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   23164篇
  免费   643篇
  国内免费   1篇
林业   4103篇
农学   1522篇
基础科学   174篇
  4573篇
综合类   969篇
农作物   2403篇
水产渔业   2496篇
畜牧兽医   3949篇
园艺   1199篇
植物保护   2420篇
  2024年   11篇
  2023年   61篇
  2022年   79篇
  2021年   153篇
  2020年   158篇
  2019年   114篇
  2018年   2968篇
  2017年   2975篇
  2016年   1494篇
  2015年   328篇
  2014年   355篇
  2013年   526篇
  2012年   1312篇
  2011年   2643篇
  2010年   2354篇
  2009年   1468篇
  2008年   1729篇
  2007年   1962篇
  2006年   363篇
  2005年   417篇
  2004年   399篇
  2003年   444篇
  2002年   317篇
  2001年   211篇
  2000年   230篇
  1999年   140篇
  1998年   40篇
  1997年   26篇
  1996年   24篇
  1995年   25篇
  1994年   22篇
  1993年   32篇
  1992年   35篇
  1991年   27篇
  1990年   37篇
  1989年   54篇
  1988年   26篇
  1987年   34篇
  1986年   16篇
  1985年   19篇
  1984年   18篇
  1983年   15篇
  1981年   9篇
  1980年   10篇
  1979年   9篇
  1977年   13篇
  1976年   12篇
  1968年   11篇
  1967年   11篇
  1966年   9篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Expansion of goat improvement programs requires exploration of the factors that influence the production system and breeding initiatives. Characterization of goat breeds or populations is crucial in providing information on prevalent goat types and their attributes and may suffice as a guideline on conservation, development, and selection for improved productivity. This study investigated the existing village goat production system and phenotypic diversity of the different village populations from four South African provinces. The study further investigated the use of phenotypic attributes to classify goats to breeds or populations. Data was collected from 142 households in 26 villages of the Eastern Cape (n?=?2 villages), KwaZulu-Natal (n?=?6 villages), Limpopo (n?=?13 villages), and North West (n?=?5 villages) provinces through a survey. Individual interviews and focus group discussions revealed that the mean goat herd size per household was least in Limpopo at 13.2?±?12.40 and highest in Eastern Cape (34.18?±?28.36). Flocks had more (p?<?0.05) adults than kids, and the distribution of breeding animals was biased towards does and less bucks. Goats were kept mainly for meat, for selling, and for ritual ceremonies. The goat production system was mainly scavenging. Goat health was a major challenge across households and villages. Qualitative traits such coat, horn, ears, and wattle characteristics were recorded for populations of village goats (n?=?319) and a feral Tankwa breed (n?=?50). The dominant coat pattern was plain (74.53%) with black as the most common coat color (31.98%). Across breeds, a majority (88.08%) of the goats had horns, and 7.59% had wattles while 56.64% had beard. Adult goats (<3 years; n?=?398) were further analyzed for five quantitative traits of chest girth, height, length, and pin bone and there were significant (p?<?0.05) breed differences in all. A stepwise discriminatory procedure was used to rank the quantitative traits according to their discriminatory power to separate breeds or populations. Significant traits were then subjected to canonical discriminant analysis for principle component analysis. Based on the quantitative traits, the Tankwa, Xhosa, and Tswana goats formed their own cluster separated from commercial meat type breeds and the Venda and Zulu ecotypes. The discriminant function analysis correctly classified 90.41% of the Zulu goats and 82.93% of the Xhosa village populations. None of the Savanna goats were correctly classified. The study demonstrated diversity in village goat populations and production systems, which would allow for within population selection in genetic improvement programs. The heterogeneity in the phenotypic traits in the village goats is reflective of the role of village production systems in the maintenance of animal diversity in local populations.  相似文献   
82.
Awareness of animal influenza and its prevention and control is important for ensuring livestock health, production and welfare. In China, a country stereotyped as a major source of emerging zoonotic infectious diseases, research on the public understanding of animal influenza is limited to the Han, the main ethnic group. The present qualitative study in Southwest China investigated awareness of animal influenza among the Dai, an ethnic minority. The participants (15 men and 10 women, ages 18–83) were smallholder farmers of pigs and poultry in rural areas of Jinghong, Xishuangbanna, Yunnan Province. A mixture of interviews and group discussions took place in homes and villages. The participants were asked about their knowledge of avian influenza (H7N9), swine influenza (H1N1), precautions taken to protect against influenza, procedures when animals were sick and perceived risk of animal influenza. The data were analysed following coding and thematic analysis. The findings demonstrated a limited understanding of animal health and welfare among participants. Specifically, they were largely unaware of animal influenza (H7N9, H1N1) including its causes, symptoms, prevention and treatment. The farmers were also uninformed of the risks they faced and unknowingly engaged in behaviours which increased direct or indirect exposure to infected animals, a risk factor for human infection. They also reported poor usage of veterinary services. In order to guarantee the health, welfare and production of their livestock, immediate action is needed to enable Dai smallholder farmers to prevent and respond to animal influenza effectively and timely.  相似文献   
83.
1. The aim of the present study was to investigate whether grain type (barley or oats) and ss-glucanase inclusion influence the performance and the gastrointestinal ecosystem of broiler chickens, taking the concentrations of short-chain fatty acids and lactic acid, pH, ileal viscosity, and the weight of the caeca and liver into consideration. 2. The inclusion of beta-glucanase in the oat-based diet improved body weight gains. Enzyme supplementation of barley-based diets improved feed conversion efficiency and reduced intestinal viscosity. 3. Irrespective of the type of cereal, beta-glucanase supplementation increased the lactic acid concentration and lowered the pH of the crop contents. No such changes in fermentation were observed in the contents of the gizzard and ileum. 4. Larger amounts of total dietary fibre and its fractions (arabinoxylans and beta-glucans) in oats decreased the weight of the caeca. 5. Molar ratios of acetate, propionate and butyrate in the caecal chyme were affected by cereal type but not by enzyme supplementation. The barley-based diet increased the butyrate:propionate ratio but the opposite effect was observed with the oat-based diet.  相似文献   
84.
85.
The prevalence of Cryptosporidium species in 1-2-year-old heifers was determined for 571 animals on 14 dairy farms in seven states on the East Coast of the United States. A fecal specimen collected directly from each heifer was processed to concentrate oocysts that were then examined by polymerase chain reaction (PCR). For every PCR-positive specimen the 18S rRNA gene of Cryptosporidium was sequenced. Cryptosporidium was identified by PCR from heifers on 13 of 14 farms. On all except four farms groups of heifers were housed in a barn or in large covered pens. Others were pastured. From many of the same farms an earlier study reported that 41% of 393 pre-weaned calves and 26.2% of 447 post-weaned calves were infected. In the present study, 11.9% of 571 heifers were infected with Cryptosporidium, 0.7% with Cryptosporidium parvum, the zoonotic species. Of 68 PCR-positive specimens characterized by gene sequencing 1, 4, 10, 24, and 29 calves were infected with Cryptosporidium suis, Cryptosporidium parvum, Cryptosporidium deer-like genotype, Cryptosporidium bovis, and Cryptosporidium andersoni, respectively. These findings demonstrate a lower prevalence of infection in 1-2-year-old dairy cattle than in younger cattle as well as a change in the diversity of species present. Consequently, the risk of humans acquiring infection with C. parvum from exposure to feces from yearling and older cattle appears much lower than from exposure to pre-weaned calves.  相似文献   
86.
The IgG antibody response to Calicophoron daubneyi (Digenea: Paramphistomidae) excretory/secretory antigens was evaluated in naturally infected cattle from Lugo (Galicia, NW Spain) by using an ELISA procedure. Two studies were conducted, first a survey in 524 cattle separated into three groups according to age, G-1 (0–2 years old), G-2 (3–5 years old) and G-3 (>6 years old). In the second study, three groups of cattle were employed: G-I, naturally infected; G-T, naturally infected and treated with oxyclozanide plus levamisole (Nilzan Plus®); G-C, cattle maintained in a farm where C. daubneyi has never diagnosed. Variations on egg-output and haematic parameters (erythrocytes, haematocrite, leukocytes and lymphocytes) were also analyzed.

The ELISA procedure showed that 61.2% of the cattle in the first study had been exposed to the trematode, but only 10.1% passed eggs in the feces. Age-association with egg-output was shown but not with the IgG values. In the second experiment, the administration of the anthelmintic reduced significantly the IgG kinetic levels and the C. daubneyi-egg-output was suppressed during 12 weeks in the G-T group. The values of red cells, haematocrite, leukocytes and lymphocytes increased significantly in the treated cattle 5 weeks after chemotherapy; however, new reduction after week 5 was recorded, as results of the challenge of these cattle.

This is the first investigation in which evaluation of the IgG humoral response against C. daubneyi in cattle has been carried out. We proved that a notable IgG response in naturally infected cattle is induced, and can be detected by using an ELISA procedure. The IgG antibodies did not increase after challenge infection. Our results proved an important percentage of cattle were exposed to this trematode in the area of study and suitable measures for preventing this relationship must be considered.  相似文献   

87.
The ruminal stability of Mepron M 85 and the effect of supplementation with Mepron M 85 on free methionine level of blood were studied in rumen-fistulated cows and rumen- and duodenum-fistulated growing bulls. In five rumen-fistulated cows in situ 69.5% and 64.6% of the methionine content of Mepron M 85 was found after ruminal incubation of 16 h and 24 h, respectively. Daily rations of the rumen-fistulated cows were supplemented with 15.0 g DL-methionine and 17.7 g Mepron M 85, which increased the free methionine level of blood from 13.64 mumol/L to 15.35 and 20.46 mumol/L, respectively, three hours after feeding. In the four rumen- and duodenum-fistulated growing bulls, supplementation with 15.0 g DL-methionine and 17.7 g Mepron M 85 increased the total methionine getting into the duodenum during 24 h from 14.99 g to 16.84 and 20.84 g, respectively. The influence of Mepron M 85 on milk production was studied in 35 pairs of Hungarian Fleckvieh x Holstein-Friesian cows. The animals were coupled on the basis of the number of finished lactations, milk production in the previous lactation, and the date of calving. Daily supplementation of 18.0 g Mepron M 85 increased daily milk production significantly (p < 0.05), by 1.24 litres. Milk fat content also increased significantly (from 3.10% to 3.19%, p < 0.05) in the experimental group. The supplementation did not influence milk protein content.  相似文献   
88.
The potential cytotoxic effects of the compounds 8-quinolinol, chloramine-T and natamycin have been studied in isolated pig hepatocytes. The relative cytotoxicity of these compounds was evaluated on the basis of the leakage of cytosolic lactate dehydrogenase (LDH), 3-(4,5 dimethyl)thiazol-2-yl,-2,5-diphenyl tetrazolium bromide (MTT) reduction by mitochondrial dehydrogenases, uptake of neutral red (NR) by cytosolic lysosomes, glutathion (GSH) depletion and oxidized glutathion (GSSG) efflux after 24 h exposure. Evaluation of the 20%, 50% and 80% reduced absorbance data obtained from the parameters NR20, NR50, and NR80, and MTT20, MTT50 and MTT80 enabled us to rank these compounds in decreasing order of cytotoxicity: 8-quinolinol > natamycin > chloramine-T. Also for the parameters LDH and GSH, chloramine-T appears to be less cytotoxic than natamycin and 8-quinolinol. Our study demonstrated that pig hepatocytes may be a useful model for examining cytotoxic events of drugs to be used in pigs, therefore avoiding possible extrapolation problems due to species differences.  相似文献   
89.
Microsporidia are a ubiquitous group of obligate intracellular parasites that infect all major animal groups. Enterocytozoon bieneusi is the most commonly identified Microsporidia in humans and has also been reported worldwide in animals with importance in veterinary medicine (e.g. cats, dogs, horses, cattle and pigs). The identification of E. bieneusi in animals has raised the question of the importance of animal reservoirs in the epidemiology of this pathogen, and the implications of the infection with this pathogen in infected animals. Considerable genetic diversity within E. bieneusi has been found with over 90 genotypes identified based on the ITS nucleotide sequence of E. bieneusi spores recovered from the feces of infected humans and animals. Both host-adapted E. bieneusi genotypes with narrow host ranges and potentially zoonotic genotypes with wide host specificity have been identified. The information presented in this review should be useful in understanding the taxonomy, epidemiology, zoonotic potential, and importance in public health of E. bieneusi.  相似文献   
90.
The use of antimicrobial agents in poultry production results in their accumulation in the body tissues and products such as milk and egg. The subsequent accumulation of these drugs and their metabolites in body cells is known as drug residue. This study was designed to determine the prevalence of antimicrobial residues in eggs from poultry farms and retail outlets in Enugu State, Nigeria. Eggs from 25 selected commercial farms and ten retail outlets were screened for the prevalence of antimicrobial residue. Also, structured questionnaires were administered to 25 commercial farms in the state to determine the management practices and the most widely used antimicrobial drugs in farms and possible association between the management practices and the occurrence of antimicrobial residues in eggs from these farms. All the 25 farms surveyed use oxytetracycline. Eggs from nine of the surveyed farms tested positive for antimicrobial residue and three of the ten surveyed farms also tested positive for antimicrobial residue. No association was observed (p 0.05; Fisher’s exact test) between the occurrence of antibiotic residues in eggs and farm size, feed source and housing systems. This study was able to demonstrate the presence of antimicrobial residues in eggs destined for human consumption. Drugs like nitrofurans which has been banned for use in food animals are still very much in use in Enugu State, Nigeria. Antibiotics given as feed additives may give rise to drug residues in food animals.  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