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   121篇
  免费   14篇
林业   23篇
农学   1篇
  9篇
综合类   17篇
农作物   5篇
水产渔业   7篇
畜牧兽医   62篇
园艺   5篇
植物保护   6篇
  2023年   1篇
  2021年   3篇
  2020年   4篇
  2019年   1篇
  2018年   6篇
  2017年   7篇
  2016年   2篇
  2015年   2篇
  2014年   6篇
  2013年   4篇
  2012年   2篇
  2011年   10篇
  2010年   9篇
  2009年   7篇
  2008年   8篇
  2007年   6篇
  2006年   9篇
  2005年   1篇
  2004年   4篇
  2003年   4篇
  2000年   4篇
  1999年   3篇
  1998年   1篇
  1997年   3篇
  1996年   3篇
  1995年   1篇
  1993年   1篇
  1987年   1篇
  1985年   1篇
  1984年   3篇
  1983年   1篇
  1982年   1篇
  1980年   2篇
  1979年   1篇
  1977年   1篇
  1976年   1篇
  1973年   2篇
  1971年   2篇
  1969年   2篇
  1965年   1篇
  1964年   1篇
  1962年   1篇
  1960年   1篇
  1959年   1篇
排序方式: 共有135条查询结果,搜索用时 15 毫秒
1.
The objective was to study the use of ultrasound as a complementary test in the breeding soundness evaluation in male pigs and study the pattern of echogenicity of the testicular parenchyma in boars of different racial groups. Twenty‐six adult boars from four different racial groups were used, 10 from the Piau breed (group 1), four from the commercial and finishing group (group 2), six Pietrain breed (group 3) and six from the Duroc breed (group 4). All animals were evaluated for breeding soundness evaluation and the ultrasound examination of the testicles. The groups of animals that were evaluated showed no difference in the main semen parameters that were evaluated, except for the sperm volume, concentration of the ejaculated sperm and the supravital staining; the lowest figures were for the animals from the Piau breed (group 1). In relation to the testicular biometrics, Duroc animals (group 4) had a greater scrotal width compared to the other groups. But when we assessed the intensity of pixels of the testicles, there was a difference between groups. The groups 2 (finishing animals), 3 (Pietrain) and 4 had no difference between themselves. Group 3 had greater pixel intensity in relation to group 1. Of the 26 animals studied, five showed an abnormality during ultrasound evaluation, like hydrocele, hyperechoic mass in the testicular parenchyma, cyst in the head of the epididymis and the presence of fluid in the head and tail of the epididymis. The various animal groups studied did not differ in the principal reproductive parameters evaluated, showing that despite the great variability of reproductive traits between breeds and within the same breed, the breeding soundness evaluation, the more complete it is, is essential for the selection of breeders and the ultrasonography of the reproductive system becomes an important addition in this examination.  相似文献   
2.
3.
Ashton MJ, Layzer JB. Summer microhabitat use by adult and young‐of‐year snail darters (Percina tanasi) in two rivers. Ecology of Freshwater Fish 2010: 19: 609–617. © Published 2010. This article is a US Government work and is in the public domain in the USA. Abstract – We characterised microhabitat availability and use by adult and young‐of‐year (YOY) snail darters (Percina tanasi Etnier 1976 ) while snorkelling in the French Broad and Hiwassee rivers, TN, USA. Both age groups of snail darters disproportionately used most microhabitat variables compared to their availability. Snail darters primarily occupied moderately deep, swift water over gravel substrates with little macrophyte coverage and no silt. Univariate comparisons indicated that adult and YOY darters occupied different habitat, but there was no marked differences between principal components analysis plots of multivariate microhabitat use within a river. Although the availability of microhabitat variables differed between the French Broad and Hiwassee rivers, univariate means and multivariate plots illustrated that the habitats used were generally similar by age groups of snail darters between rivers. Because our observations of habitat availability and use were constrained to low flow periods and depths <1 m, the transferability of our results to higher flow periods may be limited. However, the similarity in habitat use between rivers suggests that our results can be applied to low‐normal flow conditions in other streams.  相似文献   
4.
5.
This study aimed to assess the effects of different cooling curves and centrifugation regimes used in cryopreservation protocols on the post‐thaw viability of Piau‐breed wild boar (Sus scrofa) sperm using in vitro assessment tests. Two centrifugations (800  g for 10 min and 2400  g for 3 min) and two cooling curves (conventional cooling using nitrogen vapour – freezing 1 and automated cooling using a programmed freezing machine – freezing 2) were tested. Therefore, the treatments were divided into M3 – centrifugation at 2400  g for 3 min and freezing 2; M10 – centrifugation at 800  g for 10 min and freezing 2; R3 – centrifugation at 2400  g for 3 min and freezing 1; and R10 – centrifugation at 800  g for 10 min and freezing 1. No significant differences (p > 0.05) between treatments occurred post‐thawing regarding the total sperm motility means recorded. The mean values of the different treatments were not different from each other regarding the supravital staining (SV), hypo‐osmotic test (HO), sperm–egg binding assay or sperm morphology. This study showed that both the cooling curve and the centrifugation regime affected the quality of post‐thaw sperm, and centrifugation for shorter times and cooling curves using automated cooling are the most suitable for minimizing sperm injury.  相似文献   
6.
7.
Essential to progress in plant biotechnology for deliverable applications, patent and patent application data must be transparent in order to determine where there is whitespace for innovation.  相似文献   
8.
Interest in native species is growing across the tropics as reforestation of degraded lands becomes more widespread. In this study four tree species native to Panama - Cedrela odorata, Pachira quinata, Samanea saman, and Tabebuia rosea - were grown on rural farms at two dry tropical sites in Panama for up to five years. Survivorship and growth data at these “on-farm” trials are compared to data recorded from nearby experimental or “species selection” trial sites and evaluated in terms of soil fertility and management. Participant farmers were also asked about their interest in planting trees in general as well as their interest in 61 species grown in the species selection trial.Although, on-farm survivorship was variable and generally lower than that found on the species selection trial, one species (S. saman) experienced high and consistent survivorship. High survivorship combined with growth data from farms at both sites for this species suggests it would be a good candidate for extension projects working with rural farmers. Survivorship of other species appears more sensitive to farmer management and/or local site conditions. Generally lower growth on the Los Santos farms as compared to the species-selection trial is attributed to the lower soil fertility (plant available P) at the on-farm sites compared to the species selection trial. In contrast, only one species - P. quinata - had a growth variable found to be significantly lower between the on-farm and species selection trial sites in Rio Hato. C. odorata, P. quinata T. rosea can all be used in on-farm conditions with consideration to specific site and management conditions.By 2009, approximately 80% of the farmers planting trees still wished to participate in tree planting activities. All of the farmers no longer wishing to continue with the project expressed slow growth rates of trees as a principal reason. All but one of these farmers had growth rates for his/her trees markedly below those of the species selection trial nearby. Some farmers wishing to continue had very high mortality rates (>70% for all species), suggesting non-tangible benefits for participating in project activities. Other species that were not tested on-farm but grew well in the species-selection trials and were of interest to local farmers are discussed.As long as specific site and management conditions are carefully considered, data from species selection trials can be useful in informing tree planting projects with rural farmers; however, care should be taken to manage expectations.  相似文献   
9.
We report the draft genome sequence of the model moss Physcomitrella patens and compare its features with those of flowering plants, from which it is separated by more than 400 million years, and unicellular aquatic algae. This comparison reveals genomic changes concomitant with the evolutionary movement to land, including a general increase in gene family complexity; loss of genes associated with aquatic environments (e.g., flagellar arms); acquisition of genes for tolerating terrestrial stresses (e.g., variation in temperature and water availability); and the development of the auxin and abscisic acid signaling pathways for coordinating multicellular growth and dehydration response. The Physcomitrella genome provides a resource for phylogenetic inferences about gene function and for experimental analysis of plant processes through this plant's unique facility for reverse genetics.  相似文献   
10.
The asymmetric alpha-addition of relatively nonpolar hydrocarbon substrates, such as allyl and aryl groups, to aldehydes and ketones remains a largely unsolved problem in organic synthesis, despite the wide potential utility of direct routes to such products. We reasoned that well-established chiral amine catalysis, which activates aldehydes toward electrophile addition by enamine formation, could be expanded to this important reaction class by applying a single-electron oxidant to create a transient radical species from the enamine. We demonstrated the concept of singly occupied molecular orbital (SOMO) activation with a highly selective alpha-allylation of aldehydes, and we here present preliminary results for enantioselective heteroarylations and cyclization/halogenation cascades.  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