全文获取类型
收费全文 | 916篇 |
免费 | 68篇 |
国内免费 | 2篇 |
专业分类
林业 | 65篇 |
农学 | 14篇 |
基础科学 | 3篇 |
128篇 | |
综合类 | 179篇 |
农作物 | 18篇 |
水产渔业 | 106篇 |
畜牧兽医 | 376篇 |
园艺 | 41篇 |
植物保护 | 56篇 |
出版年
2024年 | 5篇 |
2023年 | 11篇 |
2022年 | 9篇 |
2021年 | 28篇 |
2020年 | 31篇 |
2019年 | 37篇 |
2018年 | 34篇 |
2017年 | 21篇 |
2016年 | 30篇 |
2015年 | 30篇 |
2014年 | 24篇 |
2013年 | 53篇 |
2012年 | 56篇 |
2011年 | 78篇 |
2010年 | 40篇 |
2009年 | 30篇 |
2008年 | 47篇 |
2007年 | 62篇 |
2006年 | 60篇 |
2005年 | 53篇 |
2004年 | 43篇 |
2003年 | 37篇 |
2002年 | 36篇 |
2001年 | 11篇 |
2000年 | 10篇 |
1999年 | 5篇 |
1997年 | 3篇 |
1995年 | 3篇 |
1993年 | 6篇 |
1992年 | 3篇 |
1991年 | 4篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1980年 | 4篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1969年 | 2篇 |
1968年 | 2篇 |
1965年 | 4篇 |
1963年 | 2篇 |
1942年 | 2篇 |
排序方式: 共有986条查询结果,搜索用时 31 毫秒
41.
Andrew S. Brierley Paul G. Fernandes Mark A. Brandon Frederick Armstrong Nicholas W. Millard Steven D. McPhail Peter Stevenson Miles Pebody James Perrett Mark Squires Douglas G. Bone Gwyn Griffiths 《Fisheries Research》2003,60(2-3):569-576
The autonomous underwater vehicle (AUV) Autosub-2 was deployed on eight missions ahead of RRS James Clark Ross in the northern Weddell Sea and in the Bransfield Strait, Southern Ocean, to assess avoidance of the research vessel by Antarctic krill Euphausia superba. The AUV was equipped with the same type of scientific echosounder as the research vessel (Simrad EK500 operating at 38 and 120 kHz) and measured the density of krill along transect acoustically (g m−2 wet mass) prior to the ship’s arrival. We hypothesised that if krill avoided the ship, perhaps in response to radiated noise, then the ship should detect less krill than the AUV which is known to have much lower noise levels than the ship. We were unable to detect any significant difference between the density of krill detected by the ship or the AUV, either at the transect level or at finer scales within transects. We conclude, therefore, that avoidance by krill of RRS James Clark Ross will not significantly bias acoustic estimates of krill abundance by this vessel. 相似文献
42.
The expansion of oil palm monocultures into globally important Southeast Asian tropical peatlands has caused severe environmental damage. Despite much of the current focus of environmental impacts being directed at industrial scale plantations, over half of oil palm land-use cover in Southeast Asia is from smallholder plantations. We differentiated a first generation smallholder oil palm monoculture into 8 different sampling zones, and further divided the 8 sampling zones into oil palm root influenced (Proximal) and reduced root influence (Distal) areas, to assess how peat properties regulate in situ carbon dioxide (CO2) and methane (CH4) fluxes. We found that all the physico-chemical properties and nutrient concentrations except sulphur varied significantly among sampling zones. All physico-chemical properties except electrical conductivity, and all nutrient content except nitrogen and potassium varied significantly between Proximal and Distal areas. Mean CO2 fluxes (ranged between 382 and 1191 mg m−2 h−1) varied significantly among sampling zones, and between Proximal and Distal areas, with notably high emissions in Dead Wood and Path zones, and consistently higher emissions in Proximal areas compared to Distal areas within almost all the zones. CH4 fluxes (ranged between −32 and 243 µg m−2 h−1) did not significantly vary between Proximal and Distal areas, however significantly varied amongst sampling zones. CH4 flux was notably high in Canal Edge and Understorey Ferns zones, and negative in Dead Wood zone. The results demonstrate the high heterogeneity of peat properties within oil palm monoculture, strengthening the need for intensive sampling to characterize a land use in the tropical peatlands. 相似文献
43.
Kristen C. Nelson Rachel F. Brummel Nicholas Jordan Steven Manson 《Agriculture and Human Values》2014,31(2):245-259
Multifunctional agricultural systems seek to expand upon production-based benefits to enhance family wellbeing and animal health, reduce inputs, and improve environmental services such as biodiversity and water quality. However, in many countries a landscape-level conversion is uneven at best and stalled at worst. This is particularly true across the eastern rural landscape in the United States. We explore the role of social networks as drivers of system transformation within dairy production in the eastern United States, specifically rotational grazing as an alternative management option. We hypothesize the importance of weak ties within farmer social networks as drivers of change. In Wisconsin, Pennsylvania, and New York, we conducted 53 interviews with confinement, low-intensity, and rotational grazing dairy producers as well as 35 interviews with associated network actors. Though confinement and grazier networks had similar proportions of strong and weak ties, confinement producers had more market-based weak ties, while graziers had more weak-ties to government agencies and other graziers in the region. These agency weak ties supported rotational graziers through information exchange and cost sharing, both crucial to farmers’ transitions from confinement-based production to grazing systems. While weak ties were integral to initial innovation, farmers did not maintain these relationships beyond their transition to grazing. Of equal importance, grazier weak-tie networks did not include environmental organizations, suggesting unrealized potential for more diverse networks based on environmental services. By understanding the drivers, we can identify barriers to expanding weak tie networks and emergent properties in order to create institutions and policies necessary for change. 相似文献
44.
Robert N. White BSc BVetMed Nicholas J. Macdonald BVetMed Carolyn A. Burton BVetMed PHD 《Veterinary radiology & ultrasound》2003,44(5):514-521
A retrospective study of the use of intraoperative mesenteric portovenography (IOMP) in the surgical management of congenital portosystemic shunts in 100 dogs and cats was performed. Each portovenogram was scored using a subjective visual analogue scale (VAS) and was assessed for the presence of portal atresia or hypoplasia. VAS scores and portal hypoplasia assessments were obtained for portovenogram images obtained for each animal both before shunt manipulation (preligation) and following the temporary, complete ligation of the vessel (postligation). In each patient, surgical records were reviewed to ascertain the degree of shunt attenuation that was achieved at surgery. Hepatic portal vasculature was significantly different on postligation compared with preligation IOMP. Sixty-two percent of animals had apparent portal hypoplasia or atresia on their preligation IOMP. The majority of these (81%) had an improvement in portal vasculature on postligation IOMP. It was concluded that both preligation and postligation IOMP provided valuable information regarding the morphology of congenital portosystemic shunts. An accurate assessment of an animal's portal vasculature could only be made following the interpretation of a postligation portovenogram, and these findings significantly influenced the surgical management of the patient. Although individuals with high postligation VAS scores were more likely to achieve full shunt attenuation at surgery, there was no quantifiable relationship between IOMP findings and the degree of shunt attenuation achieved. 相似文献
45.
Nicholas Frank Janice Sojka Nat T Messer 《Veterinary Clinics of North America: Equine Practice》2002,18(2):305-19, vii
Hypothyroidism is the most common type of thyroid gland dysfunction reported in horses. Primary, secondary, and tertiary causes of hypothyroidism are discussed. Equine hypothyroidism remains a controversial endocrine disorder because extrathyroidal factors, including the administration of drugs and systemic diseases, affect serum triiodothyronine (T3) and thyroxine (T3) concentrations in horses. Accurate diagnosis of hypothyroidism therefore requires assessment of the hypothalamic-pituitary-thyroid axis. Diagnostic procedures for evaluating thyroid gland function are outlined and results of studies utilizing experimental models are discussed. 相似文献
46.
Nicholas J. Macdonald BVetMed Carolyn A. Burton BVetMed PhD Robert N. White BSc BVetMed 《Veterinary radiology & ultrasound》2002,43(6):534-540
A visual analog scale and a numeric scoring scale were designed for the assessment of dynamic intraoperative mesenteric portovenography in the dog and cat. Two independent observers evaluated both scoring scales for reproducibility (differences between observers) and repeatability (within-observer differences) in the assessment of 60 trial portovenograms. Agreement (interchangeability) of both scales was evaluated by comparing the scores obtained in the assessment of 200 portovenograms obtained from 100 dogs and cats. There was no statistical difference between the two observers when scoring the same portovenogram for either the visual analog scale (p = .730, reproducibility coefficient = 17.85 units) or the numeric scoring scale (scores identical, reproducibility coefficient = 0). There was no statistical difference, for either of the observers, when the same portovenogram was assessed on two separate occasions using the visual analog scale (observer 1, p = .35, repeatability coefficient = 17.93 units; observer 2, p = .42, repeatability coefficient = 8.27 units) or the numeric scoring scale (scores given by both observers were identical, repeatability coefficient = 0 for both observers). The results of comparison between the visual analog scale and numeric scoring scale confirmed that the two scoring systems were not directly interchangeable. Although both scoring systems demonstrated good reproducibility and repeatability, the numeric scoring scale possessed a number of inherent deficiencies that suggested it was not the method of choice for the assessment of the subjective data obtained from dynamic intraoperative mesenteric portovenography. 相似文献
47.
48.
Cultivars of common bean (Phaseolus vulgaris L.) capable of yielding well at low levels of native or added phosphorus (P)
are highly desirable in many tropical production systems. The objective of the present study was to identify geographical
regions which might be sources of such genotypes. A total of 364 landraces, cultivars and wild genotypes, drawn from a broad
geographic range, were divided on the basis of growth habit into four field trials, each comprising two levels of P, stressed
and unstressed, on an infertile Andosol in Popayan, Colombia. The regression relationship between grain yield per plant in
the presence and in the absence of stress was determined, and each genotype's deviation from this relationship was used as
a measure of P-efficiency. There was highly significant variation in efficiency among genotypes in all growth habits, and
in climbing beans there were consistent regional differences, superior genotypes being identified with greater frequency among
those from Bolivia, West Mexico and South Mexico-West Guatemala. The latter region was promising for prostrate bush genotypes
also. Wild beans in general performed relatively poorly; it appears that P-efficiency traits in P. vulgaris have been acquired
during or after domestication. These results confirm that genetic differences in P-efficiency exist among common bean genotypes
and suggest that these are related to geographic origin. Furthermore, the use of a representative sample of germplasm can
help to identify segments of the gene bank that are especially promising as sources of desirable traits.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
49.
Data on plants from five groups of remnant prairies and forests in the prairie-forest ecotone of the midwestern United States show that: (1) Archipelagos of small sites tend to contain more species than do single large ones of equal total area. (2) No species are excluded from small sites. (3) Small sites tend to have surprisingly many species, and large sites surprisingly few, relative to a random colonisation model. (4) ‘Rare’ species (those that occur in only one site) are found more often in small sites than a random colonisation model would predict. (5) There is no evidence for any of these sites that species number is ‘relaxing’, but all these sites have been remnants for only a short time. Result (1) is consistent with results from similar studies on other taxa and suggests that there is no automatic reason to believe that species richness is maximised by single large refuges. 相似文献
50.
David J. Gilbert Jeremy R. McKenzie Nicholas M. Davies 《Journal of Agricultural, Biological & Environmental Statistics》2001,6(2):281-291
We found evidence that tagging induced trap shyness in snapper (Pagrus auratus), i.e., tagged fish had a reduced probability of recapture by the method by which they had originally been caught. Tagging experiments in 1985 and 1994 involving single release over a short period and single recapture were conducted on a closed population (SNA 1: East Northland-Hauraki Gulf-Bay of Plenty). Initial capture was by trawling and by line fishing, while recapture over an extended period included other methods. A test for trap shyness that removed the possible effects of spatial and fish size heterogeneity gave a significant result for both years. The data suggested that the trap shyness effect might have been smaller for trawl-released fish than for line-released fish. However, we estimated a single trap shyness factor (0.71). There was also some evidence for attenuation of trap shyness over time. 相似文献