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   1791篇
  免费   176篇
  国内免费   3篇
林业   97篇
农学   35篇
基础科学   8篇
  177篇
综合类   299篇
农作物   51篇
水产渔业   192篇
畜牧兽医   968篇
园艺   59篇
植物保护   84篇
  2024年   2篇
  2023年   34篇
  2022年   31篇
  2021年   45篇
  2020年   77篇
  2019年   72篇
  2018年   78篇
  2017年   69篇
  2016年   67篇
  2015年   47篇
  2014年   71篇
  2013年   89篇
  2012年   114篇
  2011年   164篇
  2010年   101篇
  2009年   67篇
  2008年   116篇
  2007年   90篇
  2006年   78篇
  2005年   70篇
  2004年   80篇
  2003年   70篇
  2002年   61篇
  2001年   16篇
  2000年   15篇
  1999年   11篇
  1998年   18篇
  1997年   18篇
  1996年   23篇
  1995年   13篇
  1994年   12篇
  1993年   17篇
  1992年   18篇
  1991年   20篇
  1990年   18篇
  1989年   17篇
  1988年   10篇
  1987年   10篇
  1986年   4篇
  1985年   3篇
  1984年   5篇
  1983年   7篇
  1982年   3篇
  1921年   2篇
  1917年   2篇
  1916年   2篇
  1913年   2篇
  1906年   1篇
  1903年   1篇
  1902年   1篇
排序方式: 共有1970条查询结果,搜索用时 109 毫秒
91.
92.
Equine chorionic gonadotropin (eCG), obtained from pregnant mares, is used for assisted reproductive technologies in laboratory rodents and livestock. The objective of the present study was to use equine follicle-stimulating hormone (eFSH) to increase the incidence of twin pregnancies, through multiple ovulations, and increase eCG. Nineteen light horse–type mares were enrolled in the study. The control group (n = 9) was bred with fresh or cooled semen and given human chorionic gonadotropin (hCG) at the time of breeding. The second group (n = 10) was given 12.5 mg of eFSH intramuscularly twice a day beginning 5–7 days after ovulation. Prostaglandin F2α was administered intramuscularly the second day of eFSH treatment. Treatment with eFSH continued until follicles were >35 mm in diameter, and mares were then given no treatment for 36 hours. The mares were then bred with fresh or cooled semen from the same stallion as the control group and given hCG. Blood samples were taken weekly from day 35 to day 105 after ovulation. Serum concentration of eCG was obtained, and data were analyzed with multivariate analysis using the mixed procedure. Significance was set at P < .05. Data were combined for all mares carrying twins and compared with those carrying singletons. The group of mares carrying twins had higher peak concentrations of eCG and higher values for area under the curve compared with mares carrying singletons (P < .05). These results suggest inducing twins could be a method used to increase eCG production.  相似文献   
93.
The length of the photoperiod has been implicated as affecting growth of sunshine bass. This study was done to determine if photoperiod length might affect insulin‐like growth factor‐I (IGF‐I), which is a major hormonal regulator of growth in fish. Growth, feed conversion, peritoneal fat content, and plasma concentrations of IGF‐I of sunshine bass were compared on fish held on a short (8 h) or a long (16 h) photoperiod, or after the fish were switched from one photoperiod to the other. Fish were fed daily to apparent satiation for up to 5 wk and were not fed for the last 2 wk of the experiment. Body weight and intraperitoneal fat content increased for the first 2 wk of the experiment and then remained steady for the remainder of the experiment, and feed consumption decreased from 3.5% during the first 2 wk to about 1.5% for the second 2 wk and finally to only 1% during the last week of feeding. Plasma IGF‐I concentrations fell steadily during the entire experiment and was at the lowest level during the last 2 wk when the fish were not fed. There was no consistent effect of the photoperiod in any of the conditions; however, feeding and IGF‐I concentrations were affected by fish density.  相似文献   
94.
95.
Artemia franciscana were hatched and tested for tolerance to ammonia at pH 6.5 and 8.5 in artificial seawater with a salinity of 17 ‰. Nauplii seemed to be less tolerant of ammonia at pH 6.5 despite the fact that more of the ammonia is in the ionized form (NH4+). It is suggested that this greater sensitivity is a result of the NH4+ competing with the Na+ during gut transfer. Nauplii hatched from decapulated cysts were more tolerant of ammonia than those hatched from whole cysts, probably because of a greater energy reserve.  相似文献   
96.
The interactive effects of salinity and temperature on development and hatching success of lingcod, Ophiodon elongatus Girard, were studied by incubating eggs at four temperatures (6, 9, 12 and 15°C) and five salinities (15, 20, 25, 30 and 35 g L?1). Hatch did not occur in any of the 15°C treatments. Degree days (°C days) to first hatch was not influenced by temperature or salinity, however, calendar days to first hatch differed significantly for temperature (P<0.0001, 61±1, 44±1 and 35±1 days for 6, 9 and 12°C respectively). Degree days to 50% (427.1±4.2) hatch was not significantly influenced by temperature but was by salinity (P=0.0324). Viable hatch (live with no deformities, 74.1±4.0%) was greatest at 9°C and 25 g L?1 but not significantly different in the range of 20–30 g L?1. Larval length (9.4±0.13 mm) was greatest at 9°C and 20–30 g L?1. Temperature and salinity significantly influenced all categories of deformities with treatments at the upper (12°C and 35 g L?1) and lower limits (6°C and 15 g L?1) producing the greatest deformities. The optimal temperature and salinity for incubating Puget Sound lingcod eggs was found to be 9°C and 20–30 g L?1.  相似文献   
97.
Fishery collapses cause substantial economic and ecological harm, but common management actions often fail to prevent overfishing. Minimum length limits are perhaps the most common fishing regulation used in both commercial and recreational fisheries, but their conservation benefits can be influenced by discard mortality of fish caught and released below the legal length. We constructed a computer model to evaluate how discard mortality could influence the conservation utility of minimum length regulations. We evaluated policy performance across two disparate fish life‐history types: short‐lived high‐productivity (SLHP) and long‐lived low‐productivity (LLLP) species. For the life‐history types, fishing mortality rates and minimum length limits that we examined, length limits alone generally failed to achieve sustainability when discard mortality rate exceeded about 0.2 for SLHP species and 0.05 for LLLP species. At these levels of discard mortality, reductions in overall fishing mortality (e.g. lower fishing effort) were required to prevent recruitment overfishing if fishing mortality was high. Similarly, relatively low discard mortality rates (>0.05) rendered maximum yield unobtainable and caused a substantial shift in the shape of the yield response surfaces. An analysis of fishery efficiency showed that length limits caused the simulated fisheries to be much less efficient, potentially exposing the target species and ecosystem to increased negative effects of the fishing process. Our findings suggest that for overexploited fisheries with moderate‐to‐high discard mortality rates, reductions in fishing mortality will be required to meet management goals. Resource managers should carefully consider impacts of cryptic mortality sources (e.g. discard mortality) on fishery sustainability, especially in recreational fisheries where release rates are high and effort is increasing in many areas of the world.  相似文献   
98.
99.
  1. Intermediate metrics of translocation success are useful for long-lived, slow to mature species where survival and reproduction happen over decades.
  2. With fewer than 150 individuals in the wild, the Critically Endangered Philippine crocodile (Crocodylus mindorensis) is one of the most threatened species on Earth. This study presents the first analysis of diet and body condition of wild Philippine crocodiles and headstarted (i.e. captive-reared) individuals released into the wild over the last decade, and uses these results to show how diet and body condition can be pertinent intermediate metrics of translocation success.
  3. Analyses of stomach contents revealed 17 different aquatic and terrestrial invertebrate and vertebrate prey species. Interestingly, 70% of Philippine crocodiles showed snails to be the predominent prey type, followed by fish (36.7%), birds (33.3%) and reptiles (33.3%). More than 50% of crocodiles consumed the invasive golden apple snail, a leading agricultural pest. Regardless of crocodile history (wild vs. headstarted) or size class (juvenile vs. adult), no evidence was found for dietary differences in percentage occurrence, percentage composition or prey diversity.
  4. Body condition was significantly higher in wild compared with headstarted individuals when analysed together in a pooled group, although neither group differed significantly from the standardized expectation, and headstarted individuals were not significantly different when body condition was derived independently for the two groups.
  5. This study provides a working example of how assessing the convergence of diet and body condition between translocated and wild individuals can provide complementary monitoring parameters to demonstrate post-release establishment of translocated crocodylians. The congruent dietary composition and comparable body condition observed in this study suggest that headstarted crocodiles adapt well following release.
  6. Crocodylus mindorensis survives in an agricultural landscape and is likely to play an ecologically important role by exploiting invasive species, reinforcing the importance of this species to local communities.
  相似文献   
100.
Ionomic profiles are primarily influenced by genetic and environmental factors. Identifying ionomic responses to varietal effects is necessary to understand the ionomic variations among species or subspecies and to potentially understand genetic effects on ionomic profiles. We cultivated 120 rice (Oryza sativa) varieties to seedling stage in identical hydroponic conditions and determined the concentrations of 26 elements (including 3 anions) in the shoots and roots of rice. Although the subspecies effects were limited by the genus Oryza pre-framework and its elemental chemical properties, we found significant differences in ionomic variations in most elements among the aus, indica and japonica subspecies. Principal component analysis of the correlations indicated that variations in the root-to-shoot ionomic transport mechanisms were the main causes of ionomic differences among the subspecies. Furthermore, the correlations were primarily associated with the screening of varieties for elemental covariation effects that can facilitate breeding biofortified rice varieties with safe concentrations of otherwise toxic elements. The japonica subspecies exhibited the strongest elemental correlations and elemental covariation effects, therefore, they showed greater advantages for biofortification than the indica and aus subspecies, whereas indica and aus subspecies were likely safer in metal(loid) polluted soils. We also found that geographical and historical distribution significantly defined the ionomic profiles. Overall, the results of this study provided a reference for further association studies to improve the nutritional status and minimize toxicity risks in rice production.  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