全文获取类型
收费全文 | 205774篇 |
免费 | 10897篇 |
国内免费 | 8068篇 |
专业分类
林业 | 14553篇 |
农学 | 16431篇 |
基础科学 | 7636篇 |
29554篇 | |
综合类 | 38772篇 |
农作物 | 8641篇 |
水产渔业 | 9565篇 |
畜牧兽医 | 82147篇 |
园艺 | 3937篇 |
植物保护 | 13503篇 |
出版年
2022年 | 2318篇 |
2021年 | 3625篇 |
2020年 | 3655篇 |
2019年 | 3814篇 |
2018年 | 3836篇 |
2017年 | 4808篇 |
2016年 | 4659篇 |
2015年 | 5075篇 |
2014年 | 5349篇 |
2013年 | 9372篇 |
2012年 | 8585篇 |
2011年 | 9861篇 |
2010年 | 7981篇 |
2009年 | 7782篇 |
2008年 | 9252篇 |
2007年 | 9093篇 |
2006年 | 8639篇 |
2005年 | 7810篇 |
2004年 | 5796篇 |
2003年 | 5675篇 |
2002年 | 4943篇 |
2001年 | 6059篇 |
2000年 | 6190篇 |
1999年 | 5587篇 |
1998年 | 3353篇 |
1997年 | 3024篇 |
1996年 | 2763篇 |
1995年 | 2946篇 |
1994年 | 2506篇 |
1993年 | 2483篇 |
1992年 | 3702篇 |
1991年 | 3685篇 |
1990年 | 3535篇 |
1989年 | 3473篇 |
1988年 | 3052篇 |
1987年 | 2857篇 |
1986年 | 2705篇 |
1985年 | 2478篇 |
1984年 | 2029篇 |
1983年 | 1779篇 |
1982年 | 1205篇 |
1979年 | 1666篇 |
1978年 | 1293篇 |
1975年 | 1165篇 |
1974年 | 1358篇 |
1973年 | 1450篇 |
1972年 | 1381篇 |
1971年 | 1196篇 |
1970年 | 1249篇 |
1969年 | 1150篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
An experiment was designed to evaluate the effects of estradiol‐17β (E17β) on follicular wave dynamics and ovulatory response in Holstein heifers receiving either a progestogen ear‐implant (Crestar®; Intervet International b.v. Boxmeer, The Netherlands) or an intravaginal progesterone‐releasing device [controlled internal drug release‐bovine device (Eazibreed, CIDR‐B®; Bodinco BV, Alkmaar, The Netherlands)]. For comparison, another group of heifers was also synchronized using Crestar plus an injection of estradiol valerate (EV) and norgestomet as recommended by the pharmaceutical company. Twenty 20–22‐month‐old cycling Holstein heifers were allocated to one of the following treatment groups at random stages of the oestrous cycle: (I) simultaneous insertion of Crestar and intramuscular injection of 3 mg norgestomet and 5 mg EV (Crestar 9 + EV 9); (II) simultaneous insertion of Crestar and intramuscular injection of 5 mg E17β (Crestar 9 + E17β 9); (III) insertion of Crestar followed 2 days later by intramuscular injection of 5 mg E17β (Crestar 9 + E17β 7); or (IV) insertion of CIDR‐B device followed 2 days later by intramuscular injection of 5 mg E17β (CIDR 9 + E17β 7). The CIDR‐B or Crestar implants were removed after 9 days and all heifers received 500 μg Cloprostenol (Estrumate®, Pitman‐Moore Nederland BV, Houten, The Netherlands). Ovarian ultrasonographic examinations were performed once daily during the synchronization period using a B‐mode scanner equipped with a 7.5 MHz linear‐array transrectal transducer. In addition, heifers were scanned every 12 h after implant/device withdrawal until 3 days after ovulation in order to monitor follicular activity, detect ovulation and subsequent early luteal formation. Detection of oestrus was performed every 6 h for 4 days after device/implant removal. Oestrus was observed 24–32 h before ovulation in all heifers. The mean hours interval from treatment withdrawal to ovulation was not significantly different (84.0 ± 16.5, 77.6 ± 4.1, 73.6 ± 4.1 and 64.0 ± 4.4 h for treatments I, II, III and IV, respectively; p > 0.1). However, the variance for heifers treated with EV + norgestomet was significantly larger (Levene’s Test; p < 0.01) than those treated with E17β. All E17β treatments resulted in dominant follicle suppression and a new wave emerged 4.1 days after treatment compared with 6.6 days for the EV + norgestomet treatment (p < 0.05). The time from emergence of the new ovulatory wave to ovulation was longer for the new wave that emerged after E17β treatment (9.2 ± 0.3 days) than after EV + norgestomet treatment (6.9 ± 0.4 days; p < 0.05). The results of this study suggest that the four treatments used were effective in inducing synchronous behavioural oestrus and ovulation. However, a higher degree of oestrus and ovulation synchrony was observed in heifers treated with E17β than in heifers treated with EV + norgestomet. Synchronization treatments with exogenous E17β or EV + norgestomet at the time of progestin device insertion (Crestar or CIDR‐B) or 2 days later in heifers can regulate a different emergence pattern of ovarian follicular development in randomly cyclic heifers. The E17β was effective in inducing follicular suppression and resulted in the consistent emergence of a new follicular wave. 相似文献
12.
H Inokuma G Nane T Uechi Y Yonahara P Brouqui M Okuda T Onishi 《The Journal of veterinary medical science / the Japanese Society of Veterinary Science》2001,63(11):1225-1227
Twelve (54.5%) of 22 free-roaming dogs in Ishigaki Island had tick infestation identified as Rhipicephalus sanguineus. There were 121 ticks recovered and consisted of 28 females, 58 males, 22 nymphs and 3 larvae. Infection of dogs possibly with canine ehrlichial pathogens was examined by both indirect immunofluorescence assay and polymerase chain reaction (PCR). Two dogs of the 13 examined were sero-positive for the human granulocytic ehrlichia agent, and one of two dogs was PCR positive for Ehrlichia platys. This dog had platelet numbers slightly lower than normal value, however, no morulae were found within platelet on peripheral blood smear stained with Giemsa. 相似文献
13.
A. Bhogal A. Bhogal M.A. Shepherd D.J. Hatch L. Brown S.C. Jarvis 《Soil Use and Management》2001,17(3):163-172
Abstract. The ability of two nitrogen cycle models, of contrasting complexity, to predict N mineralization from a range of grassland soils in the UK, was evaluated. These were NCYCLE, a simple mass balance model of the N cycle in UK grasslands, and CENTURY, a more complex model simulating long-term C, N, P & S dynamics in grassland ecosystems. The models were tested using field measurements of net N mineralization from a range of grassland soils (differing in soil type, history & management practice), obtained over a 2 year period using a soil core incubation technique. This method was considered to measure the total net release of mineral N from the soil organic matter over a specified time, including N which may have been recycled several times. NCYCLE consistently under-estimated mineralization rates at all sites. By contrast, there was some correlation between CENTURY predictions of net N mineralization and field measurements. This may have reflected the different abilities of the two models to simulate N recycling. Neither model, however, was able to predict adequately the effect of cultivation and reseeding on net N mineralization. 相似文献
14.
15.
16.
本文引用国外农业投融资的做法,提出了改善中国农业相对落后状况的办法,从认识农业投融资现状入手,构建出一套符合中国农业可持续发展的投融资体制. 相似文献
17.
泥鳅的生物学特性和人工繁殖及苗种培育 总被引:4,自引:0,他引:4
泥鳅是分布广、数量多的杂食性淡水食用鱼,其适应性强、疾病少、成活率高,且繁殖力强,有较高的食疗作用和药用价值。本文从形态特征、生态特性、食性、生长等方面系统介绍了泥鳅的生物学特性,全面介绍了泥鳅亲鱼的选育、人工催产以及苗种培育技术,为泥鳅的人工养殖提供参考资料。 相似文献
18.
19.
20.