全文获取类型
收费全文 | 484篇 |
免费 | 39篇 |
专业分类
林业 | 62篇 |
农学 | 18篇 |
93篇 | |
综合类 | 15篇 |
农作物 | 17篇 |
水产渔业 | 94篇 |
畜牧兽医 | 175篇 |
园艺 | 7篇 |
植物保护 | 42篇 |
出版年
2023年 | 9篇 |
2022年 | 15篇 |
2021年 | 41篇 |
2020年 | 39篇 |
2019年 | 55篇 |
2018年 | 36篇 |
2017年 | 35篇 |
2016年 | 36篇 |
2015年 | 17篇 |
2014年 | 19篇 |
2013年 | 30篇 |
2012年 | 29篇 |
2011年 | 39篇 |
2010年 | 19篇 |
2009年 | 21篇 |
2008年 | 12篇 |
2007年 | 11篇 |
2006年 | 19篇 |
2005年 | 10篇 |
2004年 | 8篇 |
2003年 | 8篇 |
2002年 | 4篇 |
2001年 | 1篇 |
2000年 | 1篇 |
1999年 | 3篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1969年 | 1篇 |
排序方式: 共有523条查询结果,搜索用时 15 毫秒
121.
122.
Felipe de Azevedo Silva Ribeiro Alexandre Firmino Diógenes Julio Cesar Silva Cacho Thiago Lima de Carvalho João Batista Kochenborger Fernandes 《Aquaculture Research》2014,45(4):637-646
In this study, we evaluated the production potential of the polyculture of Freshwater Angelfish (Pterophyllum scalare, Cichlidae) and Pacific white shrimp (Litopenaeus vannamei). Two experimental trials were set up. The first trial entailed the use of a randomized design to investigate three treatments options: angelfish monoculture, shrimp monoculture, and fish and shrimp polyculture in 12 experimental 15 m2 ponds. In the second trial, we investigated two treatments (polyculture of caged fish and uncaged fish) in 10 experimental units. In trial 1, polyculture negatively affected fish growth and survival. However, fish did not affect shrimp growth and the greatest profit was achieved in polyculture. In trial 2, caging fish improve the growth of both fish and shrimp. An economic analysis showed the greatest profit and benefit cost ratio for caged fish compared with uncaged fish. We conclude that growing P. scalare and L. vannamei together is a better strategy than shrimp monoculture in low‐salinity water. The use of caged fish in such a polyculture operation would enhance productivity and profitability. 相似文献
123.
Bruno Corrêa da Silva Felipe do Nascimento Vieira José Luiz Pedreira Mouriño Norha Bolivar Walter Quadros Seiffert 《Aquaculture Research》2016,47(2):612-623
Organic salts may improvement the animal performance, increasing the efficiency of nutrient utilization and modifying the intestinal microbiota. This study aimed to evaluate the effects of sodium butyrate and sodium propionate supplementation at different levels of dietary inclusions in the growth of Litopenaeus vannamei. In total, seven diets were evaluated: a control diet (without supplementation) and three diets from each sodium salt, propionate and butyrate, in concentrations of 0.5%, 1% and 2%. We used 21 tanks of 6000 L stocked with 150 shrimps (2.53 ± 0.03 g). The shrimps fed diets supplemented with propionate and butyrate, in all concentrations, increased their final weight. The feed efficiency, nitrogen retention, protein efficiency rate, survival and yield of shrimps fed the diet containing 2% butyrate were higher in comparison with control treatment. The shrimps supplemented with butyrate also showed lower counts of Vibrio sp. in the intestine. The shrimps fed the diet supplemented with butyrate and propionate also showed higher values of serum agglutination titre. Thus, it is possible to conclude that dietary supplementation with propionate and butyrate in different dietary concentrations modify the intestinal microbiota and improves the growth of L. vannamei. 相似文献
124.
125.
Background
How environmental factors affect forest regeneration is relevant for systems that depend partially or fully on natural regeneration. 相似文献126.
Rodrigo Hakamada Carmeni Giunti Neto Cristiane CZ de Lemos Sérgio R Silva Marina SG Otto Kevin B Hall 《Southern Forests》2016,78(4):275-281
Leaf area index (LAI) is a key ecophysiological parameter in forest stands because it characterises the interface between atmospheric processes and plant physiology. Several indirect methods for estimating LAI have been developed. However, these methods have limitations that can affect the estimates. This study aimed to evaluate the accuracy and applicability of a visual method for estimating LAI in clonal Eucalyptus grandis × E. urophylla plantations and to compare it with hemispherical photography, ceptometer and LAI-2000® estimates. Destructive sampling for direct determination of the actual LAI was performed in 22 plots at two geographical locations in Brazil. Actual LAI values were then used to develop a field guide with photographic images representing an LAI range of 1.0–5.0 m2 m?2 (leaf area/ground area). The visual LAI estimation guide was evaluated with 17 observers in the field. The average difference between actual LAI and visual LAI estimation was 12% and the absolute difference between the two methods was less than or equal to 0.5 m2 m?2 in 77% of plots. Pearson’s correlation coefficients were high between actual LAI and hemispherical photographs (0.8), visual estimation (0.93) and LAI-2000® (0.99) and low for the ceptometer (0.18). However, absolute values differed among methods, with the average difference between the actual and estimated LAI of [12]% for visual estimation, 28% for the LAI-2000®, 37% for the ceptometer and ?43% for hemispherical photographs. The LAI-2000® and ceptometer overestimated LAI in all plots, whereas hemispherical photographs underestimated the values in all measurements, showing that these methods need calibration to be used. No differences were observed between actual LAI and visual estimates across stand ages of 2–8 years and LAI of 1.5–5.3 m2 m?2 (P > 0.05). The results show that visual estimation of LAI in Eucalyptus stands is a practical method that is unaffected by atmospheric characteristics and can be used on an operational scale. 相似文献
127.
Pessoa OD de Carvalho CB Silvestre JO Lima MC Neto RM Matos FJ Lemos TL 《Fitoterapia》2005,76(7-8):712-714
The essential oil of fresh leaves of Lippia aff. gracillis was analyzed by GC/MS and evaluated for its antibacterial effects. The results showed a moderate antibacterial activity and confirm the traditional uses of L. aff. gracillis. 相似文献
128.
Rafael Coelho Ribeiro Walkymário de Paulo Lemos Paulo Manoel Pontes Lins Roberta de Melo Valente Antonio Agostinho Müller Alexandre Mehl Lunz José Cola Zanuncio 《Phytoparasitica》2012,40(3):239-241
The coconut palm (Cocos nucifera L., Arecaceae) has great economic and social importance for many industrial and non-industrial products. Pests can reduce its productivity and cause its death. In 2005, larvae and adults of a Curculionidae were observed damaging the floral stalk of coconut palms in commercial plantations in the municipality of Moju, Pará State, in the Brazilian Amazon. Insects were identified as the black coconut bunch weevil, Homalinotus depressus (L.) (Coleoptera: Curculionidae: Cholini). This is the first report of this pest damaging coconut palms in Brazil. 相似文献
129.
130.
Felipe García-Oliva Juan F. Gallardo Lancho Noé Manuel Montaño Pilar Islas 《Agroforestry Systems》2006,66(2):93-100
Gains and losses of soil carbon (C), have been reported when tropical forests are converted to pastures. Regional studies
are crucial for setting regional baselines and explaining each particular trend, in order to solve this controversy. Tropical
deciduous forest (TDF) is under high deforestation pressure, mainly for conversion to pastures. The present study compared
soil organic C (SOC) and nitrogen (SON) in the surface layer (0–5 cm) of forest and pasture soils in a TDF of western Mexico.
SOC and SON concentrations were 18 and 60% lower in pasture soils than in forest soils, and C:N ratio increased in pasture
soils. Furthermore, pasture soils had lower labile C and available inorganic nitrogen (N) than forest soils. These results
can be explained as a reduction in C inputs to pasture soils and management-induced disruption of soil aggregates. In forest
soils, macroaggregates (> 250 μm) were predominant (85%), whereas in pasture soils they were reduced to 35% of dry sand-free soil mass. The estimated SOC
and SON losses from the top 5 cm of soil were 3 Mg C ha−1 and 0.9 Mg N ha−1, respectively. 相似文献