Over-use of N fertilizer in crop production has resulted in a series of environmental problems in the North China Plain (NCP).  Thus, improvement of nitrogen use efficiency (NUE) in summer maize has become an effective strategy for promoting sustainable agriculture in this region.  Using twenty maize cultivars, plant dry matter production, N absorption and accumulation, yield formation, and NUE in summer maize were investigated under three N levels in two growing seasons.  Based on their yield and yield components, these maize cultivars were categorized into four groups including efficient-efficient (EE) cultivars, high-nitrogen efficient (HNE) cultivars, low-nitrogen efficient (LNE) cultivars and nonefficient-nonefficient (NN) cultivars.  In both two seasons, the EE cultivars improved grain yield together with increased plant biomass, and enhanced accumulative amounts as well as higher average grain yields than the other cultivar groups under deficient-N conditions.  Significant correlations were observed between yield and kernel numbers (KN), dry matter (DM) amount and N accumulation at both post-silking and maturity stages.  DM and N accumulation at late growth stage (i.e., from silking to maturity) contributed largely to the enhanced yield capacity and improved NUE under N-deficient conditions.  Compared with the NN cultivars, the EE cultivars also showed increased N assimilation amount (NAA) and N remobilization content (NRC), and elevated N remobilization efficiency (NRE), NUE and nitrogen partial factor productivity (PFPN).  Our investigation has revealed N-associated physiological processes and may provide guidance for cultivation and breeding of high yield and NUE summer maize under limited N conditions in the NCP.

Mycoplasma leachii was initially isolated from arthritic calves in South Queensland, Australia, and its ability to cause clinical polyarthritis in calves was demonstrated by experimental infection.  However, the source of M. leachii infection in calves and its means of spreading are not well known.  In this study, one-month-old calves were inoculated with cultures of M. leachii or joint fluid (collected from M. leachii-infected calves) through the intraarticular, intravenous, intratracheal, intranasal or oral routes.  Multidisciplinary procedures, including clinical assessment, etiology assessment, pathology and immunohistochemistry (IHC), were used to evaluate the pathogenicity of M. leachii in calves and to elucidate the transmission route of M. leachii infection in calves.  The results showed that all calves inoculated intraarticularly with cultured GN407 or joint fluid and two-thirds of the calves inoculated intravenously with joint fluid developed severe polyarthritis, and the M. leachii antigen was detected in the joints of all infected calves by IHC and PCR.  In contrast, calves inoculated with cultured M. leachii or joint fluid through the intratracheal, intranasal or oral routes did not show any M. leachii infection in the clinical assessment, etiology assessment, or pathology and IHC results.  These results indicated that polyarthritis caused by M. leachii in calves is transmitted via the blood route; however, this disease is not transmitted through the respiratory or digestive routes.  In addition, the M. leachii antigen was not detected in the lungs of all the inoculated calves using IHC and PCR, indicating that M. leachii is not associated with pneumonia, even in the calves inoculated through the respiratory duct.  These findings are important information for the prevention and control of calf polyarthritis caused by M. leachii.