全文获取类型
收费全文 | 119473篇 |
免费 | 6488篇 |
国内免费 | 123篇 |
专业分类
林业 | 5458篇 |
农学 | 3323篇 |
基础科学 | 643篇 |
13289篇 | |
综合类 | 23732篇 |
农作物 | 4655篇 |
水产渔业 | 5351篇 |
畜牧兽医 | 61322篇 |
园艺 | 1313篇 |
植物保护 | 6998篇 |
出版年
2018年 | 1641篇 |
2017年 | 1811篇 |
2016年 | 1597篇 |
2015年 | 1403篇 |
2014年 | 1786篇 |
2013年 | 4059篇 |
2012年 | 3250篇 |
2011年 | 3891篇 |
2010年 | 2480篇 |
2009年 | 2636篇 |
2008年 | 3821篇 |
2007年 | 3670篇 |
2006年 | 3492篇 |
2005年 | 3258篇 |
2004年 | 3139篇 |
2003年 | 3182篇 |
2002年 | 2955篇 |
2001年 | 3565篇 |
2000年 | 3553篇 |
1999年 | 2793篇 |
1998年 | 1096篇 |
1997年 | 1089篇 |
1996年 | 1098篇 |
1995年 | 1240篇 |
1994年 | 1084篇 |
1993年 | 1148篇 |
1992年 | 2261篇 |
1991年 | 2483篇 |
1990年 | 2348篇 |
1989年 | 2360篇 |
1988年 | 2162篇 |
1987年 | 2243篇 |
1986年 | 2297篇 |
1985年 | 2262篇 |
1984年 | 1756篇 |
1983年 | 1576篇 |
1982年 | 1071篇 |
1979年 | 1673篇 |
1978年 | 1360篇 |
1977年 | 1145篇 |
1975年 | 1191篇 |
1974年 | 1591篇 |
1973年 | 1516篇 |
1972年 | 1589篇 |
1971年 | 1532篇 |
1970年 | 1461篇 |
1969年 | 1400篇 |
1968年 | 1172篇 |
1967年 | 1224篇 |
1966年 | 1082篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
62.
Abstract – Foraging juvenile fish with relatively high food demands are usually vulnerable to various aquatic and avian predators. To compromise between foraging and antipredator activity, they need exact and reliable information about current predation risk. Among direct predator-induced cues, visual and olfactory signals are considered to be most important. Food intake rates and prey-size selectivity of laboratory-reared, naive young-of-the-year (YOY) perch, Perca fluviatilis , were studied in experiments with Daphnia magna of two size classes: 2.8 and 1.3 mm as prey and northern pike, Esox lucius , as predator. Neither total intake rate nor prey-size selectivity was modified by predator kairomones alone (water from an aquarium with a pike was pumped into the test aquaria) under daylight conditions. Visual presentation of pike reduced total food intake by perch. This effect was significantly more pronounced (synergistic) when visual and olfactory cues were presented simultaneously to foraging perch. Moreover, the combination of cues caused a significant shift in prey-size selection, expressed as a reduced proportion of large prey in the diet. Our observations demonstrate that predator-induced olfactory cues alone are less important modifiers of the feeding behaviour of naive YOY perch than visual cues under daylight conditions. However, pike odour acts as a modulatory stimulus enhancing the effects of visual cues, which trigger an innate response in perch. 相似文献
63.
Louis R. D'Abramo Cortney L. Ohs Kathleen C. E. Elgarico 《Journal of the World Aquaculture Society》2006,37(3):307-312
The effects of addition of substrate consisting of plastic mesh fencing to experimental earthen ponds were evaluated for the culture of the red swamp crawfish, Procambarus clarkii, without planted forage. Mean daily yield (kg/ha/d), number of crawfish harvested (number/ha/d), and mean individual harvest weight were compared for ponds with and without substrate during a 2‐yr period. The surface area of added substrate was equivalent to 50% of the bottom surface area of the pond. Mean daily yield in the ponds containing substrate, based on a comparison with that of the previous year, increased by 80.2% and was principally attributed to a 66.7% increase in the number of crawfish harvested. Comparative values for the ponds without substrate were ?13.1 and ?11.7%. For the second year, the treatment‐dependent response was consistent. Mean daily yield for the substrate ponds increased by 48.8% compared to 15.0% for the ponds lacking substrate. The primary benefit of the substrate appears to be provision of refuge and three‐dimensional distribution of crawfish in the pond, thereby reducing the incidence of cannibalism and increasing the total number of crawfish harvested per hectare per day. The benefit of the substrate in a commercial production setting must be weighed against its cost and longevity as well as the conditions of the particular management scenario into which it may be incorporated. 相似文献
64.
65.
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
66.
L S Pelan-Mattocks B A Pesch M E Kehrli 《American journal of veterinary research》2001,62(11):1740-1744
OBJECTIVE: To develop an efficient and reliable method that accurately differentiates bovine lymphocytes from monocytes in leukograms. SAMPLE POPULATION: Blood samples from 30 healthy cows and 1 calf with bovine leukocyte adhesion deficiency. PROCEDURE: Flow cytometric analysis of intracellular complexity and CD45 expression on bovine leukocytes was compared with results for conventional light microscopy methods. Verification of leukocyte subpopulations determined by intracellular complexity and CD45 expression was conducted, using 2-color phenotypic analysis with selected monoclonal antibodies. RESULTS: The CD45 and side-scatter properties of bovine leukocytes clearly differentiated cell types, including neutrophils, eosinophils, monocytes, and lymphocytes. CONCLUSIONS AND CLINICAL RELEVANCE: This is a rapid assay that is simple to use. More importantly, it is more accurate than the conventional method that involves the use of blood slides and light microscopy, because of the ability of the assay to readily distinguish bovine monocytes and lymphocytes. Rapid preparation of samples and short analysis times allow for efficient and reliable examination of a large number of samples, and the task of viewing slides by light microscopy is eliminated. The labor-savings benefit of this procedure is most apparent in research environments that require frequent processing of batches of blood samples. 相似文献
67.
Canine distal renal tubular acidosis and urolithiasis 总被引:1,自引:0,他引:1
D J Polzin C A Osborne F W Bell 《Veterinary Clinics of North America: Small Animal Practice》1986,16(2):241-250
Distal RTA is characterized by decreased distal renal tubular hydrogen ion secretion, decreased ability to acidify urine, hypercalciuria, hyperphosphaturia, hypocitraturia, and metabolic acidosis. Because of the resulting alterations in urine composition and pH, patients with distal RTA are predisposed to urolithiasis and renal calcification. Diagnosis of distal RTA is important because it is a potentially reversible disorder that, left untreated, may cause nephrocalcinosis, recurrent urolith formation, moderate to severe metabolic acidosis, and renal failure. 相似文献
68.
A E van de Braak A T van 't Klooster A Malestein J A Faber 《The Veterinary quarterly》1986,8(1):12-23
Forty-one dairy cows were fed a low (LCa-13 g/d) and a high (HCa-83.5 g/d) calcium ration in the 8 weeks prior to parturition and the effect on the Ca mobilization rate around parturition was studied. Plasma Ca values were stable in the LCa group around parturition. In the older cows of the HCa group a very slight decrease in the mean plasma Ca was observed: 2.58 mmol/l at 12-36 h ante partum decreased to 2.38 mmol/l at parturition. Hypocalcaemia, which commonly occurs around parturition, did not occur in 40 of the cows. A subclinical hypocalcaemia (1.8 mmol/l) occurred in one cow (parity 10) from the HCa group. To assess the efficiency of Ca mobilization, a severe hypocalcaemia (1.0 mmol/l) with clinical signs was induced by means of Na2EDTA infusion (0.90 mmol/min), starting at 10 h post-partum. The older cows in the LCa group required more Na2EDTA than those in the HCa group. Higher urinary hydroxyproline excretion in the week before parturition in the LCa than in the HCa group suggested a higher bone turnover. Plasma PTH levels around parturition were not significantly different between the groups. The amount of colostrum milked out in the first 10 h post-partum did not influence Ca homeostasis around parturition. The results contradict those of many other experiments in which hypocalcaemia was observed in cows ingesting high levels of Ca. It is concluded that the restricted feed intake prepartum possibly had a favourable effect on Ca homeostasis. 相似文献
69.
70.