全文获取类型
| 收费全文 | 2285篇 |
| 免费 | 388篇 |
| 国内免费 | 54篇 |
专业分类
| 林业 | 788篇 |
| 农学 | 96篇 |
| 基础科学 | 178篇 |
| 232篇 | |
| 综合类 | 988篇 |
| 农作物 | 83篇 |
| 水产渔业 | 26篇 |
| 畜牧兽医 | 168篇 |
| 园艺 | 54篇 |
| 植物保护 | 114篇 |
出版年
| 2025年 | 3篇 |
| 2024年 | 41篇 |
| 2023年 | 34篇 |
| 2022年 | 63篇 |
| 2021年 | 78篇 |
| 2020年 | 74篇 |
| 2019年 | 86篇 |
| 2018年 | 58篇 |
| 2017年 | 100篇 |
| 2016年 | 112篇 |
| 2015年 | 133篇 |
| 2014年 | 145篇 |
| 2013年 | 162篇 |
| 2012年 | 200篇 |
| 2011年 | 195篇 |
| 2010年 | 156篇 |
| 2009年 | 152篇 |
| 2008年 | 117篇 |
| 2007年 | 96篇 |
| 2006年 | 105篇 |
| 2005年 | 73篇 |
| 2004年 | 71篇 |
| 2003年 | 54篇 |
| 2002年 | 38篇 |
| 2001年 | 43篇 |
| 2000年 | 45篇 |
| 1999年 | 40篇 |
| 1998年 | 36篇 |
| 1997年 | 36篇 |
| 1996年 | 30篇 |
| 1995年 | 27篇 |
| 1994年 | 27篇 |
| 1993年 | 19篇 |
| 1992年 | 18篇 |
| 1991年 | 13篇 |
| 1990年 | 17篇 |
| 1989年 | 8篇 |
| 1988年 | 12篇 |
| 1987年 | 3篇 |
| 1986年 | 4篇 |
| 1984年 | 1篇 |
| 1976年 | 1篇 |
| 1956年 | 1篇 |
排序方式： 共有2727条查询结果，搜索用时 33 毫秒
11.
12.
D. Edwin Swift William Knight Martin Béland Issifi Boureima Charles P.-A. Bourque Fan-Rui Meng 《Scandinavian Journal of Forest Research》2017,32(1):45-59
In the late 1980s, large forest companies began precommercial thinning (PCT) operations in young northern hardwood cutovers in New Brunswick, Canada. To provide supporting growth and yield information, an industrial experiment was established at residual stand densities of 1300, 1600, 1900, and 2200?stems?ha?1. Stand responses were examined for measurements recorded at 0 (1987), 5 (1992), 10 (1997), 16 (2003), and 23 (2010) years after establishment. Average diameter at breast height, quadratic mean diameter, stand basal area, and stand total volume growth increased as stem density decreased from PCT. There were significant linear differences for many of these variables between treatments and time periods (year). No significant differences were detected in tree height between treatments. In 2010, the four PCT thinning treatments did not exhibit any differences in potential sawlogs at 2.4?m (8?ft) and 3.6?m (12?ft) lengths. Significant differences were observed for 4.9?m (16?ft) sawlogs that were produced at the least dense spacing (1300?stems?ha?1). Results from this study and recommendations from the European literature suggest that value-added timber products may be produced from more intense PCT treatments than are currently being practiced on sites dominated by yellow birch (Betula alleghaniensis Britt.). 相似文献
13.
Summary This second paper of the review describes the development of the potato tuber and whole plant based on research data and literature.
The development and growth, daily changes of fresh and dry matter, and of the contents of organic and inorganic components
(dry matter, starch, sugars, organic acids, ascorbic acid, nitrogenous compounds, nitrate, crude lipid, glycoalkaloids, P,
K, Ca, Mg, Mn, Na), discolouration indices, and physiological parameters (rate of assimilation and respiration, activity of
enzymes) are reviewed. 相似文献
14.
传统上,轮胎成型机成型时仅胎侧采用卷取方式,胎面采用在压出冷却线上预裁断的方式.近几年,为了提高轮胎的动平衡,轮胎成型用的半成品部件越来越多地采用卷取的方式,现胎面也开始大量采用卷取方式.但使用原有的设备存在卷取的部件厚度变化较大时,卷取速度与生产线速度不匹配的问题,为此,提出了实现方式简单和改动很小的改进的措施. 相似文献
15.
16.
Elizabeth M Messersmith Emma K Niedermayer Kara J Thornton Grant I Crawford Stephanie L Hansen 《Journal of animal science》2021,99(8)
Two hundred eight Angus-crossbred heifers (291 ± 23 kg) from four sources were used in a randomized complete block design. The objective of the study was to determine the effects of implant strategy and Zn supplementation on performance, carcass characteristics, muscle fiber diameter, and mineral status of heifers. Heifers were assigned to a 2 × 2 factorial study for 168 d, and factors included Zn and implant (IMP). Heifers were supplemented Zn (mg/kg dry matter [DM]; ZnSO4) at national (30; NRC) or industry (100; IND) recommendations. Implant strategies (Merck Animal Health, Madison, NJ) included extended-release Revalor-XH on day 0 (REV-XH; 20 mg estradiol + 200 mg trenbolone acetate) containing four uncoated pellets and six coated pellets or the uncoated implant Revalor-200 on day 0 and again on day 91 (REV-200; 20 mg estradiol + 200 mg trenbolone acetate). Heifers were blocked by weight within source to pens of five or six heifers per pen (nine pens per treatment). A corn silage-based diet was fed during the growing period (days 0–55) followed by transition to a corn-based finishing diet. Weights were taken consecutively on days −1/0, 55/56, and 167/168. Liver and muscle from the longissimus thoracis were collected from one heifer per pen on days −5, 14, 105, and 164. Data were analyzed via Mixed Procedure of SAS. Average daily gain (ADG) and liver mineral used Period as the repeated effect. Corresponding to periods of high hormone payout from each implant, days 0–28 and 91–120 ADG were greatest for REV-200, whereas REV-XH numerically peaked during days 56–91 (IMP × Period; P = 0.02). Day 91 IND body weight tended to be heavier (P = 0.06) and day 120 body weight was heavier (P = 0.05) than NRC heifers. No effect of Zn or IMP on final body weight was observed (P ≥ 0.21). Muscle fiber cross-sectional diameter on day 164 was greater (P = 0.05) in IND than NRC. Liver Mn concentrations decreased by day 14 regardless of implant, though days 105 and 164 concentrations were lesser for REV-200 than REV-XH (IMP × Period; P = 0.02). No effects of Zn, IMP, or the interaction were observed for carcass-adjusted gain to feed, days 0–168 DM intake, hot carcass weight, or ribeye area (P ≥ 0.11). The nominal differences in performance between implant strategies suggest that extended-release implants may be an effective implant strategy to replace re-implant programs in heifers, whereas the improved performance of heifers fed IND vs. NRC during times of peak hormone payout suggests a role for Zn in periods of rapid growth. 相似文献
17.
Browse of multipurpose tree species such as black locust could be used to broaden grazing options, but the temporal distribution of foliage has not been adequately studied. Our objective was to determine effects of harvest date, P fertilization (0 and 600 kg ha−1 yr−1), and pollard height (shoots clipped at 5-, 50-, and 100-cm above ground) on foliar and shoot allometry of black locust. The experiment was conducted on a naturally regenerated 2-yr-old black locust stand (15,000 trees ha−1). Basal shoot diameter and foliar mass were measured monthly in June to October 2002 and 2003. Foliar and shoot dry mass (Y) was estimated from basal shoot diameter (D) by the function Y = aD b , with regression explaining ≥95% of variance. Allometry of foliar mass was affected by harvest date, increasing at a greater rate with D in September than in June or July, but not by P fertilization or pollard height. Foliar mass was predicted best by month-specific equations for the June to October growth interval. Allometry of shoot dry mass was unaffected by harvest date, P fertilization, or pollard height. These equations could be used as a first approximation of foliar and shoot mass for pollarded black locust. 相似文献
18.
19.
Mingzhu HE 《干旱区科学》2020,12(4):701-715
In arid desert regions of northwestern China, reclamation and subsequent irrigated cultivation have become effective ways to prevent desertification, expand arable croplands, and develop sustainable agricultural production. Improvement in soil texture and fertility is crucial to high soil quality and stable crop yield. However, knowledge on the long-term effects of the conversion of desert lands into arable croplands is very limited. To address this problem, we conducted this study in an arid desert region of northwestern China to understand the changes in soil physical-chemical properties after 0, 2, 5, 10, 17, and 24 years of cultivation. Our results showed that silt and clay contents at the 17-year-old sites increased 17.5 and 152.3 folds, respectively, compared with that at the 0-year-old sites. The soil aggregate size fraction and its stability exhibited an exponential growth trend with increasing cultivation ages, but no significant change was found for the proportion of soil macroaggregates (>5.00 mm) during the 17 years of cultivation. The soil organic carbon (SOC) content at the 24-year-old sites was 6.86 g/kg and increased 8.8 folds compared with that at the 0-year-old sites. The total (or available) nitrogen, phosphorus, and potassium contents showed significant increasing trends and reached higher values after 17 (or 24) years of cultivation. Changes in soil physical-chemical properties successively experienced slow, rapid, and stable development stages, but some key properties (such as soil aggregate stability and SOC) were still too low to meet the sustainable agricultural production. The results of this long-term study indicated that reasonable agricultural management, such as expanding no-tillage land area, returning straw to the fields, applying organic fertilizer, reducing chemical fertilizer application, and carrying out soil testing for formula fertilization, is urgently needed in arid desert regions. 相似文献
20.