全文获取类型
收费全文 | 143883篇 |
免费 | 8220篇 |
国内免费 | 76篇 |
专业分类
林业 | 6428篇 |
农学 | 5319篇 |
基础科学 | 1013篇 |
18163篇 | |
综合类 | 22717篇 |
农作物 | 5396篇 |
水产渔业 | 7641篇 |
畜牧兽医 | 74207篇 |
园艺 | 1949篇 |
植物保护 | 9346篇 |
出版年
2019年 | 1334篇 |
2018年 | 2320篇 |
2017年 | 2617篇 |
2016年 | 2383篇 |
2015年 | 2012篇 |
2014年 | 2486篇 |
2013年 | 5641篇 |
2012年 | 4329篇 |
2011年 | 5287篇 |
2010年 | 3549篇 |
2009年 | 3588篇 |
2008年 | 5236篇 |
2007年 | 4861篇 |
2006年 | 4744篇 |
2005年 | 4482篇 |
2004年 | 4401篇 |
2003年 | 4340篇 |
2002年 | 3912篇 |
2001年 | 5007篇 |
2000年 | 4939篇 |
1999年 | 3933篇 |
1998年 | 1650篇 |
1997年 | 1674篇 |
1996年 | 1463篇 |
1995年 | 1671篇 |
1994年 | 1539篇 |
1993年 | 1500篇 |
1992年 | 2983篇 |
1991年 | 3097篇 |
1990年 | 3148篇 |
1989年 | 2996篇 |
1988年 | 2722篇 |
1987年 | 2624篇 |
1986年 | 2681篇 |
1985年 | 2564篇 |
1984年 | 2120篇 |
1983年 | 1860篇 |
1982年 | 1217篇 |
1979年 | 1870篇 |
1978年 | 1482篇 |
1977年 | 1273篇 |
1976年 | 1266篇 |
1975年 | 1393篇 |
1974年 | 1536篇 |
1973年 | 1551篇 |
1972年 | 1472篇 |
1971年 | 1428篇 |
1970年 | 1402篇 |
1969年 | 1415篇 |
1967年 | 1290篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Single spring dressings of N were compared with split applications over the first two growth periods of the year at ten sites in eastern England in the period 1964–67. None of the four spring periods was unusually dry and the single application of N in spring did not outyield a similar total N rate applied some before and some after the first cut. At nine out of ten sites split applications of N yielded considerably more in the second growth period than the single spring dressing. It is concluded that the split applications of N in the spring, already shown superior to single dressings under moist west country conditions, are correct for many seasons in the drier east of England. 相似文献
993.
994.
A GIS-based plant prediction model for wetland ecosystems 总被引:5,自引:0,他引:5
An existing non-spatial model for the prediction of response of wetland plant species on ecological factors has been transformed into a GIS-based prediction model which produces spatial output at the landscape scale. The input, spatial patterns of the ecological factors, were constructed with geostatistical spatial interpolation (kriging). With this GIS-based model the spatial patterns of presence and absence of 78 wetland plant species are predicted for an area with wetlands in the Netherlands of approximately 500 square kilometers. The GIS-based model has been validated, and the estimated uncertainty of the input has been propagated through the model. At the species level the output shows spatially coherent and non-random patterns. The validation is affected by the propagation of input errors through the model. The number of valid predictions declines approximately 10–20% when 95% confidence intervals are used in the validation. This study shows that it is feasible to use a geostatistical interpolation method to construct spatial patterns of ecological factors on a landscape scale and to use these patterns as input for a GIS-based prediction model. The added uncertainty on the input values however, affects the number of valid predictions of the model. 相似文献
995.
The occurrence of genetic male sterility and development of highly sterile lines have been reported. The male sterility was not accompanied by any visible chromosomal aberration. It behaved as recessive and was governed by multiple factors. It was hypothesised that three major genes with additive effect were operating to produce highly sterile forms while less sterile forms would be dependent on one or two genes. The expression of male sterility was also influenced by modifying factors and environments. 相似文献
996.
Eight maize complexes and varieties representing high and low yield levels were tested for their suitability as top cross testers by comparing the performance of the crosses in relation to the combining ability of the parents estimated by diallel analysis. The discriminating ability of the testers was compared by Schumann and Bradley test. In both cases the results clearly showed that the low performing testers were better and more reliable than the high performing complexes. The utility of per se performance as an indication of general combining ability of the complexes has been discussed. 相似文献
997.
THE DIGESTIBILITY AND PALATAB1LITY OF HIGH-MOISTURE LEGUME SILAGES WITH DIFFERENT CHEMICAL ADDITIVES
The effects of wilting and of various chemical additives on the digestibility and palatability of wet legume silages are reported. These silages showed most of the characteristics associated with the ensiling of wet herbage with a low content of soluble carbohydrate. Digestibility of all silages by wether lambs was low and exhibited little difference that might be attributed to the treatments. The untreated silage or the one sprayed with valeric/isovaleric acid, just before feeding, were more palatable to dairy heifers. Silage consumption was sharply reduced when urea was added during ensiling. 相似文献
998.
Two steers excreting similar quantities of organic matter daily were grazed at pasture during the summer of 1961. The four experiments each involved a preliminary period of five days dosing with Cr2 O3 to reach a fairly constant excretion. The steers received a daily dose of Cr2 O3 either in oil suspension or in shredded paper. The carriers and steers were interchanged for each trial. For two of the trials each steer in turn was harnessed every other day and all its dung collected in a bag. Total faecal excretion of organic matter was measured from the bagged steer and estimated from the Cr2 O3 concentration in a composite sample made up from all dung pats voided during the ten-day collection period. In the other two trials the steers were not harnessed and each dung pat was separately sampled and analysed for Cr2 O3 in the organic matter. The average recovery of Cr2 O3 in the first two trials was 99·1 (oil) or 94·4% (paper). Calculating the faecal organic matter excretion from the concentration of Cr2 O3 in the faeces and the quantity given daily overestimated it by 3·5 to 8·0%. There was no significant difference between shredded paper and oil as carriers of Cr2 O3 for estimating the quantity of organic matter excreted. There were 37% and 22% less variation between dung pats in Cr2 O3 concentration when this was administered in shredded paper than when given in oil suspension. 相似文献
999.
The oral introduction of 0·5 g histamine as the diphosphate into the rumen of sheep produced no effect on the physiological condition of the animals. The feeding of histamine, as the dihydrochloride, in levels up to 1 g/day to sheep on silage diets did not affect the dry-matter intake. 相似文献
1000.
Results of the analysis for dry-matter, nitrogen and major base content of 9 silage effluents obtained from farm silos and 3 effluents from experimental tower silos are presented. The mean values obtained for the principal plant nutrients in the 12 effluents examined were 0·19% N, 0·037% P and 0·38% K. The results indicate that effluents may be a useful source of plant nutrients, particularly when undiluted with rainwater, and that, in general, silage effluents appear to be superior to liquid manure in manurial value. 相似文献