全文获取类型
收费全文 | 12908篇 |
免费 | 506篇 |
国内免费 | 16篇 |
专业分类
林业 | 1168篇 |
农学 | 435篇 |
基础科学 | 82篇 |
2375篇 | |
综合类 | 524篇 |
农作物 | 777篇 |
水产渔业 | 961篇 |
畜牧兽医 | 5842篇 |
园艺 | 288篇 |
植物保护 | 978篇 |
出版年
2023年 | 52篇 |
2022年 | 76篇 |
2021年 | 125篇 |
2020年 | 130篇 |
2019年 | 85篇 |
2018年 | 401篇 |
2017年 | 487篇 |
2016年 | 438篇 |
2015年 | 422篇 |
2014年 | 442篇 |
2013年 | 538篇 |
2012年 | 884篇 |
2011年 | 731篇 |
2010年 | 314篇 |
2009年 | 245篇 |
2008年 | 374篇 |
2007年 | 359篇 |
2006年 | 292篇 |
2005年 | 1007篇 |
2004年 | 894篇 |
2003年 | 670篇 |
2002年 | 289篇 |
2001年 | 271篇 |
2000年 | 168篇 |
1999年 | 247篇 |
1992年 | 136篇 |
1991年 | 166篇 |
1990年 | 192篇 |
1989年 | 185篇 |
1988年 | 181篇 |
1987年 | 143篇 |
1986年 | 156篇 |
1985年 | 134篇 |
1984年 | 100篇 |
1983年 | 91篇 |
1979年 | 141篇 |
1978年 | 100篇 |
1977年 | 98篇 |
1976年 | 88篇 |
1975年 | 142篇 |
1974年 | 133篇 |
1973年 | 121篇 |
1972年 | 144篇 |
1971年 | 122篇 |
1970年 | 91篇 |
1969年 | 105篇 |
1968年 | 120篇 |
1967年 | 133篇 |
1966年 | 111篇 |
1965年 | 73篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
G Flachowsky H Wilk H J L?hnert W I Ochrimenko D Geinitz A Hennig 《Archives of Animal Nutrition》1990,40(1-2):85-100
Five experiments with 18 to 36 male calves each of the black and white dairy cattle breed (age: 14-21 days, initial live weight: approximately 45 kg per animal) were carried out in order to investigate the influence of various vitamin A supply (0-80,000 IU per 100 kg LW and day) on dry matter intake and weight gain as well as the vitamin A status of liver and blood plasma over 84 days. The calves consumed a diet free of carotene and vitamin A consisting of milk replacer, concentrate and chopped wheat straw. The calves were fed in three experiments for a longer time in order to observe the further vitamin A depletion. Nine animals consumed an unsupplemented ration, nine other one got 10,000 IU vitamin A per 100 kg LW and day. Biopsies of liver and plasma samples were taken from 4 animals per group every four weeks. The various vitamin A supplementation did not significantly influence the dry matter intake (Mean: 1.67; 1.48 to 1.80 kg DM per animal and day) and the weight gain of calves (Mean: 702, 599 to 770 g per animal and day). First vitamin A deficiency symptoms (reduced feed intake, decreased weight gain, diarrhoea etc.) were observed in animals of unsupplemented group after 100 days of experiments. After 84 days the vitamin A concentration of liver of animals of unsupplemented groups decreased to 1.3-32.2% compared with the begin of experiments (60.6-155.7 mumol/kg fresh matter). Up to 51% of initial concentration were found when 10,000 IU vitamin A per 100 kg LW and day were fed. About 25,000 IU vitamin A per 100 kg LW and day were required in order to keep the initial level of vitamin A concentration of liver. The plasma vitamin A concentration is unsuitable for estimation of vitamin A status of calves. The concentration of vitamin A of liver and plasma amounted to 114 mumol per kg and 0.25 mumol per litre at the begin of experiments. The vitamin A concentration of liver of unsupplemented group decreased to 20 mumol per kg, that of plasma increased to 0.28 mumol per 1 at the end. A strong vitamin A deficiency (liver concentration: less than 10 mumol/kg) may cause a decrease of vitamin A concentration of blood. 相似文献
52.
We derived an allometric model of the height–diameter curve for even-aged pure stands, which was a modification of the earlier
model proposed by Inoue (2000a). An individual-dependent allometric equation was used as the height–diameter curve. Using
the discriminant analysis method, all trees composed of a stand were stratified into upper and lower trees. It was assumed
that both relationships between mean tree height H
m
and upper tree height H
u
and between mean DBH D
m
and mean DBH of upper trees D
u
could be described by the time-dependent allometric power equations. The height–diameter curve showed an average relationship
between tree height and DBH of a given stand at a given time, and hence it could be assumed that the height–diameter curve
contained two points (D
m
, H
m
) and (D
u
, H
u
). With these assumptions, we derived an allometirc model of height–diameter curve, which allowed the coefficients of the
curve to be estimated from mean tree height and mean DBH. The proposed model was fitted to Japanese cedar (Cryptomeria japonica D. Don) data. The error ratio of the allometric model ranged from 2.254% to 13.412% (mean = 6.785%), which was significantly
smaller than that of the earlier model. When the error of mean tree height was ±1.0 m or less, the effect of the error of
mean tree height on the error ratio was comparatively small. This suggested that the error of ±1.0 m in mean tree height could
be accepted in the estimation of height–diameter curve using the allometric model. These features enable us to combine the
allometric model with Hirata’s vertical angle-count sampling or growth models. In conclusion, the allometric model would be
one of the most practical and convenient approaches for estimating the height–diameter relationship of even-aged pure stands. 相似文献
53.
Antti Oksanen Kjetil ?sbakk Marja Raekallio Mauri Nieminen 《Acta veterinaria Scandinavica》2014,56(1)
Background
Overwintering (breeding) reindeer (Rangifer tarandus tarandus) are commonly treated with ivermectin against parasitic infestations once yearly in autumn-winter roundups. The only preparations registered to reindeer are those for subcutaneous injection. However, also oral extra-label ivermectin administration is used. Twenty-six, 8-month-old reindeer calves were randomly allocated into three groups. Group 1 (n = 9) received oral ivermectin mixture (Ivomec® vet mixt. 0.8 mg/ml, oral ovine liquid drench formulation), Group 2 (n = 9) oral ivermectin paste (Ivomec® vet 18.7 mg/g equine paste), and Group 3 (n = 8) subcutaneous injection of ivermectin (Ivomec® 10 mg/ml vet inj.), each group at a dose of 200 μg/kg body weight. Blood samples were collected at treatment and at days 1, 2, 3, 6, 9 and 16 post treatment. Plasma concentrations of ivermectin were determined by high-pressure liquid chromatography (HPLC) with fluorescence detection.Results
The peak plasma concentration (Cmax) was reached by 2 days after each treatment. The Cmax and Area Under Curve (AUC) differed significantly between the groups: Cmax was 30.2 ± 3.9, 14.9 ± 5.7 and 63.1 ± 13.1 ng/ml, and AUC∞ was 2881 ± 462, 1299 ± 342 and 6718 ± 1620 ng*h/ml for groups 1, 2 and 3, respectively (mean ± standard deviation).Conclusions
The differences in plasma concentrations of ivermectin are concomitant with earlier observed differences in antiparasitic efficacy, which discounts the use of the equine paste in reindeer in favour of the oral ovine liquid drench formulation, or preferably, the reindeer-registered subcutaneous injection formulation. 相似文献54.
Olafsdóttir G Högnadóttir A Martinsdóttir E Jónsdóttir H 《Journal of agricultural and food chemistry》2000,48(6):2353-2359
An electronic nose was used as a rapid technique to monitor changes in the headspace gas above capelin (Mallotus villosus) during storage at 0 and 5 degrees C. At both temperatures, refrigeration was compared with an ice/seawater cooling system. At 5 degrees C, the effect of added 0.2% acetic acid was also tested. Electronic nose measurements were compared to measurements of total volatile bases (TVB), gas chromatography of volatile compounds, and sensory analysis. TVB analysis indicated less spoilage in seawater/ice systems than in refrigeration, but the other measurements indicated more spoilage in the seawater system compared to refrigeration. The possibility of using only a few sensors in the electronic nose to predict TVB was studied using partial least squares regression and a saturated generalized linear model. The results reported herein indicate that rapid electronic nose measurements, which require no sample preparation, can be used to predict the TVB value of the raw material stored under different conditions. 相似文献
55.
Javier Abadía Ana Álvarez-Fernández Adamo D. Rombolaà Manuel Sanz Massimo Tagliavini Anunciación Abadía 《Soil Science and Plant Nutrition》2013,59(7):965-971
The objective of this paper is to review the developments in the last few years in two important issues related to Fe deficiency in plants. First, the current knowledge on the possible ways to carry out the diagnosis and prognosis of Fe deficiency in plants is discussed. This includes discussion on the best ways to carry out a meaningful analysis of Fe-containing compounds in different plant parts. We will also discuss other measurement techniques that can permit to assess the Fe nutritional status in plants, including leaf chlorophyll concentrations and others. Second, the new developments in management techniques to control and remediate iron deficiency are discussed. This includes possible improved ways to supply Fe compounds available to plants, both to the soil and to the irrigation water. We also discuss possible ways to supply directly the plant with Fe containing compounds, either to the foliage or to the stem. A particular emphasis is given throughout the paper to fruit tree crops growing in Mediterranean areas. 相似文献
56.
Gerónimo Quiñonez-Barraza Dehai Zhao Héctor M. De Los Santos Posadas José J. Corral-Rivas 《Annals of Forest Science》2018,75(3):78
Key message
More accurate diameter at breast height (dbh)-growth models are needed for developing management tools for mixed-species forests in Mexico. Individual distance-dependent dbh growth models that quantify local neighborhood effects have been developed for four species groups in such forests. The performance of the models is improved by distinguishing between inter- and intraspecific group competitions.Context
The management of mixed-species forests in the northwest of Durango, Mexico, is mainly based on the selection method. Understanding the interspecific and intraspecific competition is critical to developing management tools for such mixed-species forests.Aims
An individual-based distance-dependent modeling approach was used to model the growth of dbh and to evaluate neighborhood effects for four species groups in Mexican mixed-species stands.Methods
Twenty-two species were classified into four groups: Pinus (seven species), other conifers (three species), other broadleaves (four species), and Quercus (eight species). Four methods were used to select neighboring trees and 12 competition indices (CIs) were calculated. Comparisons of the neighboring trees selection methods and CIs and tests of assumptions about neighborhood effects were conducted.Results
Intra-species-group competition significantly reduced diameter growth for all species groups, except for the Quercus group. The Pinus, other conifers, and Quercus groups had significant and negative neighborhood effects on the other broadleaves species group, and not vice versa. The Quercus group also had negative neighborhood effect on the Pinus and other conifers species groups, and not vice versa. The Pinus and other conifers species groups had negative neighborhood effects on each other. All fitted age-independent dbh growth models showed a good of fit to the data (adjusted coefficient of determination larger than 0.977).Conclusion
The growth models can be used to predict dbh growth for species groups and competition in mixed-species stand from Durango, Mexico.57.
Shinpei?WadaEmail author Chutima?Hanjavanit Osamu?Kurata Kishio?Hatai 《Fisheries Science》2005,71(3):682-684
58.
Helgason A Pálsson S Gudbjartsson DF Kristjánsson T Stefánsson K 《Science (New York, N.Y.)》2008,319(5864):813-816
Previous studies have reported that related human couples tend to produce more children than unrelated couples but have been unable to determine whether this difference is biological or stems from socioeconomic variables. Our results, drawn from all known couples of the Icelandic population born between 1800 and 1965, show a significant positive association between kinship and fertility, with the greatest reproductive success observed for couples related at the level of third and fourth cousins. Owing to the relative socioeconomic homogeneity of Icelanders, and the observation of highly significant differences in the fertility of couples separated by very fine intervals of kinship, we conclude that this association is likely to have a biological basis. 相似文献
59.
60.
María?C.?Mateo-SánchezEmail author Aitor?Gastón Carlos?Ciudad Juan?I.?García-Vi?as Jorge?Cuevas César?López-Leiva Alfredo?Fernández-Landa Nur?Algeet-Abarquero Miguel?Marchamalo Marie-Josée?Fortin Santiago?Saura 《Landscape Ecology》2016,31(6):1261-1276