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   1308篇
  免费   100篇
林业   114篇
农学   44篇
基础科学   4篇
  291篇
综合类   79篇
农作物   86篇
水产渔业   177篇
畜牧兽医   489篇
园艺   29篇
植物保护   95篇
  2023年   17篇
  2022年   23篇
  2021年   47篇
  2020年   63篇
  2019年   78篇
  2018年   74篇
  2017年   67篇
  2016年   62篇
  2015年   43篇
  2014年   76篇
  2013年   72篇
  2012年   99篇
  2011年   111篇
  2010年   51篇
  2009年   53篇
  2008年   67篇
  2007年   63篇
  2006年   54篇
  2005年   48篇
  2004年   50篇
  2003年   35篇
  2002年   39篇
  2001年   18篇
  2000年   7篇
  1999年   12篇
  1998年   3篇
  1997年   5篇
  1995年   7篇
  1994年   4篇
  1992年   3篇
  1991年   4篇
  1990年   5篇
  1988年   8篇
  1987年   3篇
  1986年   3篇
  1985年   2篇
  1984年   6篇
  1983年   3篇
  1981年   4篇
  1980年   3篇
  1979年   3篇
  1977年   1篇
  1975年   1篇
  1972年   1篇
  1971年   2篇
  1969年   1篇
  1968年   1篇
  1961年   1篇
  1959年   1篇
  1931年   1篇
排序方式: 共有1408条查询结果,搜索用时 62 毫秒
1.

Background

High latitude ecosystems are at present changing rapidly under the influence of climate warming, and specialized Arctic species at the southern margin of the Arctic may be particularly affected. The Arctic fox (Vulpes lagopus), a small mammalian predator endemic to northern tundra areas, is able to exploit different resources in the context of varying tundra ecosystems. Although generally widespread, it is critically endangered in subarctic Fennoscandia, where a fading out of the characteristic lemming cycles and competition with abundant red foxes have been identified as main threats. We studied an Arctic fox population at the Erkuta Tundra Monitoring site in low Arctic Yamal (Russia) during 10 years in order to determine which resources support the breeding activity in this population. In the study area, lemmings have been rare during the last 15 years and red foxes are nearly absent, creating an interesting contrast to the situation in Fennoscandia.

Results

Arctic fox was breeding in nine of the 10 years of the study. The number of active dens was on average 2.6 (range 0–6) per 100 km2 and increased with small rodent abundance. It was also higher after winters with many reindeer carcasses, which occurred when mortality was unusually high due to icy pastures following rain-on-snow events. Average litter size was 5.2 (SD = 2.1). Scat dissection suggested that small rodents (mostly Microtus spp.) were the most important prey category. Prey remains observed at dens show that birds, notably waterfowl, were also an important resource in summer.

Conclusions

The Arctic fox in southern Yamal, which is part of a species-rich low Arctic food web, seems at present able to cope with a state shift of the small rodent community from high amplitude cyclicity with lemming dominated peaks, to a vole community with low amplitude fluctuations. The estimated breeding parameters characterized the population as intermediate between the lemming fox and the coastal fox ecotype. Only continued ecosystem-based monitoring will reveal their fate in a changing tundra ecosystem.
  相似文献   
2.
The use of phytotherapy to treat human diseases has its roots in pre-historical times. Despite the modern advances achieved in the field of synthetic chemistry, the most efficient drugs available have their genesis directly or indirectly related with the vegetal kingdom. Indigenous communities have long used plant extracts to treat illnesses. Many of these extracts have shown effective action, with new bioactive compounds being extracted and screened every year. These extracts have also proven to be good sources of therapeutic agents to the treatment of Leishmaniasis. This work highlights some of these agents, while trying to emphasize the importance of plants as a source of new and powerful drugs against this widespread disease.  相似文献   
3.
'Meleira', or 'sticky disease', is currently the most damaging papaya disease in the mid-eastern Brazilian growing regions. Consistent disease transmission via latex injection, presence of similar isometric particles in the laticiferous vessels of diseased plants, and detection of double-stranded DNA in naturally and experimentally infected papaya trees suggest that a virus is the causal agent. Conclusive evidence for viral aetiology was previously lacking, mostly because every attempt to purify the putative virus from infected papayas had failed. Following the successful purification and partial characterization of the meleira virus, healthy papaya seedlings injected with purified virus particles later developed typical symptoms of the disease. Negatively stained, isometric, full and 'empty' purified virus particles measured 42 and 38 nm, respectively. The viral genome was a single dsRNA molecule of about 12 kbp. Several capsid proteins, ranging in size from 14·4 to 45 kDa, were consistently revealed by PAGE. Papaya meleira virus (PMeV) appears to represent a novel group of viruses, with no known similar counterpart among known plant-, vertebrate-, invertebrate- or prokaryote-infecting viruses.  相似文献   
4.
Tropical Animal Health and Production - This study evaluated the effect of two doses of prostaglandin at different intervals on reproductive parameters of crossbred ewes. In Experiment 1, 30 ewes...  相似文献   
5.

Using a cross-sectional survey, we determined the prevalence and risk factors associated with bovine brucellosis in herds under extensive production system in southwestern Nigeria. Antibodies to Brucella species in serum samples were tested using the Rose Bengal test (RBT) and competitive enzyme-linked immunosorbent assay (cELISA); for milk, the milk ring test (MRT) and indirect-ELISA (i-ELISA) were used. Questionnaire was administered to cattle herdsmen to determine factors predisposing the animals to bovine brucellosis. Data were analyzed using STATA 12. From 513 serum and 635 milk samples tested among 120 herds, overall animal-level prevalence of 10.1% (95% CI 7.5–12.7%) and 20.2% (95% CI 17.1–23.3%) were recorded by RBT and MRT, respectively; while 9.4% (95% CI 6.9–11.9%) and 17.8% (95% CI 14.8–20.8%) were obtained using cELISA and i-ELISA, respectively. In all, from the 120 herds tested, 29.2% and 43.3% were positive by RBT and MRT, respectively. Multivariable logistic regression revealed that herd location (OR?=?8.12, 95% CI 1.68–38.90) and improper disposal of placenta/fetus (OR?=?17.33, 95% CI 4.81–62.33) were predictors for a seropositive herd using RBT; while herd location (OR?=?5.13, 95% CI 1.27–20.28), large herd size (OR?=?2.62, 95% CI 1.15–5.85), and occurrence of abortion for a year or more (OR?=?4.62, 95% CI 1.53–13.71) were predictors of seropositivity to antibodies to Brucella spp. using MRT. We found high prevalence of brucellosis in cattle herds under extensive management system in southwestern Nigeria. Urgent and coordinated control strategies are required to mitigate this problem.

  相似文献   
6.
Tropical Animal Health and Production - A study was conducted to determine the effects of varied brooding regimes on the performance of small bronze turkey poults and their grow-outs. One hundred...  相似文献   
7.
The effects of corn replacement by cassava dreg in diets of crossbred goat kids were evaluated. We tested the impacts of 0, 33, 66 and 100% replacement on intake, digestibility, feeding behaviour, performance and carcass characteristics. Thirty-six goat kids, aged between 4 and 5 months and with initial body weights of 17.61 ± 1.98 kg, were used in a completely randomised design. Analysis of regression revealed a negative linear effect on neutral detergent fibre (NDF) intake and a positive linear effect on non-fibrous carbohydrates (NFC) and hydrocyanic acids (HCN) intake. Cassava dreg use had a positive linear effect on organic matter digestibility and non-fibrous carbohydrates. Based on our results, cassava dreg use did not negatively impact animal performance, feeding behaviour and carcass characteristics, suggesting that it may replace corn up to 100% in the diets of confined goat kids.  相似文献   
8.
To compare the effects of PGF2α (dinoprost tromethamine) and d-cloprostenol in a two-dose protocol for estrus synchronization in hair sheep during breeding season in Yucatán, México, two experiments were conducted. In experiment 1, 61 cyclic hair sheep were divided into two groups: G1 (control n?=?30), two doses of 50 μg of dinoprost tromethamine IM with 12 days between applications, and G2 (n?=?31), two doses of 50 μg of d-cloprostenol IM at the same time interval. For determination of progesterone levels, 16 ewes from each group were randomly selected. In experiment 2, 70 cyclic hair sheep were assigned at the same treatments (G1 and G2, n?=?35) and 48 h after the second application, the ewes in estrus were detected by two vasectomized rams. Sheep with detected estrus were inseminated, and 45 days after, pregnant animals were identified by ultrasonography. An exact Fisher’s test was performed for the analysis of ewes in estrus (experiments 1 and 2) and number of pregnant ewes (experiment 2); for the comparison of time between end of treatment-estrus presentation, a survival analysis was used. Duration of estrus in hours was analyzed using a generalized mixed model (GLM) ANOVA whereas plasma progesterone concentrations were analyzed by non-linear regression. There were significant differences (P?<?0.05) in the proportion of ewes in estrus upon treatments (G1, 57% vs G2, 87% and G1, 37.1% vs G2, 65.7% in experiments 1 and 2, respectively), and between the end of treatment-onset estrus interval (P?<?0.01), survival curves showed the highest number of sheep in estrus between 40 and 48 h (G1, 43.7?+?8.05 h vs G2, 42.9?+?6.7 h, experiment 1). There were no significant differences (P?>?0.05) in duration of estrus (G1, 42?+?6.1 h, vs G2, 41.1?+?11.2 h, experiment 1) and pregnancy in the ewes that presented estrus, and were inseminated (G1, 38.4% vs 52.1%, experiment 2). With regard to concentrations of progesterone, significant differences (P?<?0.01) were found between treatments, and progesterone levels before the second application of d-cloprostenol were higher. In consideration of the results, it can be concluded that in a two-dose protocol of a luteolytic agent, more ewes presented estrus in response to d-cloprostenol compared to dinoprost tromethamine with similar pregnancy rates.  相似文献   
9.
In this study, we evaluated the effects of including cassava wastewater in the diet on the feeding behavior of feedlot lambs in 35 male uncastrated Santa Inês × Dorper crossbred lambs at an approximate age of 3 months, with an average live weight of 20.0?±?3.4 kg. Diets were formulated with hay of cassava shoots (roughage) and a concentrate based on corn and soybean, with a roughage:concentrate ratio of 50:50, plus inclusion of cassava wastewater at the levels of 0, 12, 24, 36, or 48 g/kg of the total diet. Feeding behavior was evaluated between the 46th and 52nd days of the experiment. Increasing cassava wastewater levels in the diet reduced (P?<?0.05) the intakes (kg/day) of dry matter and neutral detergent fiber as well as the efficiency of rumination (g/cud and g/h) of dry matter and neutral detergent fiber. The other behavioral parameters were not affected by wastewater inclusion in the diet. Therefore, the inclusion of up to 48 g/kg of cassava wastewater on fresh matter of diets is not recommended for feedlot lambs.  相似文献   
10.
We evaluated the prevalence of mycobacterial infections (i.e., Mycobacterium bovis and non-tuberculous mycobacteria [NTM]) and their associated risk factors among cattle herds and trade cattle in southwestern Nigeria. Through cross-sectional study design, cattle herds from three locations were screened using the single intradermal comparative cervical tuberculin test based on two diagnostic standards; more than 4 mm (? 4 mm) and more than 2 mm (? 2 mm) cut-off points. Abattoir study involved screening trade cattle for tuberculous lesions. Overall, 515 cattle from 45 herds were screened. Using >?4 mm, animal level and herd prevalence of 11.7 and 46.7% were recorded, respectively. Applying the ? 2 mm cut-off, animal level and herd prevalence increased to 31.1 and 60.0%, respectively. Significantly, using the ? 2 mm cut-off, cattle in medium size herds/extensive management system (OR?=?1.6; 95% CI 1.1–2.5) and Sokoto Gudali (OR?=?2.3; 95% CI 1.4–3.8) were more at risk of being positive reactors, while Rahaji (OR?=?0.3; 95% CI 0.1–0.7) breeds of cattle and cows in the peri-urban area (OR?=?0.4; 95% CI 0.2–0.9) were less at risk of being positive reactors. Again, M. avium reactor of 21.7% was observed. In the abattoir, 1797 cattle were examined with 126 lesions suggestive of tuberculosis (TB). Culture/molecular analyses confirmed 2.2% M. bovis and 0.9% NTM infections. Risk factors associated with bovine TB among trade cattle were sex (OR?=?4.0; 95% CI 1.2–13.5) and age (OR?=?0.3; 95% CI 0.1–0.9). We confirm 11.7% prevalence of mycobacterial infections among populations of cattle screened with breed and herd size being major risk factors.  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