全文获取类型
收费全文 | 34475篇 |
免费 | 1377篇 |
国内免费 | 52篇 |
专业分类
林业 | 4977篇 |
农学 | 1988篇 |
基础科学 | 262篇 |
6500篇 | |
综合类 | 1981篇 |
农作物 | 2904篇 |
水产渔业 | 3117篇 |
畜牧兽医 | 9245篇 |
园艺 | 1414篇 |
植物保护 | 3516篇 |
出版年
2023年 | 159篇 |
2022年 | 259篇 |
2021年 | 472篇 |
2020年 | 465篇 |
2019年 | 481篇 |
2018年 | 3235篇 |
2017年 | 3205篇 |
2016年 | 1851篇 |
2015年 | 642篇 |
2014年 | 736篇 |
2013年 | 1127篇 |
2012年 | 2039篇 |
2011年 | 3266篇 |
2010年 | 2650篇 |
2009年 | 1781篇 |
2008年 | 2333篇 |
2007年 | 2531篇 |
2006年 | 902篇 |
2005年 | 989篇 |
2004年 | 922篇 |
2003年 | 896篇 |
2002年 | 766篇 |
2001年 | 529篇 |
2000年 | 522篇 |
1999年 | 408篇 |
1998年 | 124篇 |
1997年 | 111篇 |
1996年 | 97篇 |
1995年 | 119篇 |
1994年 | 76篇 |
1993年 | 85篇 |
1992年 | 164篇 |
1991年 | 121篇 |
1990年 | 124篇 |
1989年 | 126篇 |
1988年 | 128篇 |
1987年 | 101篇 |
1986年 | 97篇 |
1985年 | 106篇 |
1984年 | 79篇 |
1983年 | 78篇 |
1982年 | 48篇 |
1981年 | 41篇 |
1979年 | 74篇 |
1978年 | 43篇 |
1974年 | 49篇 |
1973年 | 48篇 |
1972年 | 47篇 |
1971年 | 44篇 |
1970年 | 52篇 |
排序方式: 共有10000条查询结果,搜索用时 23 毫秒
141.
Margot Van de Velde Kim Roels Cyrillus Ververs Ilse Gerits Jan Govaere 《Reproduction in domestic animals》2018,53(5):1027-1032
In recent years, the interest in equine foetal gender determination (FGD) during gestation increased remarkably. Ultrasonographic FGD can be performed in two different periods during gestation. The earliest examination can take place at a gestational age of 60–70 days, whereby the genital tubercle is used to differentiate between male and female foeti. The time window of the second approach is wider (120–210 days), and there are more characteristics to take into consideration. In this article, the feasibility and accuracy of ultrasonographic FGD in mid‐ to late gestation are evaluated. One hundred twenty‐one mares from different breeds with a pregnancy stage between 120 and 270 days were examined once, using B‐scale ultrasonography (Esaote MyLab™ClassC). None of the mares were sedated nor shaved, and the procedure was completed within 15 min. Diagnosis was firstly based on the gonads. The final judgement was made based on all visible foetal reproductive organs. In three cases with a pregnancy stage beyond 257 days, FGD was not possible. All of the examined mares in which a FGD could be performed gave birth to a healthy foal. In 98% of the examinations (116/118), the diagnosis was correctly made. In both cases of misdiagnosis, only one characteristic was seen during the procedure and wrongly interpreted. Beyond 210 days of pregnancy, the extremities can preclude a good visualization of the inguinal region. In conclusion, equine FGD in mid‐ to late gestation is an accessible and accurate technique, although a good ultrasound device is a prerequisite and experience and expertise is necessary. 相似文献
142.
Five horses with a primary surgical lesion of the small (descending) colon were diagnosed with eosinophilic colitis based on visual and histopathological examination. These were evident as visibly striking, hyperaemic, focal lesions of the small colon, with serosal petechiation, oedema and marked thickening of the intestinal wall at the site. Areas of focal necrosis were also evident. The gross appearance of the lesions were considered to be sufficiently severe in all cases to merit resection, due to concerns about intestinal necrosis and septic peritonitis. An inability to fully exteriorise the affected portion of intestine to perform a resection and anastomosis necessitated intraoperative euthanasia of one horse. A total of three horses survived to hospital discharge. Eosinophilic colitis lesions are a rare cause of severe small colon disease, but should be considered in cases with similar visual characteristics. 相似文献
143.
144.
145.
146.
Pedotransfer functions to assess adsorbed phosphate using iron oxide content and magnetic susceptibility in an Oxisol
下载免费PDF全文
![点击此处可从《Soil Use and Management》网站下载免费的PDF全文](/ch/ext_images/free.gif)
L. A. Camargo J. Marques Jr. G. T. Pereira L. R. F. Alleoni A. S. R. de S. Bahia D. De B. Teixeira 《Soil Use and Management》2016,32(2):172-182
Adsorbed phosphate in soils can be chemically extracted; however, this process is both time‐consuming and not cost‐effective if large numbers of samples have to be analysed. Indirect assessment of adsorbed phosphate by pedotransfer functions (PTFs) can help optimize fertilizer strategies. This study aimed to evaluate the spatial variability of adsorbed phosphate (Pads), iron oxides and magnetic susceptibility (MS) in oxisols and to calibrate PTFs to predict Pads. A total of 308 soil samples were collected from Hapludox and Eutrudox soils formed from sandstone in Brazil. The contents of clay (196–607 g/kg), iron oxides (40–165 g/kg), MS (1.2–29 × 10?6 m3/kg) and Pads (327–842 mg/kg) were in the range of typical values for these highly weathered soils. This study showed that the attributes studied were spatially dependent. Geomorphic surfaces enabled understanding of spatial variability and helped to develop a more efficient sampling scheme to calibrate PTFs. Moreover, the adsorbed phosphate in these oxisols could be predicted by a PTF using iron oxides and MS as predictors. The MS attribute enabled the most accurate prediction (concordance coefficient = 0.95, root‐mean‐square error = 46 mg/kg and relative improvement in root‐mean‐square error = ?4.12) of spatial variability through PTF compared to other predictors. 相似文献
147.
Huimin Yang Murray Unkovich Ann McNeill Xianzhi Wang 《Biology and Fertility of Soils》2011,47(4):377-385
Symbiotic N2 fixation by lucerne (Medicago sativa) has capacity to provide significant inputs of N to agro-ecosystems, and the species has also been shown to scavenge soil
mineral N and thus act as a sink for excess reactive N. The balance between these two N cycle processes was investigated in
an extensive irrigated lucerne growing region where nitrate contamination of groundwater has been reported. We sampled 18
permanent pure lucerne stands under irrigation for standing dry matter, total shoot N, and N2 fixation using 15N natural abundance along with activity of the inducible enzyme nitrate reductase as indicators of use of soil NO3− by lucerne. On average 65% of lucerne N was obtained from symbiotic N2 fixation. Converting standing dry matter estimates to annual N2 fixation amounts we calculated average N2 fixation of 311 kg N/ha, including N in roots and nodules. Uptake of N from soil by lucerne was calculated to be 181 kg N/ha/year.
We were not able to identify the source of this soil mineral N, although nitrate reductase activity of lucerne was higher
than that of non-N2 fixing species examined. 相似文献
148.
G. Caria D. Arrouays E. Dubromel C. Jolivet C. Ratié M. Bernoux B. G. Barthès D. Brunet C. Grinand 《Soil Use and Management》2011,27(3):333-339
We studied the black carbon (BC) content of ca. 405 samples from French topsoil and artificial soil and carbonate mixtures. Our protocol involved three main steps: (i) decarbonation by HCl, (ii) elimination of non‐pyrogenic organic carbon in a furnace at 375 °C, and (iii) quantification of residual carbon by CHN analysis. BC content increased for calcareous soils according to their carbonates content. Subsequent analyses confirmed the existence of a methodological artefact for BC determination only in calcareous soils. Decarbonation changes the thermal properties of organic matter, creating more recalcitrant carbon than in the initial sample. Higher CaCO3 and organic carbon content results in a more pronounced artefact. The reversal of the first two steps of the chemo‐thermal oxidation method (i.e. thermal oxidation before soil decarbonation) eliminates this artefact. Overall, our results suggest that BC content may have been overestimated in a large number of studies on calcareous soils. 相似文献
149.
150.