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   2110篇
  免费   71篇
  国内免费   95篇
林业   89篇
农学   150篇
基础科学   13篇
  108篇
综合类   1060篇
农作物   151篇
水产渔业   62篇
畜牧兽医   469篇
园艺   51篇
植物保护   123篇
  2024年   3篇
  2023年   11篇
  2022年   44篇
  2021年   59篇
  2020年   57篇
  2019年   74篇
  2018年   58篇
  2017年   100篇
  2016年   97篇
  2015年   80篇
  2014年   117篇
  2013年   109篇
  2012年   194篇
  2011年   156篇
  2010年   118篇
  2009年   127篇
  2008年   138篇
  2007年   133篇
  2006年   94篇
  2005年   79篇
  2004年   67篇
  2003年   64篇
  2002年   48篇
  2001年   29篇
  2000年   33篇
  1999年   28篇
  1998年   34篇
  1997年   27篇
  1996年   32篇
  1995年   14篇
  1994年   15篇
  1993年   4篇
  1992年   5篇
  1991年   3篇
  1990年   5篇
  1989年   5篇
  1988年   2篇
  1987年   4篇
  1986年   2篇
  1985年   1篇
  1984年   2篇
  1981年   2篇
  1978年   1篇
  1956年   1篇
排序方式: 共有2276条查询结果,搜索用时 31 毫秒
131.
为了探讨在饲粮中添加不同水平紫苏籽提取物对蛋鸡产蛋高峰后期生产性能、生殖激素及免疫功能的影响,本试验选用体重、产蛋率基本一致的48周龄海兰褐蛋鸡480只,采用单因素完全随机试验设计,随机分为4组,每组6个重复,每个重复20只。对照组饲喂基础饲粮,试验Ⅰ、Ⅱ、Ⅲ组分别在基础饲粮中添加0.01%、0.02%、0.03%的紫苏籽提取物,试验期30 d。结果表明:1)与对照组相比,饲粮中添加0.01%、0.02%、0.03%紫苏籽提取物均能降低料蛋比,但差异不显著(P0.05),4组间日均采食量、破软蛋率无显著差异(P0.05),添加0.03%紫苏籽提取物能极显著提高蛋鸡产蛋率和日均产蛋量(P0.01);2)与对照组相比,添加0.01%、0.02%、0.03%紫苏籽提取物能极显著提高蛋鸡血清中孕酮(第15天)和雌二醇(第15和30天)的含量(P0.01),添加0.03%紫苏籽提取物能极显著降低蛋鸡血清中睾酮含量(P0.01);3)与对照组相比,添加0.01%、0.02%、0.03%紫苏籽提取物能极显著提高蛋鸡血清中白细胞介素-2(IL-2)含量(P0.01),添加0.02%、0.03%紫苏籽提取物能极显著提高蛋鸡血清中免疫球蛋白G(Ig G)含量(P0.01),添加0.03%紫苏籽提取物能极显著提高蛋鸡血清中免疫球蛋白A(Ig A)含量(P0.01)。由此可知,饲粮中紫苏籽提取物添加水平为0.03%时,能极显著提高蛋鸡产蛋高峰后期产蛋率、日均产蛋量和免疫功能。  相似文献   
132.
Pretreatment of somatic cells with undifferentiated cell extracts, such as embryonic stem cells and mammalian oocytes, is an attractive alternative method for reprogramming control. The properties of induced pluripotent stem cells (iPSCs) are similar to those of embryonic stem cells; however, no studies have reported somatic cell nuclear reprogramming using iPSC extracts. Therefore, this study aimed to evaluate the effects of porcine iPSC extracts treatment on porcine ear fibroblasts and early development of porcine cloned embryos produced from porcine ear skin fibroblasts pretreated with the porcine iPSC extracts. The ChariotTM reagent system was used to deliver the iPSC extracts into cultured porcine ear skin fibroblasts. The iPSC extracts-treated cells (iPSC-treated cells) were cultured for 3 days and used for analyzing histone modification and somatic cell nuclear transfer. Compared to the results for nontreated cells, the trimethylation status of histone H3 lysine residue 9 (H3K9) in the iPSC-treated cells significantly decreased. The expression of Jmjd2b, the H3K9 trimethylation-specific demethylase gene, significantly increased in the iPSC-treated cells; conversely, the expression of the proapoptotic genes, Bax and p53, significantly decreased. When the iPSC-treated cells were transferred into enucleated porcine oocytes, no differences were observed in blastocyst development and total cell number in blastocysts compared with the results for control cells. However, H3K9 trimethylation of pronuclear-stage-cloned embryos significantly decreased in the iPSC-treated cells. Additionally, Bax and p53 gene expression in the blastocysts was significantly lower in iPSC-treated cells than in control cells. To our knowledge, this study is the first to show that an extracts of porcine iPSCs can affect histone modification and gene expression in porcine ear skin fibroblasts and cloned embryos.  相似文献   
133.
In this paper, thirty Yili mares aged 11 to 12 years with weighing 410 kg ± 30 kg, parity of 4 to 5 in third lactation month were selected, and were randomly divided into six groups, each group with five repeats.Two factors quadratic regression orthogonal rotational combination design was adopted to evaluate the effects of feeding different dietary CP and DE to mares on growth development and blood biochemical parameters of foals.Daily DE and CP feeding levels of the 6 groups were 92 MJ/d and 1.20 kg/d, 125 MJ/d and 1.20 kg/d, 92 MJ/d and 1.55 kg/d, 107 MJ/d and 1.35 kg/d, 125 MJ/d and 1.44 kg/d, 115 MJ/d and 1.55 kg/d, respectively.The results showed that body weight, body height, chest circumference and cannon circumference of foals of four and five months were not significant affected by maternal protein and energy intake (P>0.05).As compared with ration contained high DE (125 MJ/d) and low CP (1.20 kg/d), feeding with ration contained moderate DE (107 MJ/d) and CP (1.35 kg/d) improved body length of foals of four and five months significantly (P<0.05).From the blood biochemical parameters, such as total protein, albumin, triglyceride, high-density lipoprotein cholesterol, low density lipoprotein cholesterol, urea nitrogen, creatinine, alanine aminotransferase, there were no significant differences among all groups (P>0.05).Mares fed with ration contained moderate DE (107 MJ/d) and CP (1.35 kg/d) obviously improved the aspartate aminotransferase and uric acid level.In summary, Yili mares during lactation late stage fed with 1.35 kg/d CP and 107 MJ/d DE daily could significantly improve body length development of foals, and foals nitrogen metabolism was altered by maternal dietary nutrition through increasing blood aspartate aminotransferase activity.  相似文献   
134.
【目的】 研究沙葱及其提取物对小尾寒羊生产性能、胴体特征,肉品质及血清生化指标的影响,为舍饲肉羊饲料添加剂的开发提供试验依据。【方法】 选取3月龄、体重相近的小尾寒羊公羊60只,随机分为4组,每组3个重复,对照组(CK)饲喂基础日粮,试验组分别在基础日粮中添加沙葱粉(AMR),沙葱水提物(AWE)和沙葱醇提物(AFE),添加剂量分别为10、3.4和2.8 g/(只•d)。试验持续75 d,其中预饲期15 d,正饲期60 d。正饲期内测定小尾寒羊生产性能,正饲期结束空腹颈静脉采血测定血清生化指标,随后每个重复随机选2只羊进行屠宰,立即测定胴体特征(胴体热重、屠宰率、肾周脂肪重、皮下脂肪厚度及硬度),取胴体左侧背最长肌测定肉品质,取肾周脂肪、尾部脂肪、背部皮下脂肪和大网膜脂肪测定脂肪肉色。【结果】 与对照组相比,①沙葱醇提物组的平均日采食量显著降低(P=0.001),饲料转化率显著升高(P=0.039),平均日增重无显著性变化(P>0.05);②日粮添加沙葱粉及水提物能够显著降低羊肉的剪切力(P=0.029);③沙葱粉组、水提物组和醇提物组肾周脂肪的黄度值(b*)无显著性差异(P>0.050),醇提物组肾周脂肪(P=0.096)和大网膜脂肪(P=0.087)的b*值与沙葱粉组相比有升高的趋势,而红度值(a*)(P=0.044)和饱和度(C*)(P=0.021)显著高于沙葱粉组;④添加沙葱及其提取物能够显著升高血清总蛋白(P=0.006)的含量,沙葱及其醇提物能够显著提高血清白蛋白(P=0.006)的含量,沙葱醇提物能够显著降低血清尿素氮(P=0.024)的含量。【结论】 沙葱及其提取物对舍饲小尾寒羊胴体特征和脂肪肉色无显著影响;日粮中添加沙葱醇提物能够显著提高小尾寒羊饲料转化率,沙葱粉及水提物能够显著降低羊肉的剪切力,提高羊肉嫩度;同时沙葱及其醇提物能够显著提高血清中总蛋白和白蛋白的含量,促进肉羊对蛋白质的吸收。  相似文献   
135.
通过试验给出了对虾、鲈鱼、毛蚶和文蛤四种渔业经济生物不同受试天数的油臭阈限值,并指出受试天数越短,油臭阈限值越大;受试天数越长,油臭阈限值越小,油臭阈限值与渔业生物的受试天数呈负相关关系。  相似文献   
136.
U.D Chima  G. Vure 《林业研究》2014,25(4):915-921
The study evaluated the impact of crude oil pollution on natural regeneration of plant species in a major oil-producing community in the Niger Delta region of Nigeria. Three sites—unpolluted site(US), polluted and untreated site(PUS), and polluted and treated site(PTS)—were purposively chosen for the study. The seedling emergence method was used to evaluate soil seed banks in the various sites at two depths, 0 to 10 cm and 10 to 20 cm. Woody-plant species richness, abundance, and diversity were higher in the US seed bank than in the PUS and PTS seed banks. The highest number of non-woody plants was observed in the US, followed by the PTS, and then the PUS. Both species richness and diversity of non-woody plants were highest at the US, followed by the PUS, and lowest in the PTS. Woody species in the US seed bank were 87.5% and 80% dissimilar with those of the PUS and PTS at 0–10 cm and 10–20 cm respectively. No variation was observed between woody species in the PUS and PTS seed banks. Non-woody species at 0-10 cm US seed bank were 73.08% dissimilar with those of PUS at the two soil depths and 81.48/88.46% dissimilar with those of the 0–10/10–20 cm of the PTS respectively. At 10–20 cm, non-woody species of the US were 69.66% dissimilar with those from each of the two soil depths in PUS; and 73.91/81.82% dissimilar with those of 0–10/10–20 cm of the PTS respectively. Non-woody species variation between the PUS and PTS was higher at 10–20 cm than 0–10 cm. The poor seed bank attributes at the polluted sites demonstrates that crude oil pollution negatively affected the natural regeneration potential of the native flora because soil seed banks serve as the building blocks for plant succession. Thorough remediation and enrichment planting are recommended to support the recovery process of vegetation in the polluted areas.  相似文献   
137.
甘肃瑞香提取物对菜粉蝶幼虫生物活性的影响   总被引:1,自引:0,他引:1  
为探明甘肃瑞香(Daphne tangutica Maxim)对菜粉蝶幼虫的生物活性,以甘肃瑞香的全株、叶、茎皮、茎木质部、根皮和根木质部为研究对象,以甲醇、乙酸乙酯、丙酮、氯仿、苯和石油醚为溶剂,采用索氏法提取其活性成分,明确其最佳的杀虫活性部位和提取溶剂.结果表明:在干粉100mg/mL的质量浓度下,各溶剂瑞香全株的提取物均对菜粉蝶幼虫表现出明显的拒食、胃毒和触杀作用,其中苯的提取效果最好,其粗提物对菜粉蝶5龄幼虫24h的拒食率为92.29%,胃毒作用的LD50为19.56μg/头,7d后触杀作用的LD50为52.56μg/头.甘肃瑞香不同部位的苯提取物对菜粉蝶5龄幼虫均表现出明显的拒食、胃毒和触杀作用,其活性部位表现为甘肃瑞香叶>根皮>茎皮>茎木质部>根木质部.甘肃瑞香叶粗提物杀虫活性最高,其粗提物对菜粉蝶5龄幼虫24h的拒食率为94.59%,胃毒作用的LD50为8.47μg/头;7d后触杀作用的LD50为50.41μg/头.  相似文献   
138.
为研究不同饲粮粗蛋白质水平对20~35 kg淮猪生长性能和血清生化指标的影响,选择48头遗传背景相似、初始体重约20 kg的淮猪,按随机区组设计分为3组,每组4个重复,每个重复4头淮猪,公母各半。3个试验组等能量水平下(12.78 MJ/kg)分别饲喂低(14.53%)、中(15.55%)、高(16.62%)3种不同粗蛋白质水平的饲粮。猪体重达35 kg左右结束试验。饲养试验共49 d,其中预试期7 d,正试期42 d。结果显示,日采食量,中等蛋白质组分别比低蛋白质组和高蛋白质组高8.55%和5.77%;日增重,低蛋白质组和中等蛋白质组相同,均比高蛋白质组高9.09%;料重比,低蛋白质组分别比中等蛋白质组和高蛋白质组低4.19%和8.63%。血清尿素氮含量,低蛋白质组分别比中等蛋白质组和高蛋白质组低16.25%(P〈0.05)和11.96%(P〉0.05),说明低蛋白质组猪对饲料蛋白质的利用率高于中等蛋白质组和高蛋白质组;中等蛋白质组血清中葡萄糖、总胆固醇、甘油三酯含量略低于其它试验组,而血清总蛋白、球蛋白含量略高于其它试验组;低、中、高蛋白质组血清中白蛋白含量依次递减。本试验结果表明,20~35 kg淮猪饲粮蛋白质适宜水平为14.53%。  相似文献   
139.
本试验旨在研究饲粮代谢能(ME)和粗蛋白质(CP)水平对蛋鸭产蛋期不同阶段产蛋性能的影响。采用3×3两因子完全随机试验设计,ME水平分别为10.88、10.46和10.04 MJ/kg,CP水平分别为18.26%、17.07%和16.42%,选取刚开产的福建龙岩麻鸭1 674羽,随机分为9个组,每组6个重复,每个重复31羽。试验共持续32周。结果表明:1)产蛋初期(16~18周龄),饲粮ME和CP水平均未显著影响蛋鸭产蛋率、平均蛋重、日产蛋重和料蛋比(P>0.05)。饲粮ME和CP水平对蛋鸭平均蛋重有显著的交互作用(P<0.05)。2)产蛋高峰期(19~39周龄),饲粮ME水平显著影响蛋鸭平均蛋重(P<0.05)。饲粮ME水平为10.88和10.46 MJ/kg组的平均蛋重显著高于10.04 MJ/kg组(P<0.05)。饲粮CP水平未显著影响蛋鸭产蛋率、平均蛋重、日产蛋重和料蛋比(P>0.05)。饲粮ME和CP水平对蛋鸭产蛋率、平均蛋重、日产蛋重和料蛋比没有显著的交互作用(P>0.05)。3)产蛋后期(40~47周龄),饲粮ME水平为10.46 MJ/kg组的产蛋率和日产蛋重显著高于10.04 MJ/kg组(P<0.05);10.88 MJ/kg组的平均蛋重显著高于10.46和10.04 MJ/kg组(P<0.05)。饲粮CP水平为18.26%组的产蛋率、日产蛋重显著高于17.07%和16.42%组(P<0.05),料蛋比显著低于17.07%和16.42%组(P<0.05)。饲粮ME和CP水平对蛋鸭日产蛋重有显著的交互作用(P<0.05)。综合考虑产蛋性能指标,产蛋初期、高峰期和后期龙岩麻鸭饲粮适宜的ME和CP水平分别为10.88、10.46、10.46 MJ/kg和17.07%、17.07%、18.26%,每日ME和CP需要量分别为1.47、1.60、1.48 MJ和22.96、26.16、25.81 g,饲粮的蛋能比分别为15.62、16.35、17.44 g/MJ。  相似文献   
140.
This study was conducted to investigate the effects of increasing levels of extracts of Byrsonima crassifolia, Celtis pallida, Enterolobium cyclocarpum, Fraxinus excelsior, Ficus trigonata, Phoradendrom brevifolium and Prunus domestica on in vitro gas production (GP) and ruminal fermentation of a high concentrate diet. Plant extracts were prepared at 1 g dry matter (DM)/8 mL of solvent mixture (methanol : ethanol : water, 1:1:8) and added at levels of 0, 0.6, 1.2 and 1.8 mL/g DM of a high concentrate diet. In vitro GP was recorded at 2, 4, 6, 8, 10, 12, 24, 48 and 72 h of incubation. Increasing addition of extracts linearly increased (P < 0.001), the GP24, GP48 and GP72 (mL/g DM), and linearly decreased (P < 0.001), the discrete GP lag time. Moreover, increasing extract doses linearly increased (P < 0.001) the asymptotic GP and decreased (P < 0.001) the rate of GP. GP6 was not impacted by treatments and GP12 increased linearly (P = 0.01) with increasing addition of extracts. Rumen pH declined linearly (P < 0.05) with increasing doses of extracts added. As no interactions (P > 0.05) occurred between the extracts and doses, it could be conclude that all extracts positively modified rumen fermentation at doses of 1.2 to 1.8 mL extract/g diet DM.  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