全文获取类型
收费全文 | 312130篇 |
免费 | 17405篇 |
国内免费 | 653篇 |
专业分类
林业 | 24849篇 |
农学 | 14359篇 |
基础科学 | 3014篇 |
47186篇 | |
综合类 | 36764篇 |
农作物 | 17520篇 |
水产渔业 | 18533篇 |
畜牧兽医 | 135893篇 |
园艺 | 6522篇 |
植物保护 | 25548篇 |
出版年
2020年 | 3006篇 |
2019年 | 3772篇 |
2018年 | 4654篇 |
2017年 | 5095篇 |
2016年 | 5287篇 |
2015年 | 4620篇 |
2014年 | 6026篇 |
2013年 | 16620篇 |
2012年 | 8490篇 |
2011年 | 10966篇 |
2010年 | 8460篇 |
2009年 | 8799篇 |
2008年 | 10213篇 |
2007年 | 9465篇 |
2006年 | 9603篇 |
2005年 | 8719篇 |
2004年 | 8526篇 |
2003年 | 8376篇 |
2002年 | 7582篇 |
2001年 | 8942篇 |
2000年 | 8619篇 |
1999年 | 7197篇 |
1998年 | 4114篇 |
1997年 | 4167篇 |
1996年 | 4021篇 |
1995年 | 4732篇 |
1994年 | 4150篇 |
1993年 | 3822篇 |
1992年 | 5794篇 |
1991年 | 6046篇 |
1990年 | 5834篇 |
1989年 | 5836篇 |
1988年 | 5208篇 |
1987年 | 5213篇 |
1986年 | 5106篇 |
1985年 | 5238篇 |
1984年 | 4618篇 |
1983年 | 4229篇 |
1982年 | 3313篇 |
1981年 | 3128篇 |
1980年 | 3041篇 |
1979年 | 3982篇 |
1978年 | 3454篇 |
1977年 | 3100篇 |
1976年 | 2924篇 |
1975年 | 2947篇 |
1974年 | 3208篇 |
1973年 | 3232篇 |
1972年 | 2936篇 |
1971年 | 2834篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Shahid Iqbal Shahzad M. A. Basra Irfan Afzal Abdul Wahid Muhammad S. Saddiq Muhammad B. Hafeez Sven‐Erik Jacobsen 《Journal of Agronomy and Crop Science》2019,205(1):13-21
Quinoa is recently introduced to Pakistan as a salt‐tolerant crop of high nutritional value. Open field trials were conducted to evaluate its performance on normal and salinity/sodicity‐degraded lands at two locations of different salinity/sodicity levels, S1 (UAF Farm, Normal Soil), S2 (Paroka Farm UAF, saline sodic), S3 (SSRI Farm, normal) and S4 (SSRI Farm, saline sodic) during 2013–2014. Two genotypes (Q‐2 and Q‐7) were grown in lines and were allowed to grow till maturity under RCBD split‐plot arrangement. Maximum seed yield (3,062 kg/ha) was achieved by Q‐7 at normal field (S1) soil which was statistically similar with yield of same genotype obtained from salt‐affected field S2 (2,870 kg/ha). Furthermore, low yield was seen from both genotypes from both S3 and S4 as compared to S1 and S2. Q‐7 was best under all four conditions. Minimum yield was recorded from Q‐2 (1,587 kg/ha) at S4. Q‐7 had higher SOD, proline, phenolic and K+ contents, and lower Na+ content in leaves as compared to Q‐2. High levels of antioxidants and K+/Na+ of Q‐7 helped to withstand salt stress and might be the cause of higher yields under both normal and salt‐affected soils. Seed quality (mineral and protein) did not decrease considerably under salt‐affected soils even improved seed K+, Mg2+ and Mn2+. 相似文献
992.
Shiva Bakhshandeh Paola E. Corneo Liming Yin Feike A. Dijkstra 《Journal of Agronomy and Crop Science》2019,205(2):157-167
Drought and high temperature are major environmental stress factors threatening wheat production during grain filling stage resulting in substantial yield losses. Four wheat genotypes (Suntop, IAW2013, Scout and 249) were planted under two temperature levels (25 and 30°C) and two water levels (15% and 25% soil moisture content). Wheat yield, leaf δ13C, plant rhizodeposition, shoot biomass and root traits were examined. Low moisture (drought stress) and high temperature (heat stress) decreased the grain yield of all wheat genotypes, in particular 249, while combined drought and temperature stresses had the most pronounced negative effect on plant biomass and grain yield. Decreasing soil water availability decreased the allocation of plant‐derived C to soil organic carbon (SOC) and to microbial biomass through rhizodeposition. Leaf δ13C decreased with increased yield, suggesting that higher yielding genotypes were less water stressed and allocated less C to SOC and microbial biomass through rhizodeposition. Wheat genotypes with lower root/shoot ratios and thinner roots were more efficient at assimilating C to the grain, while genotypes with higher root/shoot ratios and thicker roots allocated more C belowground through rhizodeposition at the expense of producing higher yield. Therefore, improving these traits for enhanced C allocation to wheat grain under variable environmental conditions needs to be considered. 相似文献
993.
Guanglong Zhu Yutiao Chen Evangelina S. Ella Abdelbagi M. Ismail 《Journal of Agronomy and Crop Science》2019,205(2):235-247
Stagnant flooding (SF) during vegetative growth triggers stem elongation usually at the cost of tiller production in rice, reducing grain yield. To explore physiological mechanisms associated with tillering suppression under SF, three contrasting genotypes (Swarna and Swarna‐Sub1, both sensitive and IRRI154, tolerant) were evaluated under standing water depths of 0, 5, 30 and 50 cm. SF significantly suppressed tiller formation but increased plant height, root biomass, shoot elongation (ratio of plant height before and after flooding), leaf emergency and non‐structural carbohydrate (NSC) concentration (in root–shoot junction) in all genotypes at the early stage of development. Chlorophyll concentration in the upper leaves (upper most fully expanded leaf at top) was higher than in lower leaves (lowest green leaf at base), but decreased under SF in both. SF increased hydrogen peroxide (H2O2) at the early stage of treatment, with concomitant increase in malondialdehyde (MDA) production by stems and leaves. MDA concentration in root–shoot junction increased but delayed. Tiller number correlated negatively with plant height, shoot elongation, leaf emergency, MDA concentration in leaves and root–shoot junction, root biomass, and NSC concentration in the root–shoot junction. The results suggested existence of compensatory mechanisms between tiller growth and shoot elongation in rice for resilience under SF, where energy is mainly diverted for shoot elongation to escape flooding. The SF‐tolerant genotype produced less H2O2 and maintained energy balance for higher survival and better growth under stagnant flooding. 相似文献
994.
Foot-hold trapping is an important tool used in pest management programs in countries such as Australia, New Zealand and in North America. Research on humane trapping methods including the addition of sedatives (Tranquilizer Trap Device) and toxins (Lethal Trap Device) to foot-hold traps to improve the welfare of trapped pest animals is important. Lethal Trap Devices (LTD) are being tested in Australia to determine if deploying a toxin with a foot-hold trap is effective at delivering a lethal dose of toxin to trapped predators. This study aimed to test whether fitting an LTD to two different foot-hold jaw traps (Victor Soft catch #3 and Bridger #5) would affect the jaw closure time and as such affect capture rates. We found that two spring Victor Soft catch traps were faster (20.91, SD 0.72 ms) than four spring Bridger #5 traps (26.79, SD 0.48 ms) even when fitted with a Lethal Trap Device. Fitting a Lethal Trap Device to either of these trap models did not affect closure time and as such would not have any effect on capture efficacy. 相似文献
995.
M. A. Dadzie A. Oppong K. Ofori J. S. Y. Eleblu B. E. Ifie E. T. Blay E. Obeng-Bio Z. Appiah-Kubi M. L. Warburton 《Plant pathology》2019,68(8):1565-1576
Fungal mycotoxins are important contaminants of agricultural commodities that pose serious concerns to producers, consumers and exporters. Aflatoxin is a carcinogenic metabolite produced by the fungi Aspergillus flavus and A. parasiticus. These fungi and other mould species living in the soil contaminate several crops including maize. This study was undertaken with the following objectives: (i) to assess the presence of A. flavus and other moulds in Ghanaian soil; (ii) to determine the distribution and density of A. flavus under three agro-ecologies noted for major maize production; and (iii) to assess the effect of percentage of carbon (C) and nitrogen (N) and soil pH on A. flavus population densities and diversity in the isolates found across the agro-ecologies. The data showed seven moulds that were common across the agro-ecologies. Significant differences (P < 0.05) were observed in the A. flavus density and distribution within and across these agro-ecologies. Fumesua soils recorded the highest levels of A. flavus (1.185 × 103 cfu g−1) while Akomadan recorded the least (9.76 × 102 cfu g−1). Percentage available C, N and soil pH did not significantly influence A. flavus density. The A. flavus isolates identified in this study varied in genetic sequence within the aflatoxin gene cluster, but these differences were not distinguishable by origin. 相似文献
996.
Swadling Daniel S. Knott Nathan A. Rees Matthew J. Davis Andrew R. 《Landscape Ecology》2019,34(10):2337-2352
Landscape Ecology - Whilst the composition and arrangement of habitats within landscape mosaics are known to be important determinants of biodiversity patterns, the influence of seascape patterning... 相似文献
997.
Carlos Yanes‐Roca Mayra E. Toledo‐Cuevas Lizbeth J. Sánchez Ana Born‐Torrijos Nicole Rhody Kevan L. Main 《Journal of the World Aquaculture Society》2018,49(3):612-624
Black snook, Centropomus nigrescens, have been identified as a promising candidate for aquaculture although, like many of the Centropomid species, high mortality associated with early larval stages presents a significant bottleneck to their commercialization. The digestive capacity of black snook larvae throughout the first 37 d after hatch (d.a.h.) was evaluated by quantifying digestive enzyme activities using biochemical techniques. Results showed that black snook larvae have alkaline proteases at hatching, which are known to be important during the first days of feeding for digestion. Toward the end of the study, acid proteases concentration increased (37 d.a.h.). Enzymes for lipid digestion, pancreatic lipase and bile salt‐activated lipase, were already present in the larvae before exogenous feeding commenced, and their activity increased with age and growth (length). Intracellular digestion, measured as the activity of leucine‐alanine peptidase, was high early on (5 d.a.h.) and decreased as development progressed (next 32 d). In contrast, alkaline phosphatase activity was lowest at first feeding and subsequently increased with age. Overall patterns in enzyme activity suggest the possibility of live feed weaning before 32 d.a.h. if artificial diets can be properly balanced. 相似文献
998.
Use of Porcine Meal in Plant‐based Practical Diets for Pacific White Shrimp,Litopenaeus vannamei 下载免费PDF全文
Two growth trials were conducted to evaluate and confirm the efficacy of a porcine meal (PM) with high protein content (>90%) as an alternative feed ingredient in commercial‐type feed formulation for Pacific white shrimp, Litopenaeus vannamei. Six experimental diets were formulated for the two growth trials. The first five diets contained increasing levels (0, 1, 2, 4, and 6%) of PM as a replacement for soybean meal in a plant‐based diet with low inclusion level (6%) of fish meal (FM). The last experimental diet was produced utilizing 4.2% PM to completely replace FM. In Trial 1, shrimp (1.5 g initial mean weight, 20 shrimp/tank, n = 4) were offered test diets for 6 wk in a semirecirculation system. At the end of Trial 1, shrimp fed with the diet containing 6% PM exhibited significantly enhanced weight gain (WG), feed conversion ratio (FCR), and survival compared to those fed with the diet devoid of FM. As survival was poor across all treatments and different densities could mask growth results the trial was repeated. In Trial 2, shrimp (0.85 g, 15 shrimp/tank, n = 4) were offered diets for 6 wk. Dietary supplementation of PM at 6% significantly improved WG, FCR, and apparent net protein retention in contrast with the treatment devoid of FM, confirming the same trends in Trial 1. No significant difference was detected in protein, lipid, moisture, and mineral profiles of whole‐body shrimp as well as survival across all the treatments. Results of this study indicate that PM is a good high protein source in shrimp feeds, which can be included up to 6% in the low FM‐based diet without compromising the growth of shrimp. 相似文献
999.
Krystina D. Mossop Nicholas P. Moran David G. Chapple Bob B. M. Wong 《Landscape Ecology》2017,32(5):1065-1078
Context
Dispersal has important fitness consequences for individuals, populations, and species. Despite growing theoretical insights into the evolution of dispersal, its behavioral underpinnings remain empirically understudied, limiting our understanding of the extent and impact of responses to landscape-level heterogeneity of environments, and increasing the risk of inferring species-level responses from biased population sampling.Objectives
We asked if predictable ecological variation among naturally fragmented arid waterbodies is correlated with disparate dispersal responses of populations of the desert goby Chlamydogobius eremius, which naturally inhabits two habitat “types” (permanent springs, ephemeral rivers), and different levels of hydrological connectivity (high and low) that potentially convey different costs and benefits of dispersal.Methods
To test for possible behavioral divergence between such populations, we experimentally compared the movement behaviors (correlates of emigration and exploration) of wild-caught fish. We used two biologically relevant spatial scales to test movement relevant to different stages of the dispersal process.Results
Behavior differed at both spatial scales, suggesting that alternative dispersal strategies enable desert gobies to exploit diverse habitat patches. However, while emigration was best predicted by the connectivity (flood risk) of fish habitats, exploration was linked to their habitat type (spring versus river).Conclusions
Our findings demonstrate that despite a complex picture of ecological variation, key landscape factors have an overarching effect on among-population variation in dispersal traits. Implications include the maintenance of within-species variation, potentially divergent evolutionary trajectories of naturally or anthropogenically isolated populations, and the direction of future experimental studies on the ecology and evolution of dispersal behavior.1000.
Maria Garcia-Martin Nora Fagerholm Claudia Bieling Dimitris Gounaridis Thanasis Kizos Anu Printsmann Matthias Müller Juraj Lieskovský Tobias Plieninger 《Landscape Ecology》2017,32(11):2133-2150