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   132460篇
  免费   7269篇
  国内免费   93篇
林业   5381篇
农学   4269篇
基础科学   831篇
  17035篇
综合类   21482篇
农作物   5393篇
水产渔业   6688篇
畜牧兽医   67961篇
园艺   1800篇
植物保护   8982篇
  2020年   1177篇
  2019年   1375篇
  2018年   2031篇
  2017年   2330篇
  2016年   2095篇
  2015年   1742篇
  2014年   2224篇
  2013年   5290篇
  2012年   4114篇
  2011年   5093篇
  2010年   3245篇
  2009年   3389篇
  2008年   4971篇
  2007年   4666篇
  2006年   4353篇
  2005年   4088篇
  2004年   3850篇
  2003年   3973篇
  2002年   3542篇
  2001年   4150篇
  2000年   4252篇
  1999年   3372篇
  1998年   1474篇
  1997年   1387篇
  1996年   1288篇
  1995年   1529篇
  1994年   1358篇
  1993年   1231篇
  1992年   2584篇
  1991年   2700篇
  1990年   2591篇
  1989年   2611篇
  1988年   2368篇
  1987年   2348篇
  1986年   2427篇
  1985年   2340篇
  1984年   1881篇
  1983年   1677篇
  1982年   1149篇
  1979年   1748篇
  1978年   1397篇
  1977年   1151篇
  1976年   1212篇
  1975年   1292篇
  1974年   1500篇
  1973年   1514篇
  1972年   1405篇
  1971年   1304篇
  1970年   1251篇
  1969年   1283篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The aim of this study was to investigate the effects of different levels of Desmodium intortum (Desmodium) hay supplementation in sheep fed fixed amounts of enset pseudostem or corm and a basal diet of wheat straw on intake, digestibility and nitrogen utilization. Eighteen male sheep with a mean (± SD) live weight of 20.5 ± 1.45 kg were assigned to six treatments in a completely randomized design and fed either 108 g dry matter (DM) enset pseudostem or 165 g DM enset corm each with three levels (100, 200 and 300 g) of hay supplementation. For the pseudostem diets, there was no significant difference in total DM intake. Total crude protein (CP) intake and N retention increased with increasing levels of hay in both pseudostem and corm diets. The apparent digestibility of DM, OM, CP, acid detergent fibre and neutral detergent fibre (NDF) and microbial nitrogen supply (MN) at 100 g was lower that other levels of supplementation. For the corm diets, total DM and OM intake and MN supply increased with increasing levels of hay. The digestibility decreased (p < 0.001) with increasing levels of supplementation. The results suggest that at least 300 g (395 g/kg dietary DM) of Desmodium hay is required in pseudostem diets, whereas 200 g (337 g/kg dietary DM) may be sufficient in corm diets for efficient nutrient utilization.  相似文献   
992.
The aims of this study were to verify the efficacy of delayed hormonal treatments performed on day 25 post‐insemination on pregnancy rate at 45 and 70 days in buffalo. The trial was performed on 385 buffaloes synchronized by the Ovsynch/TAI protocol and submitted to artificial insemination (AI). Twenty‐five days after AI, pregnant animals were assigned to four treatments: (1) GnRH agonist (n = 52), 12 μg of buserelin acetate; (2) hCG (n = 51), 1500 IU of human chorionic gonadotrophin; (3) Progesterone (n = 47), 341 mg of P4 intramuscular (im) every 4 days for three times; (4) Control (n = 54), treatment with physiological saline (0.9% NaCl). Milk samples were collected on days 10, 20 and 25 after AI in all buffaloes to determine progesterone concentration in whey by radioimmunoassay method. Statistical analysis was performed by anova . Pregnancy rate on day 25 after AI was 52.9%, but declined to 41.8% by day 45, indicating an embryonic mortality (EM) of 21%. If only control group is considered, the incidence of EM was 38.9%. Pregnant buffaloes had higher (p < 0.01) progesterone concentrations on day 20 and 25 after AI than both non‐pregnant buffaloes and buffaloes that showed EM. The treatments on day 25 increased (p < 0.01) pregnancy rate, although in buffaloes with a low whey progesterone concentration on day 20 and 25 after AI (n = 22); all treatments were ineffective to reduce EM.  相似文献   
993.
The economics of dairy production are in great part dictated by the reproductive efficiency of the herds. Many studies have reported a widespread decrease in fertility of dairy cows. In a previous work ( Rocha et al. 2001 ), we found a very poor oestrus detection rate (38%), and consequently a delayed calving to 1st AI and calving to conception intervals. However, a good conception rate at 1st AI was noted (51%) resulting in a low number of inseminations per pregnancy (IAP) (1.4). Here, results from a subsequent fertility time trend assessment study carried out in the same region for cows born from 1992 to 2002 are reported. Statistical linear models were used to analyse the data. Estimate linear contrasts of least square means were computed from each model. The number of observations per studied index varied from 12 130 (culling rate) to 57 589 (non‐return rate). Mean age at first calving was 28.9 ± 0.14 months, without (p > 0.05) variation over time. There was a small, but significant (p < 0.05), deterioration of all other parameters. Non‐return rates at 90 days and calving rate at 1st AI decreased 0.3% per trimester, with a consequent increase of 0.04 IA/parturition. Oestrus detection rate decreased 0.13% per year, and calving at 1st AI and calving‐conception intervals increased 0.17 and 0.07 days/year respectively, while intercalving interval increased 1.7 days per year. From 12 130 cows calving, only 1 816 had a 4th lactation (85% culling/losses). The data was not meant to draw conclusions on the causes for the decreased fertility over time, but an increase of milk production from 6537 kg to 8590 kg (305 days) from 1996 to 2002 is probably one factor to take into consideration. Specific measures to revert or slow down this trend of decreasing fertility are warranted. Available strategies are discussed.  相似文献   
994.
Overexpression of high mobility group A (HMGA) genes was described as a prognostic marker in different human malignancies, but its role in canine haematopoietic malignancies was unknown so far. The objective of this study was to analyse HMGA1 and HMGA2 gene expression in lymph nodes of canine lymphoma patients. The expression of HMGA1 and HMGA2 was analysed in lymph node samples of 23 dogs with lymphoma and three control dogs using relative quantitative real‐time RT‐PCR. Relative quantity of HMGA1 was significantly higher in dogs with lymphoma compared with reference samples. HMGA2 expression did not differ between lymphoma and control dogs. With the exception of immunophenotype, comparison of disease parameters did not display any differences in HMGA1 and HMGA2 expression. The present findings indicate a role of HMGA genes in canine lymphoma. This study represents the basis for future veterinary and comparative studies dealing with their diagnostic, prognostic and therapeutic values.  相似文献   
995.
996.
Nine lactating dairy cows were evaluated because of eye lesions. Examination revealed mild to severe ulceration of the lower and/or upper eyelids, mild to severe swelling surrounding affected eyes, and profuse lacrimation. Lesions typically affected 1 eye, and involved the eyelid skin and conjunctiva. Oxytetracycline treatment led to cure within 2 wk.  相似文献   
997.

Food irradiation is a preservation method that has been approved by the European Union for a limited number of foods or food products and within a limited range of doses. European Community legislation states that all foods or food ingredients treated in this manner and authorized for sale in the European Union must be clearly labeled, and that market-level inspections must be carried out to ensure compliance with this regulation. To improve detection of irradiated foods, we evaluated the efficacy of electron spin resonance (ESR) spectroscopy for the identification of seafood that had been subjected to irradiation. Three fish species were tested: gilthead sea bream, cod, and swordfish. For each species, 18 samples of flesh-containing bone were analyzed after either irradiation at 0.5, 1, or 3 kGy or no irradiation (control). Induced radicals in irradiated samples produced distinct ESR signals that allowed differentiation from non-irradiated samples. Within the dose range tested, a linear dose-response relationship was observed (R 2 > 0.94). The gilthead sea bream displayed the highest radiosensitivity. The analytical procedure was validated and successfully used for the routine monitoring of locally marketed seafood. The main aims of the study were to develop a reliable tool for the enforcement of the proper labeling of irradiated foods and to obtain preliminary data on compliance with current laws in the Italian market.

  相似文献   
998.
999.
1000.
The key role of records in a production medicine practice   总被引:2,自引:0,他引:2  
The needs of dairy farmers for veterinary assistance have changed and are still changing. It appears that many future opportunities in dairy practice will involve active consultation on herd management factors. The ability to evaluate herd records will be crucial to this effort. This article discusses organizing and then utilizing a production medicine program based on systematic records analysis.  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