全文获取类型
收费全文 | 52006篇 |
免费 | 1486篇 |
国内免费 | 12篇 |
专业分类
林业 | 5187篇 |
农学 | 2473篇 |
基础科学 | 368篇 |
8507篇 | |
综合类 | 5742篇 |
农作物 | 3468篇 |
水产渔业 | 3264篇 |
畜牧兽医 | 18879篇 |
园艺 | 1499篇 |
植物保护 | 4117篇 |
出版年
2021年 | 293篇 |
2020年 | 339篇 |
2019年 | 422篇 |
2018年 | 3585篇 |
2017年 | 3647篇 |
2016年 | 1887篇 |
2015年 | 502篇 |
2014年 | 643篇 |
2013年 | 1771篇 |
2012年 | 1929篇 |
2011年 | 3702篇 |
2010年 | 3390篇 |
2009年 | 2517篇 |
2008年 | 2888篇 |
2007年 | 3310篇 |
2006年 | 1106篇 |
2005年 | 1023篇 |
2004年 | 1020篇 |
2003年 | 1036篇 |
2002年 | 848篇 |
2001年 | 783篇 |
2000年 | 822篇 |
1999年 | 570篇 |
1998年 | 261篇 |
1995年 | 284篇 |
1993年 | 242篇 |
1992年 | 444篇 |
1991年 | 426篇 |
1990年 | 439篇 |
1989年 | 470篇 |
1988年 | 370篇 |
1987年 | 365篇 |
1986年 | 390篇 |
1985年 | 350篇 |
1984年 | 335篇 |
1983年 | 305篇 |
1979年 | 481篇 |
1978年 | 330篇 |
1977年 | 389篇 |
1976年 | 473篇 |
1975年 | 487篇 |
1974年 | 531篇 |
1973年 | 483篇 |
1972年 | 439篇 |
1971年 | 356篇 |
1970年 | 374篇 |
1969年 | 437篇 |
1968年 | 461篇 |
1967年 | 454篇 |
1966年 | 439篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
991.
I. V. Hunt 《Grass and Forage Science》1973,28(3):171-180
The effects of 59, 118 or 177 kg N/ha were measured on 7 occasions from 8 to 57 days after application to a perennial ryegrass sward in terms of changes in composition. The first increase Dt of N significantly raised percentages of CP, non-protein N, K, P, Ca, Na and Mg and significantly depressed percentages of DM, OM and OM digestihility. The second increment of N resulted in a further rise in the percentages of CP, true protein, non-protein N, K, P, Ca, Na and Mg; the percentages of DM and OM were further depressed but OM digestihility was not significantly depressed. Percentages of P and Na showed increased responses for the more mature herbage; while percentage non-protein N showed a more marked response in younger herbage. As herbage matured percentages of OM and Na rose, the latter only from days 16 to 45. All other attributes showed falling percentages to levels which could be inadequate for a 500 kg dairy cow yielding 15 kg milk. CP percentage reached this critical level (1.4% N) at days 37, 45 or 56 given 59, 118 or 177 kg N/ha; P percentage became critically low (< 0.36%) at days 27, 35 or 38, respectively; Mg percentage fell to the critical level (0.12%) at days 23, 35 and 46. Na percentage showed the reverse trend, being deficient (< 0.13%) in the earliest samples and becoming satisfactory from days 23 or 27 at 118 and 177 kg N/ha but it remained inadequate at 59 kg N/ba. 相似文献
992.
Abstract. Starry flounder Platichthys stellatus (Pallas) from the Duwamish River estuary, which has received large volumes of pollutants in the past, and starry flounder from two relatively unpolluted reference areas were examined for pathological conditions. Gross and microscopic examination of starry flounders from the Duwamish River estuary revealed one or more of the following unusual liver characteristics in 92% (monthly range 85–100%) of the fish: liver discolouration, liver enlargement, severe fatty vacuolation of the hepatocytes, centrolobular cellular degeneration and necrosis of hepatocytes, megalocytic hepatocytes, proliferating perivascular fibroblasts displacing adjacent liver parenchyma, hepatic structural disarray and increased basophilia of hepatocytes in localized areas. Livers of starry flounders, usually spent spawners, from one of the reference areas also had one or more of these characteristics, although at a much lower frequency (<13%), and these included liver discolouration, increased fatty vacuolation of the hepatocytes and hepatic structural disarray. 相似文献
993.
A thin layer chromatographic (TLC) method is described for the determination of citrinin in feeds. Citrinin is extracted from feed with methanol and water, the mixture is made alkaline with 10% sodium carbonate, and the aqueous solution is filtered and extracted with chloroform to remove most of the interfering materials. The aqueous layer is acidified with 2N HCl and extracted with chloroform. The chloroform extract is concentrated and spotted on a thin layer chromatographic (TLC) plate which is developed in chloroform-acetone-ethanol-water (60 + 40 + 10 + 1). The citrinin is viewed under ultraviolet light after TLC. Either visual or fluorodensitometric quantitation is used. Recoveries of citrinin from various feed samples spiked at levels of 2.0--5 micrograms/g were 75--92%. The proposed method can detect 0.5 micrograms/g feed, including corn, silage, ready mixed feeds, and feed pellets. 相似文献
994.
D W Fink R P Martin J V Pivnichny J S Shim 《Journal of the Association of Official Analytical Chemists》1978,61(5):1078-1082
An analytical method has been developed for the determination of arprinocid (9-(2-chloro-6-fluorophenylmethyl)-9H-purin-6-amine) in feed, based upon measurement of the absorbance of the diazo chromophore formed from a product of zinc reduction of the drug in acidic solution. The analyte is extracted from the feed into chloroform in the presence of a pH 7 phosphate buffer and isolated by adsorption chromatography on alumina, followed by partitioning between hexane and 0.15M HCl. The reduction product in the aqueous phase is then treated for colorimetric measurement. This procedure has been applied to determining 0.0010--0.0080% arprinocid in feed with a precision of less than 5% relative standard deviation near the middle of this concentration range. Of 32 feed additives examined, only zoalene and sulfamethazine were serious interferences. A study and discussion of several factors, e.g., reaction time, pH, and amount of zinc metal, that affect the analytical reactions are also included. 相似文献
995.
V.Rajaramamohan Rao 《Soil biology & biochemistry》1978,10(4):319-321
The effect of various carbon sources on asymbiotic N2 fixation in a paddy soil was studied by C2H2 reduction and 15N tracer techniques. N2 fixation increased with increasing concentrations of cellulose and rice straw, but the former was more effective. N2 fixation was considerably enhanced in soils amended with glucose, sucrose, succinate, acetate, butyrate, pyruvate or n-propanol, while n-butanol, ethanol, lignin or paraffin were ineffective. Succinate at low moisture content and butyrate under flooded conditions stimulated N2 fixation during a 15-day incubation. 相似文献
996.
Sampling of the benthic fauna showed that dredging operations produced relatively short-term effects on invertebrate population levels. The behaviour of the animals was immediately affected, the differences between species were noted and the implications of dredging at different times of year are discussed. 相似文献
997.
DM yield and N uptake data are presented from primary growth and two successive regrowths of perennial ryegrass treated with combinations of three N treatments for the primary growth viz. 0, 50 and 100 kg N/ha (PN) and four N treatments applied for the second growths, viz. 0, 33, 66 and 100 kg N/ha (SN). Primary growth gave a response of 24·2 kg DM/kg applied N to PN50 with only a further l±0 kg DM/kg applied N from the second PN increment. A significant interaction between PN and SN treatments was shown in second growth. The residual DM response to PN was highest at SN0 and reduced as the level of SN was raised. Substantially greater residual responses to PN treatments were shown in the third growth. Third growth DM responses to SN treatments were high. High apparent recovery of fertilizer N reached 111% of primary N where SN66 followed PN50. High available soil N is partly responsible for both high apparent recovery of N and high DM response. The latter appears to be associated with inclusion of growths given no fertilizer N so that the full residual effects of fertilizer N and ‘priming’ of available soil N can be realised. 相似文献
998.
T.M. Aronovich S.I. Doroshev L.V. Spectorova V.M. Makhotin 《Aquaculture (Amsterdam, Netherlands)》1975,6(3):233-242
The embryonic and larval development of three White Sea cold-water fish species, rate of yolk sac absorption, age at first feeding and their survival and growth when fed different food organisms, were studied.Eggs were obtained from spawners in the Bay of Kandalaksha, White Sea, and incubated in troughs and aquaria at a mean temperature of 1.5 °C, slightly above that of the sea. The incubation period for polar cod eggs lasted 35 days, for arctic flounder, 42 days and for navaga eggs, 48 days. Emergent larvae were 5.5–6.0 mm long and began feeding at 2–4 °C, 5–6 days (navaga) and 12–14 days (polar cod) after hatching, when their yolk sac was still fairly large.They were fed day-old Artemia nauplii and zooplankton taken from the sea and consisting of Calanus and Pseudocalanus nauplii 400–600 μ in length.The period of establishing first feeding is the most critical for larvae. 相似文献
999.
The capacitance meters available in the past for estimating herbage yields have lacked sensitivity at the extremes of their range of measurement. The preliminary evaluation of a highly sensitive capacitance meter described here indicates that accurate measurement is possible in the range 0.1100 kg DM/ha. The implications of this development for the measurement of herbage of low availability are discussed. 相似文献
1000.
I. V. Hunt 《Grass and Forage Science》1972,27(2):119-128
Fifteen out of 36 possible combinations of 3 cutting dates for hay, 3 rates of N application and 4 intervals between N application and cutting date were compared to study the effects of pre-harvest application of N on the N, P, K, Mg and Ca contents of the aftermath and the effectiveness of utilization of these elements. According to the method of calculation given by ARC (1), the control haycrop harvested on 16 June was deficient in N, Ca, P and Mg, and the aftermath which was given no further N after harvest was of low bulk, but adequate in mineral content. Earlier harvesting improved the quality of the hay, and increased the bulk and reduced the mineral content of the aftermaths. Applying 125.6 kg N/ha after harvest resulted in a satisfactory increase in bulk, plus an improved mineral content in the aftermath. Applying N before harvest resulted in increased uptake of N, K, Ca, P and Mg in the hay and in the increased percentage of N, K, and Ca in the DM of the hay. Very early application of pre-harvest N increased the percentage of Mg. The general effect of pre-harvest N on the aftermath was to reduce yield and uptake of N, K, Ca, P and Mg. Doubling the pre-harvest N raised the mineral uptake. Very early pre-harvest application resulted in low mineral uptake in the aftermath. The percentage of N, K and P in the aftermath was reduced by pre-harvest N applied at both levels and on both dates. Deferring the application of fertilizer N until 4 days after harvesting the hay crop produced the highest mineral contents in the aftermath, without reducing its bulk. Mineral uptake in the regrowths was linearly related to N uptake at 1.43 kg K, 0.34 kg Ca, 0.157 kg P, 0.006 kg Mg/kg N uptake. 相似文献