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   201篇
  免费   12篇
  国内免费   10篇
  2022年   3篇
  2021年   4篇
  2020年   12篇
  2019年   11篇
  2018年   11篇
  2017年   9篇
  2016年   13篇
  2015年   14篇
  2014年   10篇
  2013年   13篇
  2012年   7篇
  2011年   10篇
  2010年   8篇
  2009年   10篇
  2008年   10篇
  2007年   9篇
  2006年   9篇
  2005年   8篇
  2004年   3篇
  2003年   7篇
  2002年   5篇
  2001年   4篇
  2000年   9篇
  1999年   4篇
  1998年   4篇
  1997年   2篇
  1995年   2篇
  1994年   1篇
  1989年   1篇
  1988年   1篇
  1984年   3篇
  1982年   1篇
  1981年   1篇
  1979年   2篇
  1978年   2篇
排序方式: 共有223条查询结果,搜索用时 31 毫秒
1.
A total of 627 cattle representing seven breeds from south central Nebraska, USA were tested for 37 BoLA antigens which behave as products of 37 distinct alleles of the class I BoLA-A locus. Four antigens were absent from all breeds tested. The other antigens showed marked and statistically significant differences in breed distribution. There was no evidence for blank (null) alleles. The number of alleles in each breed ranged from 10 to 20. The Hereford and Simmental populations tested were less polymorphic than the Angus, Brown Swiss, Charolais, Gelbvieh and Limousin populations.  相似文献   
2.
The breeding season was 157, 154, <126, 210 and 217 days for Rambouillet, Columbia, Suffolk, Rambouillet x Finnish Landrace and Columbia x Finnish Landrace ewes respectively. Treatment of cyclic ewes with pregnant mare serum gonadotropin (PMSG) (500 IU), following a 12-day treatment with progestin-containing intravaginal sponges, did not affect fertility, but did decrease the time from sponge removal to estrus, (control 48.0 +/- 3.1 hr; PMSG 39.4 +/- 1.8 hr) to the preovulatory surge of LH (control 52.7 +/- 2.8 hr; PMSG 39.0 +/- 1.7 hr) and FSH (control 52.3 +/- 2.9 hr; PMSG 42.8 +/- 1.6 hr) and caused an elevation of serum LH levels prior to the preovulatory surge (control 1.25 +/- 0.18 ng/ml; PMSG 2.31 +/- 0.22 ng/ml). Exposure of the purebred ewes to 18 hours of daylight in January, decreasing by 30 minutes a week subsequently, counteracted the seasonal reduction in the number of ewes lambing following induced breeding under natural daylight in May. Prolificacy was greatest in crossbred ewes and their fertility was not affected by season. Gestation period was longer for fall-bred ewes and varied with breed.  相似文献   
3.
The records of 1596 embryo collections were retrospectively analysed to investigate the influence of breed on embryo production and pregnancy rates. The breed of the donor cow was a significant source of variation in the results of embryo transfer. Total embryo production per collection ranged between breeds from 6.0 to 16.2, number of transferable embryos from 2.8 to 6.6 and percent transferable from 37% to 68%. The percent of pregnancies per collection ranged from 2.0 to 4.0 and pregnancy rates from 37% to 75%. The percent of transferable embryos and the pregnancy rate was independent of the total embryos collected. Care should be taken when interpreting the breed differences, since the donor cows were not randomly representative of the breeds and were selected on different bases. Brangus (16.2), Simbrah (15.8) and Beefmaster (13.0) produced the most embryos. The most transferable embryos were collected from Simmentals (6.6), Brangus (6.6), Chianina (6.2), Beefmaster (6.1), Simbrah (6.1) and Saler (6.0) cows. Angus (68%) had the highest percent transferable, followed by Saler (64%), Chianina (60%), Limousin (52%) and Simmental (51%). Pregnancy rates were highest in the Herefords (75%), Saler (57%), Zebu (56%), Charolais (53%), Longhorns (52%) and Simbrah (50%). Estrus response to superovulation varied between breeds, but this did not account for all the breed differences in embryo production.  相似文献   
4.
This study investigates how the ecosystem services (ES) linked to livestock grazing are perceived across countries. A total of 82 case studies collected from 42 countries via survey (53.7% cases from Europe and 46.3% from outside of Europe) have been analysed through a multivariate approach. In all, 18 non-provisioning ES were considered. Overall, the reported impacts of livestock grazing on the different ES were much more positive than negative. Notably, a large proportion of respondents reported either positive or very positive impacts for some cultural ES, namely cultural, historic and natural heritage (84%), knowledge systems and educational values (77%), landscape values (74%), and for some supporting and regulating ES, namely habitat provision (66%), nutrient cycling (65%), and bush encroachment/fire control (66%). Based on multiple regression analysis, geographic origin, stakeholder type and species category, as well as protection status of the grazing area, had significant effects on the perception of the impacts. Respondents reported those impacts as more positive in Europe, in protected areas and where several species were present in the grazing area. A significantly larger proportion of respondents reported recognition of ES provided by the grazing livestock population in European countries (40.9%) compared with non-European countries (23.7%). Based on the survey responses it appears that in non-European countries absence of formal recognition, especially by policy makers, is a major challenge for the continued provision of ES in grazing systems. In Europe, where such recognition is already often included in legislation, the long-term sustainability of related policies and incentives to provide such services is viewed as a major issue by the respondents.  相似文献   
5.
B. Brenig 《Animal genetics》2020,51(4):590-594
Autochthonous cattle breeds are an important part of cultural heritage and reservoir of genetic diversity. Usually, such breeds have been selected over centuries and reflect adaptation to a specific local environment and human demands. However, owing to low effective population size in conjunction with increased inbreeding and genetic drift, many of these lineages are threatened with extinction. The Jochberger Hummel (‘Jochberger Bumblebee’) is such an endangered subtype of the Pinzgauer cattle originating from a single polled female calf. To evaluate the suspected uniqueness of this subtype and to assess whether it should be kept separate or managed together with the Pinzgauer cattle as one population, I examined the genetic diversity in a set of 844 cattle (Angus, Charolais, Holstein, Jochberger Hummel, Pinzgauer, Uckermaerker and Tux-Zillertaler) using principal component and biogeographical ancestry analysis. The analysis showed that the Jochberger Hummel is still a distinct subtype of Pinzgauer cattle with less than 5% admixture and a low inbreeding coefficient.  相似文献   
6.
《Anthrozo?s》2013,26(3):196-207
Abstract

In a previous study of canine temperament (Hsu and Serpell 2003), a distinct “trainability” factor was identified, characterized by a dog's willingness to attend to its owner and obey simple commands, combined with a high “fetch” motivation, and low levels of distractibility and/or resistance to correction. This paper explores the distribution of this trait in a large sample of dogs in relation to breed, sex and neuter status. The owners of 1,563 dogs belonging to 11 common breeds were invited to assess them for “trainability” using a standardized questionnaire (C-BARQ©). Highly significant breed differences in trainability were detected. In two breeds with distinct field and show bred lines, show bred dogs obtained significantly lower trainability scores. Although no overall sex differences in trainability were detected, male Dachshunds and West Highland White Terriers were found to be significantly more trainable than females. Neutering was not associated with any differences in trainability in female dogs in any breed, but was associated with positive effects on trainability in male Shetland sheepdogs. The findings suggest that there is scope for improving trainability in most breeds of dog, and emphasize the dangers of generalizing among breeds with respect to sex differences in trainability or the benefits of neutering. The biological basis of the trainability trait is also discussed in light of recent research on the evolution of canine social cognition.  相似文献   
7.
Data from 113 Dutch organic farms were analysed to determine the effect of cross-breeding on production and functional traits. In total, data on 33 788 lactations between January 2003 and February 2009 from 15 015 cows were available. Holstein–Friesian pure-bred cows produced most kg of milk in 305 days, but with the lowest percentages of fat and protein of all pure-bred cows in the data set. Cross-breeding Holstein dairy cows with other breeds (Brown Swiss, Dutch Friesian, Groningen White Headed, Jersey, Meuse Rhine Yssel, Montbéliarde or Fleckvieh) decreased milk production, but improved fertility and udder health in most cross-bred animals. In most breeds, heterosis had a significant effect (P < 0.05) on milk (kg in 305 days), fat and protein-corrected milk production (kg in 305 days) and calving interval (CI) in the favourable direction (i.e. more milk, shorter CI), but unfavourably for somatic cell count (higher cell count). Recombination was unfavourable for the milk production traits, but favourable for the functional traits (fertility and udder health). Farm characteristics, like soil type or housing system, affected the regression coefficients on breed components significantly. The effect of the Holstein breed on milk yield was twice as large in cubicle housing as in other housing systems. Jerseys had a negative effect on fertility only on farms on sandy soils. Hence, breed effects differ across farming systems in the organic farming and farmers can use such information to dovetail their farming system with the type of cow they use.  相似文献   
8.
9.
10.
sam βada is a genome–environment association software, designed to search for signatures of local adaptation. However, pre‐ and postprocessing of data can be labour‐intensive, preventing wider uptake of the method. We have now developed R.SamBada, an r ‐package providing a pipeline for landscape genomic analysis based on sam βada , spanning from the retrieval of environmental conditions at sampling locations to gene annotation using the Ensembl genome browser. As a result, R.SamBada standardizes the landscape genomics pipeline and eases the search for candidate genes of local adaptation, enhancing reproducibility of landscape genomic studies. The efficiency and power of the pipeline is illustrated using two examples: sheep populations from Morocco with no evident population structure and Lidia cattle from Spain displaying population substructuring. In both cases, R.SamBada enabled rapid identification and interpretation of candidate genes, which are further discussed in the light of local adaptation. The package is available in the r CRAN package repository and on GitHub (github.com/SolangeD/R.SamBada).  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