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   1111篇
  免费   65篇
  国内免费   2篇
林业   42篇
农学   20篇
基础科学   15篇
  164篇
综合类   167篇
农作物   20篇
水产渔业   34篇
畜牧兽医   634篇
园艺   8篇
植物保护   74篇
  2021年   10篇
  2020年   8篇
  2019年   20篇
  2018年   17篇
  2017年   25篇
  2016年   18篇
  2015年   20篇
  2014年   21篇
  2013年   37篇
  2012年   32篇
  2011年   49篇
  2010年   34篇
  2009年   21篇
  2008年   36篇
  2007年   42篇
  2006年   45篇
  2005年   34篇
  2004年   32篇
  2003年   33篇
  2002年   25篇
  2001年   34篇
  2000年   24篇
  1999年   26篇
  1996年   8篇
  1995年   12篇
  1994年   8篇
  1992年   18篇
  1991年   29篇
  1990年   24篇
  1989年   12篇
  1988年   21篇
  1987年   24篇
  1986年   15篇
  1985年   28篇
  1984年   19篇
  1983年   17篇
  1982年   13篇
  1981年   13篇
  1980年   8篇
  1979年   12篇
  1978年   13篇
  1977年   18篇
  1974年   10篇
  1973年   25篇
  1972年   19篇
  1971年   12篇
  1970年   9篇
  1969年   10篇
  1967年   11篇
  1966年   15篇
排序方式: 共有1178条查询结果,搜索用时 22 毫秒
1.
  1. Understanding population size and trend is critical information in species management and conservation. To enable accurate population trend estimates, consistent robust monitoring of a species is essential, particularly for a species such as the New Zealand (NZ) sea lion, Phocarctos hookeri , which has experienced an almost continuous decline in pup production since the late 1990s.
  2. This research examines the pup production estimates for all known breeding sites for this species, and using a stage‐structured matrix population model, estimates population size and trend between 1995 and 2015.
  3. Overall, it is estimated that 2,316 pups were born in 2015, a decrease of 13% since 1995 and a 27% decline since the highest pup production estimate in 1998. This decline has been driven by the significant decline of 48% at the main breeding area, the Auckland Islands since 1998.
  4. Using the stage‐structured matrix population model a total species population size of 11,767 sea lions (95% CrI: 10,790–12,923) was estimated. This is the lowest population size of any sea lion species. Trend data for the Auckland Islands indicated that pup and population numbers have decreased at 1.9% yr?1 in the last 20 yr, while total species population decline is 0.6% yr?1.
  5. Estimates of population trends for this species have been hindered by inconsistent monitoring at most breeding sites. This study strengthens the growing field of research highlighting the need for consistent long‐term monitoring for the conservation management of endangered species.
  相似文献   
2.
A 10‐year‐old, domestic shorthair cat was presented for acute lameness of the left forelimb accompanied by severe pain, swelling, skin necrosis, malodorous discharge and pyrexia. Following a presumptive diagnosis of necrotising fasciitis aggressive surgical debridement of the affected soft tissues of the antebrachium and negative pressure wound treatment of the open defect were performed. Surgical findings supported the tentative diagnosis of necrotising fasciitis and Streptococcus canis was isolated from the wound. A free skin graft was performed 29 days after admission, and augmented by 3 days of negative pressure wound therapy to facilitate graft incorporation. Healing was achieved without complications and no functional or aesthetic abnormalities remained.  相似文献   
3.
4.
5.
The effectiveness of windbreaks in windspeed reduction is often evaluated without regard to the objects to be protected. In fact, many objects may have different sensitivities to wind and often require different degrees of wind protection. Since commonly used indexes do not consider the sensitivity to wind, a concept of specific protection efficiency is developed. A critical windspeed is used to represent the sensitivity of each object. Windspeeds greater than this value are considered damaging. A dimensionless protection index is defined to evaluate windbreak efficiency. The maximum index value is 1 for the highest protection, and the index is negative when sheltered windspeed is greater than the critical windspeed. This index can be compared, summed, and averaged across different windbreaks, objects, and leeward locations. A sample of critical windspeed values was compiled from the literature. The index was evaluated using actual wind data measured under both sheltered and open conditions. The results indicated that the index can be used for evaluating windbreak effectiveness in terms of objects protected under various conditions. This model could be used as a tool for windbreak-related research and policy making.  相似文献   
6.
7.
Beech (Fagus sylvatica L.) seedlings were cultivated from seeds sown in pots or directly in the ground in outdoor chambers that were transparent to solar radiation, and provided either ambient air or CO(2)-enriched air (ambient + 350 &mgr;mol mol(-1)). The rooting volume was high in all experiments. In the short-term experiment, potted plants were assigned to a factorial CO(2) x nutrient treatment (optimal nutrient supply and severe nutrient shortage) for 1 year. In the long-term experiment, plants were grown directly in the ground and received an optimal supply of water and nutrients in both CO(2) treatments for 3 years. Nutrient stress caused carboxylation capacity (V(m)) to decrease in the potted seedlings exposed to CO(2)-enriched air during their first growing season. In the long-term experiment with optimal nutrient supply, CO(2)-enriched air did not affect V(m), but caused an upward acclimation of maximum electron transport rate (J(m)). Consequently, there was a 14% increase in the J(m)/V(m) ratio, indicating nitrogen reallocation to maintain an equilibrium between RuBP consumption and RuBP regeneration. Both V(m) and J(m) decreased during the growing season in both CO(2) treatments. Although upward acclimation of J(m) was no longer apparent at the end of the third growing season, plants in CO(2)-enriched air maintained a higher J(m)/V(m) ratio than plants in ambient air, indicating that photosynthetic acclimation always occurred. Second flush leaves appeared during each growing season. When expressed on the basis of foliar nitrogen concentration, their photosynthetic characteristics (V(m) and J(m)) were enhanced compared with other leaves. Because the number of second flush leaves was also increased in the elevated CO(2) treatment, this response should be taken into account when modeling the effects of elevated CO(2) concentration on canopy photosynthesis. Stomatal conductance decreased in response to atmospheric CO(2) enrichment; however, the stomatal response to irradiance followed a single relationship based on two stomatal conductance models.  相似文献   
8.
Summary Oven dried basswood was vacuum impregnated with methyl methacrylate monomer containing various amounts of free radical catalyst and trifunctional crosslinking agent. The wood-monomer composite was polymerized at 60°C in an air oven and the rate of heat generated was followed by means of a thermocouple placed in the center of the sample.Increasing the concentration of Vazo free radical catalyst in the basswood-MMA impregnated samples reduces the time to the exothermic peak and increases the exothermic peak temperature. High Vazo concentrations also decraase the percentage conversion of monomer to polymer.Trimethyl propane trimethacrylate (TMPTMA) crosslinker reduces the time to the exothermic peak and dramatically increases the exothermic peak temperature. The use of a crosslinker rapidly increases the initial viscosity of the system, and therefore, the autoacceleration of the polymerization is more pronounced.Our thanks are due to DuPont Company and Sartomer Resins for supplying the monomers and catalysts. Research funds were provided by Organized Research Budget of the College of Forestry.  相似文献   
9.
Target spot of soybean has spread in Brazil, the southeastern United States and Argentina in the last decade. A collaborative network of field Uniform Fungicide Trials (UFT) in Brazil was created in 2011 to study the target spot control efficacy of fungicides, including azoxystrobin + benzovindiflupyr (AZ_BF), carbendazim (CZM), fluxapyroxad + pyraclostrobin (FLUX_PYRA), epoxiconazole + FLUX_PYRA (EPO_FLUX_PYRA), mancozeb (MZB) and prothioconazole + trifloxystrobin (PROT_TRIF). Network meta-analysis was used to conduct a quantitative synthesis of UFT data collected from 2012 to 2016 and to evaluate the effects of disease pressure (DP, low ≤ 35% target spot severity in the nontreated control < high) and year of experiment on the overall mean efficacy and yield response to each of the tested fungicides. Based on mean percentage control of target spot severity, the tested fungicides fall into three efficacy groups (EG): high EG, FLUX_PYRA (76.2% control relative to the nontreated control) and EPO_FLUX_PYRA (75.7% control); intermediate EG, PROT_TRIF (66.5% control) and low EG, MZB (49.6% control), AZ_BF (46.7% control) and CZM (32.4% control). DP had a significant effect on yield response. At DPLow, the highest response was due to PROT_TRIF (+342 kg ha−1, +12.8%) and EPO_FLUX_PYRA (+295.5 kg ha−1, +11.2%), whereas at DPHigh, EPO_FLUX_PYRA and FLUX_PYRA outperformed the other treatments, with yield responses of 503 kg ha−1 (+20.2%) and 469 kg ha−1 (+19.1%), respectively. The probability of a positive return on fungicide investment ranged from 0.26 to 0.56 at DPLow and from 0.34 to 0.66 at DPHigh.  相似文献   
10.
Summary Different soils are known to affect the amount and distribution of both available water and roots. Optimising irrigation water use, especially when shallow water-tables are present requires accurate knowledge of the root zone dynamics. This study was conducted to determine the effect of two soil types on root growth, soil water extraction patterns, and contributions of a water-table to crop evaporation (E). Two weighing lysimeters (L1 and L2) with undisturbed blocks of soil were used. The soil in L1 had higher hydraulic conductivity and lower bulk density than that in L2. Well watered conditions were maintained by irrigation for the first 110 days from sowing (DFS). Root length density (RLD) was calculated from observations made in clear acrylic tubes installed into the sides of the lysimeters. Volumetric soil water contents were measured with a neutron probe. A water-table (EC = 0.01 S m-1) was established 1 m below the soil surface 18 DFS. RLD values were greater in L1 than L2 at any depth. In L1, maximum RLD values (3 × 104 m m-3) were measured immediately above the water-table at physiological maturity (133 DFS). In L2, maximum RLD values (1.5 × 104 m m-3) were measured at 0.42 m on 120 DFS and few roots were present above the water-table. From 71 to 74 DFS, 55 and 64% of E was extracted from above 0.2 m for L1 and L2, respectively. In L2, extraction was essentially limited to the upper 0.4 m, while L1 extraction was to 0.8 m depth. Around 100 DFS the water-table contributed 29% (L1) and 7% (L2) of the water evaporated. This proportion increased rapidly as the upper soil layers dried following the last substantial irrigation 106 DFS. Over the whole season the water-table contributed 24% in L1 and 6.5% in L2 of total E.  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