were observed in all tested groups, suggesting an impact of basal dietary lipid on transport/metabolism of fat in the fish. Overall, the present results suggest that the doses of isoflavones tested do not compromise rainbow trout reproduction, growth and health.  相似文献   
558.
This study aimed to describe the response variability of female silver eels in terms of gonad development and eggs production to a standardized gonadotropic treatment (Carp pituitary extracts—CPE), and to relate this variability to population characteristics. For this purpose, sexual maturation, ovulation, and fertilization were induced in two eel populations coming from different locations in Adriatic Sea (Comacchio—CM and Marano-Grado—MG lagoons), and after that, their reproductive capacity was valuated. External (Silver index—SI, Eye index—EI, Pectoral fin length index—PFLI, Condition factor—K) and hormonal (17β-estradiol—E2, testosterone—T) parameters were measured, and some subject/group were killed for histological and lipid analysis and age determination. Morphometric parameters showed the CM-Group to have highest values of Body weight (BW), Body length (BL), and K, while MG-Group presented highest PFLI and Gonadosomatic index (GSI) values. Regarding hormonal analysis, the CM-Group showed significantly higher T and E2 levels than the MG-Group, both groups showed considerably rapid increase at T5 (5th injection). A positive trend in gonadal development was found through histological evaluation; a more regular maturation was observed in the MG-Group, whereas the CM-Group presented an exponential oocytes development starting from T10 (10th week), which led to an anticipated spawning. Lipid content showed significant differences in T0 (start study), post-ovulation, and Control (30th week) between CM and MG eels. As to zootechnical performances, while MG eels released spontaneously into the water, the CMs were stripped in order to check ovulation. The MG eels were statistically the most productive with 40.1 ± 6.33 % BW of eggs released. Furthermore, CM females ovulated mainly between the 19th and 22nd week (77.8 % spawned eels) instead in the MG’s ovulation goes from the 24th to the 28th week (100 % spawned eels). As fertilization is of concern, in both groups fertilized eggs were obtained with no difference in larvae production. These results seem to indicate that bigger dimensions, higher K, and larger lipid content (Comacchio eels) could fasten gonadic maturation without positively influencing reproductive performance of animals, both in term of quantity and quality of produced eggs. Smaller females with a highest SI (Marano-Grado eels) presented a more regular gonadic development, leading the animals to spontaneous spawning.  相似文献   
559.
To ensure the proper use of plant protection products, the European Directive 2009/128/CE requires sprayer inspection throughout Europe. The required tests and methods for inspection of agricultural sprayers are outlined in the European Standard EN 13790: 2003 in which nozzle flow rate measurement is one of the main requirements. Currently, most of the sprayer inspection workshops are equipped with a nozzle flow rate measurement device which consists of air-tight nozzle adaptors connected to 2-3 m hoses which discharge to a bank of graduated cylinders. However, these adaptors block the air holes of the air-induction nozzles, disrupt the spray atomization and change nozzle output. The main objective of this study was to investigate the variation of nozzle flow rate due to the use of air-tight adaptors. A series of nozzle flow rate measurements were made using different kinds of nozzle adaptors with various sizes of air induction, extended range and Turbo TeeJet® nozzles.The use of air-tight nozzle adaptors increased the flow rates of air induction and extended range nozzles. Plugging the air holes of air-induction nozzles increased the flow rate. The increase in flow rate decreased with nozzle pressure. Overall, the magnitude of the flow rate increase depended on the nozzle type but remained constant for all of the same type. These differences should be accounted for in sprayer calibration.  相似文献   
560.
Using theonellasterol as a novel FXR antagonist hit, we prepared a series of semi-synthetic derivatives in order to gain insight into the structural requirements for exhibiting antagonistic activity. These derivatives are characterized by modification at the exocyclic carbon-carbon double bond at C-4 and at the hydroxyl group at C-3 and were prepared from theonellasterol using simple reactions. Pharmacological investigation showed that the introduction of a hydroxyl group at C-4 as well as the oxidation at C-3 with or without concomitant modification at the exomethylene functionality preserve the ability of theonellasterol to inhibit FXR transactivation caused by CDCA. Docking analysis showed that the placement of these molecules in the FXR-LBD is well stabilized when on ring A functional groups, able to form hydrogen bonds and π interactions, are present.  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