全文获取类型
收费全文 | 93650篇 |
免费 | 4996篇 |
国内免费 | 66篇 |
专业分类
林业 | 4136篇 |
农学 | 3035篇 |
基础科学 | 452篇 |
10939篇 | |
综合类 | 17198篇 |
农作物 | 3310篇 |
水产渔业 | 4696篇 |
畜牧兽医 | 48203篇 |
园艺 | 1066篇 |
植物保护 | 5677篇 |
出版年
2019年 | 799篇 |
2018年 | 1368篇 |
2017年 | 1469篇 |
2016年 | 1321篇 |
2015年 | 1143篇 |
2014年 | 1403篇 |
2013年 | 3268篇 |
2012年 | 2622篇 |
2011年 | 3098篇 |
2010年 | 2072篇 |
2009年 | 2097篇 |
2008年 | 3066篇 |
2007年 | 3008篇 |
2006年 | 2832篇 |
2005年 | 2633篇 |
2004年 | 2559篇 |
2003年 | 2563篇 |
2002年 | 2313篇 |
2001年 | 3205篇 |
2000年 | 3211篇 |
1999年 | 2517篇 |
1998年 | 958篇 |
1997年 | 916篇 |
1996年 | 867篇 |
1995年 | 1061篇 |
1994年 | 938篇 |
1993年 | 884篇 |
1992年 | 1943篇 |
1991年 | 2004篇 |
1990年 | 1946篇 |
1989年 | 1872篇 |
1988年 | 1774篇 |
1987年 | 1839篇 |
1986年 | 1774篇 |
1985年 | 1745篇 |
1984年 | 1424篇 |
1983年 | 1262篇 |
1982年 | 819篇 |
1979年 | 1204篇 |
1978年 | 993篇 |
1977年 | 859篇 |
1976年 | 820篇 |
1975年 | 904篇 |
1974年 | 1153篇 |
1973年 | 1051篇 |
1972年 | 1133篇 |
1971年 | 1056篇 |
1970年 | 953篇 |
1969年 | 1005篇 |
1967年 | 844篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
T W Murphy W H McCollum P J Timoney B W Klingeborn B Hyllseth W Golnik B Erasmus 《Veterinary microbiology》1992,32(2):101-115
Equine arteritis virus (EAV), a non-arthropod borne togavirus, has been shown to have a global distribution. To date, no major antigenic variation has been demonstrated between EAV isolates from different geographic origins. In this study, the genomic RNA of EAV isolates obtained from horses of different breeds in various countries around the world was oligonucleotide fingerprinted. Comparisons of these fingerprints were used to determine the extent of genomic variation among such isolates. Comparisons among isolates from North American horses revealed, for the most part, oligonucleotide homologies of less than 60%. Only 29 of the 98 comparisons revealed greater than 60% oligonucleotide homology. Nonetheless, several comparisons indicated a close epidemiologic relationship between isolates from horses of different breeds located in different states. Though all European isolates were of Standardbred origin and were from horses located in northern European countries, the majority had oligonucleotide homologies of less than 60%. Where oligonucleotide homology was apparent, it was, with one exception, greater than 70%. The two isolates from New Zealand had 93.2% oligonucleotide homology. This is indicative of an extremely close epidemiologic relationship. Comparisons between EAV isolates from around the world revealed oligonucleotide homologies between viruses from North America, Europe and New Zealand. In several instances, this homology was greater than 70% and in one case greater than 80%. No oligonucleotide homology was evident in comparisons involving the virus from South Africa. The high level of genomic conservation between certain EAV isolates of disparate geographic origins may reflect dissemination of the virus associated with the international movement of horses. The extent of genomic variation demonstrated between most of the EAV isolates used in this study confirms the need for further investigation of genomic heterogeneity among strains of this virus before techniques that rely upon nucleic acid hybridization can be effectively applied as diagnostic procedures. 相似文献
42.
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
43.
44.
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
45.
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
46.
47.
48.
49.
50.