THE EFFECT OF STAGE OF GROWTH AT DEFOLIATION ON WHITE CLOVER IN MIXED SWARDS

Four experiments are described in which various defoliation methods and frequencies were imposed on grass/white-clover swards. Clover was not reduced in any of these hy increasing the length of growth period or hy cutting at a late stage of growth, but intensive sheep grazing reduced clover compared nitb cutting and taking a silage cut in a grazing sequence improved clover yield. These results do not support the contention that cutting a mixed sward for hay or silage suppresses clover because of competition for light: with a ‘big white’ type of clover infrequent defoliation may cause no more shading than frequent. Alternatively, any additional shading may be compensated for by an increase in clover growth, stimulated hy the longer growth period. It is suggested that, in practice, competition for nutrients and moisture also contrihute towards clover suppression when swards are cut for hay or silage.     

FERTILIZER N AND THE YIELD OF CUT GRASS SWARDS IN EASTERN ENGLAND

Single spring dressings of N were compared with split applications over the first two growth periods of the year at ten sites in eastern England in the period 1964–67. None of the four spring periods was unusually dry and the single application of N in spring did not outyield a similar total N rate applied some before and some after the first cut. At nine out of ten sites split applications of N yielded considerably more in the second growth period than the single spring dressing. It is concluded that the split applications of N in the spring, already shown superior to single dressings under moist west country conditions, are correct for many seasons in the drier east of England.     

EFFECTS OF LOW-LEVEL EXPOSURE TO IONIZING RADIATION: CURRENT CONCEPTS AND CONCERNS FOR VETERINARY WORKERS*

The issue of biological effects caused by low-level exposure to ionizing radiation (0.1Sv/yr or 10 rem) is a concern of veterinary radiologists, clinicians and radiotherapists. Cellular damge from ionizing radiation is mediated by injury to DNA, some of which can be repaired. The most important harmful effects attributed to low-level exposure are carcinogenesis and mutation. Ionizing radiation may also cause embryonic and fetal damage, life span shortening and cataractogenesis. The estimated excess risk of mortality for all human cancers is 0.8% (800 deaths per 100,000) per 0.10 Sv of acute exposure; at low dose-rates, risk is presumed to be less. Ionizing radiation increases the frequencey of spontaneous genic and chromosomal mutations in man and animals. Studies of atomic bomb survivors imply that at least 1 Sv would be needed to double the spontaneous mutation rate for acute exposure to low-LET radiation. However, the doubling dose for chronic, low-level exposure may be 4 Sv or more. The human embryo and fetus are sensitive to excessive doses of ionizing radiation, with mental retardation the main concern in the low dose range. Paradoxically, there is scientific evidence that in some instances, chronic low-level exposure to ionizing radiation causes a beneficial or hormonic effect. For veterinary workers, occupational exposures appear to be well below the maximum permissible dose limit and the risk or probability of harmful effects is low if stringent radiation safety practices are maintained.     

Pasteurella haemolytica as a co-pathogen in pullets and laying hens

Pasteurella haemolytica was isolated from internal organs of laying hens and pullets of four flocks in which five incidents of increased mortality had occurred. There was inflammation of the trachea in all cases, and infectious laryngotracheitis virus was identified in three of the incidents. Rapid response to antimicrobial therapy was seen in two out of three outbreaks in the pullets. P. haemolytica was regarded as a significant secondary invader in these cases.     

The contribution of livestock to urban resilience: the case of Bamako,Mali

Urban livestock keeping is increasing in many sub-Saharan African cities, but detailed contextual information on its extent, challenges, and potential is limited. A cross-sectional household study was done in 2010 in Bamako, Mali. Thirty-two of 67 quarters were randomly selected with selection probability proportional to the size of the human population of the communes. Questionnaire interviews were done with a head of household in 1141 households, comprising 19,816 people in total. Sheep were kept by 16% (95% CI 14–18), while 21% (95% CI 17–24) kept poultry. The sheep to human ratio was 4:100, with an extrapolated city-wide population of 67,636 sheep (95% CI 61,018–75,595). The poultry to human ratio was 11:100, with an extrapolated city-wide population of 191,802 chickens (95% CI 176,212–208,772). For urban livestock holders, household-level enterprise gross margins were calculated for sheep production at USD 103 and poultry production at USD 50 annually. The annual gross margin was estimated at USD 35 per sheep and USD 17 per chicken. Based on these figures, the city-wide urban livestock total gross margin for Bamako in 2010 was estimated at USD 5.6 million. Detailed population data help clarify the urban livestock animal human interface in diverse contexts and highlight the important contributions that urban small-holder production adds to food security and resilience. The potential for urban livestock production informs decision-makers in developing adapted, sustainable policies in resource-constrained environments.

     

A microbiological assay to estimate the antimicrobial activity of parenteral tildipirosin against foodborne pathogens and commensals in the colon of beef cattle and pigs

Tildipirosin (TIP) is a novel 16‐membered‐ring macrolide authorized for the treatment of bovine and swine respiratory disease. The pH dependency of macrolide antimicrobial activity is well known. Considering that the pH in the colon contents of growing beef cattle and pigs is usually below pH 7.0, the minimum inhibitory concentrations (MIC) of TIP against foodborne bacterial pathogens such as Campylobacter (C.) coli, C. jejuni and Salmonella enterica and commensal species including Enterococcus (E.) faecalis, E. faecium and Escherichia coli were determined under standard (pH 7.3 ± 1) or neutral as well as slightly acidic conditions. A decrease in pH from 7.3 to 6.7 resulted in an increase in MICs of TIP. Except for the MICs > 256 μg/mL observed in the resistant subpopulation of the C. coli and the Enterococcus species, the MIC ranges increased from 2–8 μg/mL to 64–> 256 μg/mL for Salmonella enterica and E. coli, from 8–16 μg/mL to 32–128 μg/mL for the two Campylobacter species, and from 4–32 μg/mL to 128–> 256 μg/mL for both Enterococcus species. To estimate the antimicrobial activity of TIP in the colon contents of livestock during recommended usage of the parenterally administered TIP (Zuprevo^®), and to compare this with the increased MICs at the slightly acidic colonic pH, we developed and validated a microbiological assay for TIP and used this to test incurred faecal samples collected from cattle and pigs. Microbiological activity of luminal TIP was determined in aqueous supernatants from diluted faeces, using standard curves produced from TIP‐spiked faecal supernatants. The limit of quantification (LOQ) for TIP was 1 μg/mL (ppm). In a cattle study (n = 14), 3 of 28 faecal samples collected 24 and 48 h post‐treatment were found to contain TIP above the LOQ (concentrations of 1.3–1.8 ppm). In another cattle study (n = 12) with faecal samples collected at 8, 24 and 48 h post‐treatment, TIP concentrations were above the LOQ in 4 of the 8 h samples (1.2–2.6 ppm) and one of the 24‐h samples (1.3 ppm). In a pig study (n = 12) with faecal samples collected 24, 48 and 72 h post‐treatment, only one sample contained TIP above the LOQ (concentration 1.5 ppm). In another pig study (n = 12), with samples collected at 8, 24 48 and 96 h post‐treatment, TIP concentrations were above the LOQ in one 8‐h sample (1.1 ppm) and two 24‐h samples (2.3 and 2.5 ppm). None of the 48‐h and 96‐h samples from these 4 studies contained measurable TIP concentrations. Thus, in cattle and pigs, only a small fraction of faecal samples collected up to 24 h postdosing contained measurable microbiologically active TIP, with its maximum limited to 2.6 μg/mL. This is several log₂ dilution steps below the MICs of TIP against foodborne pathogens and commensals collected under acidic conditions comparable with those in the colonic contents and may explain a lack of intestinal dysbacteriosis with parenteral tildipirosin in livestock.