全文获取类型
收费全文 | 252篇 |
免费 | 37篇 |
专业分类
林业 | 35篇 |
农学 | 7篇 |
48篇 | |
综合类 | 15篇 |
农作物 | 6篇 |
水产渔业 | 101篇 |
畜牧兽医 | 65篇 |
园艺 | 4篇 |
植物保护 | 8篇 |
出版年
2023年 | 2篇 |
2021年 | 6篇 |
2020年 | 6篇 |
2019年 | 7篇 |
2018年 | 15篇 |
2017年 | 14篇 |
2016年 | 14篇 |
2015年 | 10篇 |
2014年 | 10篇 |
2013年 | 26篇 |
2012年 | 18篇 |
2011年 | 21篇 |
2010年 | 11篇 |
2009年 | 4篇 |
2008年 | 18篇 |
2007年 | 10篇 |
2006年 | 6篇 |
2005年 | 5篇 |
2004年 | 9篇 |
2003年 | 9篇 |
2002年 | 9篇 |
2001年 | 8篇 |
2000年 | 3篇 |
1999年 | 4篇 |
1998年 | 5篇 |
1997年 | 1篇 |
1996年 | 8篇 |
1995年 | 1篇 |
1994年 | 7篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1969年 | 1篇 |
1950年 | 1篇 |
排序方式: 共有289条查询结果,搜索用时 15 毫秒
1.
2.
The 3-O-beta-glucopyranosides of delphinidin, petunidin, and malvidin (1-3) and cyanidin 3-O-beta-galactopyranoside (4) dissolved in deuterated methanolic solutions without and with acid (5%, CF3COOD) were identified by homo- and heteronuclear NMR techniques. The hemiacetal forms of all the four anthocyanins were characterized as two epimeric 2-hydroxy-hemiacetals on the basis of assignments of both proton and carbon NMR signals together with chemical shift considerations. This is the first report of 13C NMR assignments of two epimeric anthocyanin hemiacetal forms. No 4-hydroxy-hemiacetal form was detected for any of the pigments. For each anthocyanin dissolved in deuterated methanol, the equilibrium between each of the two epimeric hemiacetals and the corresponding flavylium cation was confirmed by the observed positive exchange cross-peaks in the 2D 1H NOESY spectra. The molar proportions of the flavylium cation and the two hemiacetals of 1-4 in deuterated methanol were very similar for all pigments, even during storage for weeks. The majority of the anthocyanins reported to occur in fruits have the same or similar structures as 1-4. These pigments have been proposed to exist predominantly as hemiacetals in slightly acidic to neutral solvents, which is a relevant pH range in plants and in the human gastrointestinal tract. 相似文献
3.
Østerberg J Ekwall SJ Nilsson I Stampe M Engvall A Wallgren P 《Berliner und Münchener tier?rztliche Wochenschrift》2001,114(9-10):331-334
An integrated SPF herd with 320 sows was found infected with Salmonella Yoruba during an annual control among sows, aiming to verify freedom from Salmonella infections. It is believed that the infection was introduced to the herd by purchase of feed. The herd performed an age segregated rearing system. Sows and piglets were reared at a central farm, while growers (25-100 kg body weight) were reared at sub-estates. The growers were free from the infection, and as a consequence a specially designed eradication program was designed. Farrowing and weaning were defined as periods of risk for sows and piglets, respectively. Consequently sows were isolated and individually tested for presence of Salmonella one week before and one week after farrowing. The offspring were tested one week post weaning. To verify freedom from disease among piglets they were also tested another time before transfer to the uninfected sub-estates. Piglets with undefined status regarding Salmonella were denoted animals at risk and not transferred to the sub-estates. Instead they were transferred to a third estate, rented to house pigs at risk. The program was successful. It allowed full production during performance, and the herd was declared free from S. Yoruba seven and a half months after the initial diagnosis. 相似文献
4.
The objectives of this study were to elucidate at which age tonsillar colonisation by Actinobacillus pleuropneumoniae occurs in pigs and relate this occurrence to the presence of colostral antibodies to A. pleuropneumoniae. The infection patterns were studied in an isolated cohort of pigs, which consisted of the offspring from five sows originating from a conventional pig herd. The sows were transferred to isolated research facilities before farrowing. A. pleuropneumoniae was detected on the tonsils of all sows. After a nursing period of 3 weeks, the pigs were weaned and reared isolated from other pigs until slaughter. The pigs were examined repeatedly for the presence of A. pleuropneumoniae on the tonsils and for antibodies to A. pleuropneumoniae using bacteriological and serological techniques, respectively.A. pleuropneumoniae was detected in the tonsils of one pig as early as 11 days after birth, showing that A. pleuropneumoniae can be transmitted from sow to offspring during a 3-week nursing period. The cumulative proportion of pigs carrying A. pleuropneumoniae in their tonsils increased significantly between the age of 4-12 weeks. This age period corresponded to the age at which the proportion of pigs with detectable levels of colostral antibodies to the different serotypes of A. pleuropneumoniae was declining. Since these two events take place in the same age period, we expect a possible biological association between the level of the passive immunity and the degree of tonsillar colonisation. The median duration of tonsillar colonisation was estimated to approximately 7-8 weeks. 相似文献
5.
6.
Two-year-old estern white pine (Pinus strobus L.) seedlings were measured for Root Growth Potential (RGP) in a hydroponic system over two years in both a greenhouse and an environmentally controlled growthroom. Analyses of variance showed no significant differences (alpha = 0.05) between testing environments for either year. This study demonstrates the validity of using greenhouse RGP testing for 2+0 eastern white pine. 相似文献
7.
Spices constitute an important sub-sector of the economy of Sri Lanka. Most perennial spices are cultivated in agroforestry types of farming systems. A field survey was conducted covering 127 agroforestry farmers in six divisional secretariats in Matale district during the period of October to December 2002. Technical efficiency of spice based agroforestry systems was estimated in order to identify the potential increase in production without incurring additional costs for farm inputs. The factors affecting technical efficiency and constraints and potential of the agroforestry system were also investigated. According to a stochastic frontier production function using a Cobb–Douglas model, hired labour, organic fertilizer, inorganic fertilizer, land size, and soil conservation measures showed significant positive effects on the agroforestry production. The mean technical efficiency of the spice based agroforestry systems was 84.32%. According to the inefficiency model the efficiency increased significantly as a result of farm visits by extension officers, participation in farmer training, less sloping lands, more experience, and higher diversity of the agroforestry system. Technical efficiency decreased, however, with higher education level of the farmer and with higher off-farm income. Unsustainability of the agroforestry system is seen as a result of a␣range of constraints related to productivity, market, technology and institutions. 相似文献
8.
Pressure-volume curves were constructed and shoot water potentials measured for +20-year-old black spruce (Picea mariana (Mill.) BSP) trees from four full-sib families growing on a moist site and a dry site at the Petawawa Research Forest, Ontario, to determine whether differences in diurnal water relations traits were related to productivity. To assess the basis for the observed diurnal patterns, we analyzed effects of environmental and internal water stress variables on diurnal water relations traits. Among the water relations traits examined, turgor pressure was the most sensitive, responding to site, family and environmental variables and displaying the strongest diurnal responses to varying soil water availability and atmospheric vapor pressure deficit (VPD). Overall, there was an 84% drop in turgor pressure with increasing VPD: turgor pressure fell 46% in response to the first 0.75 kPa increase in VPD, and 9.7% in response to a second 0.75 kPa increase in VPD. The families differed in water relations responses to moderate water stress, but not in responses to minor or more extreme water stresses. Thus, at a VPD of 0.5 kPa, there was an estimated 83% greater family difference in turgor pressure on the dry site compared with the moist site. Soil and atmospheric water stress appeared to exert effects in tandem to elicit these responses (r(2) = 0.728). A comparison of the mechanisms of response to water deficit indicated that osmotic adjustment was more important than change in cell wall elasticity. We used a conceptual water relations model to illustrate the differences between tolerant and intolerant families in their mechanisms of water stress response. We conclude that, because genetic responses to site factors are dynamic, the integrated response over time contributes to the observed genetic x environmental interaction in growth. 相似文献
9.
Marianne Frantzen Arne M. Arnesen Brge Damsgrd Helge Tveiten Helge K. Johnsen 《Aquaculture (Amsterdam, Netherlands)》2004,240(1-4):561-574
Four groups of Arctic charr (age 3+), previously exposed to natural photoperiod, were subjected to different photoperiod regimes from February 20: Simulated natural photoperiod (SNP; Tromsø 70°N), continuous light (LD 24:0) followed by short day (LD 6:18) from either May 1 (LFeb–May) or June 25 (LFeb–June), or continuous light (LD 24:0) throughout the experiment (LContinuous). In females, peak levels of estradiol-17β and testosterone were 6 and 10, 6 and 7, and 3 and 5 weeks advanced, in LFeb–May, LFeb–June and LContinuous, respectively, whereas in males plasma levels of 11-ketotestosterone peaked 8 weeks earlier in LFeb–May and LFeb–June than in SNP and LContinuous. Median ovulation time in LFeb–May and LFeb–June was advanced by 10 weeks compared to SNP and LContinuous, and median spermiation time (first observation of running milt in each individual) in LFeb–May, LFeb–June and LContinuous was advanced by 10, 7 and 5 weeks, compared to SNP. A switch from long to short day early in the reproductive cycle (LFeb–May and LFeb–June) apparently resulted in stronger synchronization of both ovulation and spermiation. Ovulation in LFeb–May and LFeb–June occurred over 3 weeks (LFeb–May) and 8 weeks (LFeb–June), as compared to 10 weeks in the SNP group, whereas all males (except one individual) began spermiating over a period of 3 weeks (LFeb–June), or less (LFeb–May), as compared to 7 weeks in the SNP group. Lack of a short day stimulus (LContinuous), on the other hand, resulted in larger variation in the timing of final maturation, with ovulation and spermiation being spread over 15 weeks. Low egg survival in LFeb–May (17% versus 60% and 97% for LContinuous and SNP) indicate that a long day signal of 10 weeks early in the reproductive cycle may be too short to ensure good egg and sperm quality. Photoperiod manipulation did not affect the proportion of maturing fish. 相似文献
10.