全文获取类型
收费全文 | 197391篇 |
免费 | 11243篇 |
国内免费 | 794篇 |
专业分类
林业 | 7756篇 |
农学 | 6061篇 |
基础科学 | 1569篇 |
23544篇 | |
综合类 | 36796篇 |
农作物 | 7456篇 |
水产渔业 | 9612篇 |
畜牧兽医 | 100464篇 |
园艺 | 2640篇 |
植物保护 | 13530篇 |
出版年
2019年 | 1836篇 |
2018年 | 2658篇 |
2017年 | 2990篇 |
2016年 | 2752篇 |
2015年 | 2539篇 |
2014年 | 2950篇 |
2013年 | 7336篇 |
2012年 | 5660篇 |
2011年 | 7011篇 |
2010年 | 4541篇 |
2009年 | 4435篇 |
2008年 | 6618篇 |
2007年 | 6341篇 |
2006年 | 5982篇 |
2005年 | 5617篇 |
2004年 | 5375篇 |
2003年 | 5345篇 |
2002年 | 5182篇 |
2001年 | 5972篇 |
2000年 | 5873篇 |
1999年 | 4736篇 |
1998年 | 1945篇 |
1997年 | 1911篇 |
1995年 | 2192篇 |
1994年 | 1976篇 |
1993年 | 1900篇 |
1992年 | 3936篇 |
1991年 | 4229篇 |
1990年 | 4263篇 |
1989年 | 4196篇 |
1988年 | 3845篇 |
1987年 | 3992篇 |
1986年 | 4114篇 |
1985年 | 3862篇 |
1984年 | 3303篇 |
1983年 | 2918篇 |
1982年 | 1991篇 |
1981年 | 1874篇 |
1979年 | 2944篇 |
1978年 | 2419篇 |
1977年 | 2044篇 |
1976年 | 1913篇 |
1975年 | 2020篇 |
1974年 | 2683篇 |
1973年 | 2695篇 |
1972年 | 2632篇 |
1971年 | 2411篇 |
1970年 | 2254篇 |
1969年 | 2101篇 |
1967年 | 1798篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
The state of mucus synthesis in the goblet cells of the small intestine was studied in conventional piglets infected with a dose of 200,000 oocytes of the coccidium Isospora suis the first and fifth day after parturition. The synthesis of mucus and its chemical characteristics undergo significant changes during the third and fourth day after infection. The activity of acid and neutral mucous substances declines; their level and the physiological synthetic function of goblet cells begin to return to the normal during the period starting on the eight to tenth day after infection. However, there were no fully functioning goblet cells in the broken numerical ratio even at the end of the period of investigation, i.e. the 13th day after infection. The thin surface layer of mucus remained almost unchanged within the whole extent of the small intestine parts studied. 相似文献
102.
CERES-Maize (Vl. 0) predicted no grain yield when 100% defoliation occurred during vegetative growth. This result conflicted with field observations where 100% defoliation early in the vegetative stage did not materially reduce grain yield. CERES-Maize was modified to realistically predict yields when defoliation greater than 50% occurred during vegetative growth by taking into account stem (stem proper and sheath) photosynthesis and translocation of dry matter from the stem to the developing leaves. With these modifications, predictions from CERES-Maize were in good agreement with field measurements when defoliation occurred both early and later in the vegetative stage. This modified version of CERES-Maize can be the basis of a decision support system for defoliating pests of corn where yields can be evaluated under different management strategies and climate scenarios. 相似文献
103.
This article reviews recent diagnostic procedures that have arisen over the last 10 years. Videoendoscopy of horses on a high-speed treadmill allows observation of some of the changes that take place in a horse's airway during exercise. Measurements of upper airway airflows and transupper airway pressure, the use of an esophageal balloon and a Ventigraph to measure changes in pleural pressure, and pulmonary function testing are new techniques that aid the researcher in understanding the mechanics and pathologic characteristics of airway diseases and help the practitioner in assessing the severity of a problem, measuring response to therapy, and accurately determining a prognosis. 相似文献
104.
Totally 13575 ewes of two different breeds, Dala and Spel, were inseminated with semen, frozen in straws and thawed at 70°C for 8 sec. An insemination dose of 0.2 ml containing approx. 150 × 106 spermatozoa with at least 45 to 50% progressive motility was imerted 5 to 12 mm into the cervix. The insemination was performed once between 12 and 30 h after the onset of heat. The NR rate of the Dala ewes increased significantly during the season. The NR rate of the ewes inseminated before 15. November was 44.3%, from 15. to 20. November 52.2%. from 20. to 25. November 55.3% and from 25. November and later 61.4%. The corresponding values for the ewes of the Spel breed were 57.3, 58.7, 61.5 and 71.0% respectively, and only the difference between the two last values was statistically significant. The difference between the fertility of the two breeds was significant within each of the periods . 相似文献
105.
Cellular alterations in level of expression of mRNA encoding for prostaglandin endoperoxide synthase were quantified within ovarian tissues of sheep obtained before, during and after induction of the preovulatory surge of LH and ovulation with LHRH. This was accomplished by isotopic in situ hybridization using a selective cRNA probe to ovine prostaglandin endoperoxide synthase mRNA. A significant elevation in mRNA was detected within the theca interna of the preovulatory follicle at 8, 16 and 24 hr following administration of LHRH. Very close to the time of ovulation (ie., at 24 hr post-LHRH) a marked rise in mRNA was observed in association with epithelial cells covering the apical surface of the follicle. Ovarian cyclooxygenase metabolites of arachidonic acid produced during the ovulatory process in the ewe originate within the thecal layer and germinal epithelium of the follicle destined to ovulate. 相似文献
106.
Y Sugawara K Hirasawa M Takeda J S Han K Doi 《The Journal of veterinary medical science / the Japanese Society of Veterinary Science》1991,53(3):463-468
One-month-old Syrian hamsters of the APA and Std: golden strains were inoculated intraperitoneally with 10(5) PFU/head of the D variant of encephalomyocarditis (EMC) virus and examined virologically and pathologically up to 7 days after inoculation. APA hamsters developed apparent hyperglycemia due to pancreatic islet cell damage while Std:golden hamsters did not. Hamsters of both strains showed clear histopathologic changes in the testis with prominent viral replication as well as in the brain, heart and exocrine pancreas. The susceptibility to EMC virus-infection was higher in males than in females and in APA than in Std: golden hamsters. 相似文献
107.
R. Iori O. Leoni L. Lazzeri G. Mosca S. Palmieri 《Journal of Agronomy and Crop Science》1991,167(2):91-95
Samples of rapeseed from three Italian growing environments (Bologna, Perugia and Palermo) were analysed for glucose content and dry weight of 1000 seeds every three or four days starting from the end of flowering until complete ripening. In addition, the content of oil, soluble and total proteins, glucosinolates and myrosinase activity was determined in samples of mature seeds. The cultivars used were jet Neuf and Lingot (type 0) and Tandem, Jade and Santana (type 00). From the results it emerged that the point of intersection of the two branches of the linear regression plots for different glucose-consumption kinetics found during seed filling, in addition to being strongly affected by the climate of the test environment, is correlated with quantitative and qualitative production, independently of the genotype. 相似文献
108.
109.
110.
In the domestic pig, a circadian rhythm of plasma cortisol occurs, with greatest concentrations in the morning and lowest concentrations in the afternoon. However, photic entrainment of the rhythms of ACTH and melatonin in pigs have not been defined clearly. This experiment was designed to evaluate free-running rhythms of ACTH, cortisol and melatonin in pigs housed in constant light (LL) and constant darkness (DD). Twelve crossbred barrows, maintained under ambient photoperiod, were catheterized and tethered individually in two environmentally controlled rooms, one with LL and the other with DD. For animals in LL, fluorescent lights provided 202 +/- 15 (mean +/- standard deviation) lux of light at 65 cm above the floors. Incandescent nightlights equipped with 7 watt red bulbs provided 7 +/- 2 lux and were illuminated continuously in both rooms. Pigs were given at least 14 d exposure to LL and DD, then samples of plasma and serum were obtained at hourly intervals for 48 hr. Plasma was assayed for ACTH, and serum for cortisol and melatonin. Periodograms were constructed to analyze the data. For this type of analysis, a statistic, Qp, is calculated, and circadian periodicity is suggested if maximum Qp (Qp max) occurs at or near 24 hr. The period of the free-running rhythms (tau) at Qp max for ACTH, cortisol and melatonin for pigs in LL (23.80 +/- .01, 23.78 +/- .01, and 23.21 +/- .02 hr, respectively) did not differ significantly from those for pigs in DD (23.39 +/- .01, 23.20 +/- .01, and 22.55 +/- .02 hr, respectively).(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献