首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   6273篇
  免费   377篇
  国内免费   1篇
林业   1104篇
农学   190篇
基础科学   13篇
  1359篇
综合类   570篇
农作物   187篇
水产渔业   251篇
畜牧兽医   2404篇
园艺   129篇
植物保护   444篇
  2023年   40篇
  2022年   46篇
  2021年   104篇
  2020年   104篇
  2019年   73篇
  2018年   211篇
  2017年   194篇
  2016年   180篇
  2015年   157篇
  2014年   180篇
  2013年   419篇
  2012年   373篇
  2011年   402篇
  2010年   194篇
  2009年   171篇
  2008年   323篇
  2007年   352篇
  2006年   343篇
  2005年   290篇
  2004年   277篇
  2003年   290篇
  2002年   239篇
  2001年   183篇
  2000年   177篇
  1999年   102篇
  1998年   38篇
  1997年   35篇
  1996年   49篇
  1995年   74篇
  1994年   43篇
  1993年   23篇
  1992年   26篇
  1991年   24篇
  1990年   26篇
  1989年   16篇
  1988年   25篇
  1986年   34篇
  1985年   29篇
  1984年   23篇
  1983年   22篇
  1982年   15篇
  1980年   26篇
  1979年   15篇
  1977年   22篇
  1975年   16篇
  1966年   19篇
  1955年   14篇
  1941年   14篇
  1937年   16篇
  1936年   15篇
排序方式: 共有6651条查询结果,搜索用时 15 毫秒
21.
Zusammenfassung Es wurde die Ableitung eines Verfahrens dargestellt, das es gestattet, eine Probe einer bestimmten Kartoffelpartie durch eine Koch- und Geschmacksprüfung einem bestimmten Type zuzuordnen. Die Eigenschaftskombinationen der benutzten Typen wurden eingehend beschrieben. Dabei wurde berücksichtigt, da? die Kombinationen den Vorstellungen der Praxis entsprechen sollen. Durch eine vereinfachte Auswertung der Testergebnisse (Schablonen) kann für den Tester das Ergebnis sehr schnell ermittelt werden. Eine sorgf?ltigere Auswertung bedingt darüberhinaus nur eine Addition von 9 Zahlen und das Nachschlagen in einer Tabelle. An mehreren Stellen wurden verschiedene Variationsm?glichkeiten des Verfahrens angedeutet.
Summary In the interest of European potato research the Section “Varieties” of the E.A.P.R. has defined a number of the commonly accepted quality factors, and their gradations, so that co-operating countries can understand each other when discussing the results of cooking tests. There was, however, in addition to this, a demand from some quarters for the inclusion of further factors from the culinary and organoleptic complexes in order that samples could be assignedwith certainty to accurately defined “table quality types” (“Speisetypen”). Such characterisation should, as far as is possible, conform to practical requirements and should include the maximum number of factors regarded as decisive in assessing cooking quality. To demonstrate the method, the types listed inTable 1 were defined so that either individual samples could be classified or, by using mean values, varieties. As in the existing tests the various factors are found in different combinations, there are numerouspossibilities of coincidence. Table 2 shows the combination when only two factors (tuber shape and consistency) are considered, andTable 3 the possibletypes. The possibilities resulting from the inclusion of a further factor are shown inTable 4, each type being indicated by a number. Continuing in this way a considerable catalogue of numbered types is finally achieved. Copies of such a list can be obtained from the authors on request. It will be appreciated that there are many combinations, not always of equal value, which lead to the assignment of a sample to the same type, particularly as variations within characters are permitted (Table 5). An additional figure (index number), therefore, denotes whether a sample is “typical” of the type or merely approximates to it. The character ratings shown inTable 5 are used to calculate this “index” (Table 7). For the determination of table quality types the use of the test schedule (Fig. 1) is recommended; assessment is simplified by the use of appropriate stencils (Fig. 2). The types listed and the factors taken into consideration are examples and can be reduced or increased as required, furthermore, the values assigned to the characters can be varied.

Résumé Dans le cadre de l'Association Européenne pour la Recherche sur la Pomme de terre, la section “Varieties” a défini des notions simplifiées et leur mode de cotation de sorte que les pays intéressés aux déterminations de type culinaire disposent d'une base commune d'appréciation. En plus, certains pays demandent à soumettre à l'examen d'autres qualités du complexe “cuisson et go?t” afin de pouvoir classer uniformément les échantillons examinés dans des types culinaires nettement définis. Une telle classification doit répondre le plus possible aux exigences de la pratique et contenir la plupart des propriétés déterminant d'une fa?on essentielle la valeur culinaire d'un lot de pommes de terre. Pour illustrer la méthode on a rapporté dans leTableau 1 les types définis à partir desquels il est possible de caractériser les lots différents, voire les variétés sur la base des valeurs moyennes. Vu que les qualités peuvent être liées entre elles de fa?ons différentes selon les lots, il en ressort queles possibilités d'association sont nombreuses. LeTableau 2 montre les associations possibles pour deux propriétés seulement (forme du tubercule et consistance). LeTableau 3 reprend le nombre detypes possibles en considérant les deux facteurs précités. En ajoutant un facteur supplémentaire, on obtient les possibilités d'association reprises dans leTableau 4, caractérisées chacune par un chiffre. En continuant de cette fa?on, on obtient finalement tout un catalogue de chiffres que l'auteur peut transmettre sur demande aux intéressés. Il va de soi que plusieurs combinaisons, qui ne sont pas nécessairement de valeur égale, mènent au même type. Ceci résulte surtout du fait que pour chaque qualité analysée il existe des fluctuations (Tableau 5). C'est pourquoi un chiffre complémentaire (chiffre indicatif) détermine dans chaque cas s'il s'agit d'un type “caractéristique” ou d'un type “approximatif”. Pour établir ce chiffre indicatif (Tableau 7) on a ajouté l'appréciation des qualités reprises dans leTableau 6. Pour l'exécution pratique de la détermination du type culinaire, l'auteur propose un formulaire (Fig. 1) pour un schéma de test et, pour la simplification de l'utilisation, il recommande l'emploi de gabarits (Fig. 2). Les types cités et les qualités considérées ne l'ont été qu'à titre d'exemple; ils peuvent être réduits ou augmentés selon les besoins. De même les cotations peuvent être modifiées avec la définition des qualités.
  相似文献   
22.
23.
24.
25.
Most traits in animal breeding, including feed efficiency traits in pigs, are affected by many genes with small effect and have a moderately high heritability between 0.1 and 0.5, which enables efficient selection. Since the microbiota composition in the gastrointestinal tract is also partly heritable and was shown to have a substantial effect on feed efficiency, the host genes affect the phenotype not only directly by altering metabolic pathways, but also indirectly by changing the microbiota composition. The effect of the microbiota composition on the breeding value of an animal is the conditional expectation of its breeding value, given the vector with microbiota frequencies, that is The breeding value of an animal can therefore be decomposed into a heritable contribution that arises from an altered microbiota composition and a heritable contribution that arises from altered metabolic pathways within the animal, so Instead of selecting for breeding value , an index comprising the two components and with appropriate weights, that is , can be used. The present study shows how this breeding strategy can be applied in pig genomic selection breeding scheme for two feed efficiency traits and daily gain.  相似文献   
26.
The change in the Common Agricultural Policy (CAP) of the European Union from product to producer support, including requirements for ‘good agricultural and environmental conditions’ and ‘greening’, is excellent. However, these requirements are now defined in rather general terms. Questions can be raised about suitable indicators, and there is a recognized need for effective management recommendations to support farmers in achieving the required ‘good’ conditions. These recommendations are bound to be quite different for different soils in different countries. A study of Dutch clay soils was based on a storyline describing current problems and management options for improvement, which were quantified using a soil–water–crop simulation model. Indicators were defined for agricultural conditions and suggestions made for the use of the model in a predictive mode to help farmers improve their soil management. Environmental conditions were judged by current environmental guidelines for water and air. When modelling, implicit assumptions that soils are homogeneous were shown to be unrealistic for these clay soils, requiring development of innovative methods and procedures, presenting a challenge for soil research.  相似文献   
27.
In a field experiment, we examined the effects of structural complexity in the form of added artificial plastic plants and shredded plastic bags on growth and abundance of juvenile brown trout (Salmo trutta). Just after emergence, the added complexity had a positive effect on the density, biomass and condition factor of young‐of‐the‐year (0+) brown trout. This difference in density was not present six weeks later. In contrast, both young‐of‐the‐year and older brown trout generally tended to be larger in the simple habitat. Hence, our data suggest that increased complexity initially is beneficial for young‐of‐the‐year individuals probably due to lower risk of predation and increased densities of prey. However, as density increases in the complex environment, it may induce negative density‐dependent effects, here reflected in smaller sized fish in the complex environment. This might force fish to redistribute to habitats with lower densities of conspecifics as they grow larger. We propose that habitat complexity can increase survival of yearlings in early phases and thereby also affect the overall population structure of brown trout in natural streams.  相似文献   
28.
29.
30.
This study aims to reveal the morphological properties of facial nerve and the middle ear in Van cats. Study material was composed of 6 female Van cats. Dissections were performed under a Zoom Stereo Microscope. There was no plexus buccalis in Van cats. The chorda tympani was observed to pass through an opening in the tympanic cavity, emerge through a small opening just behind the retroarticular process, and join the lingual nerve. A rounded anatomical formation with a size of 2.75 ± 0.3 mm was found to be located within the mastoid process of the temporal bone between the facial nerve and the auricular branch of the vagus nerve. The stapes nerve was not present. The geniculate ganglion was very prominent and about 1.00 mm high. The deep petrosal nerve was observed to emerge from the plexus tympanicus. The bulla tympanica was 18.96 ± 0.10 mm long, 13.03 ± 0.20 mm wide and 13.16 ± 0.20 mm high. After leaving the mandibular nerve, the n.tensoris tympani coursed caudally around the a.maxillaris, formed an ansa, entered the tympanic cavity through the canalis musculotubarius and reached an end in the m. tensor tympani. Due to the scarcity of studies on the middle ears of Van cats, it is thought that this study will fill a gap in the field of veterinary anatomy.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号