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We have directly observed the effects of roton-roton scattering by crossing two beams of ballistic rotons in liquid "^He at a temperature of -0.1 K. One beam is weak and is used as the probe. Its attenuation is measured as it passes through the stronger scattering beam which is predominantly perpendicular to the probe beam. The beams are pulsed and the dispersion of the roton velocities enables the momenta of the rotons to be resolved by time-of-flight measurements. It appears that the scattering is strongly wave-vector dependent.PACS numbers: 67.40.-w, 63.2aHpThe excitation model of liquid "^He envisages a ground state and excitations from this ground state. The excitations, phonons and rotons, are well defined with an inverse lifetime T<^(o, where (o is the energy of the excitation. The excitation picture is well established and the dispersion curve of the excitations has been measured by neutron scattering (see Fig. 1). The lifetime of the excitations is limited by scattering between the excitations and so the inverse lifetime is directly proportional to the prevailing density of excitations. Therefore T increases with temperature as the ambient excitation density increases. Various methods of measuring the lifetimes confirm this. ^"^ However, these measurements can only be made at high temperatures where the lifetime is short enough to be detectable. At low temperatures the lifetimes are orders of magnitude longer and this can only be seen by time-of-flight measurements.^The scattering measurements to date rely on a thermal distribution of excitations, so the excitation lifetimes which are obtained are necessarily averages over the energies of the excitations and the scattering angles. As it is likely that scattering cross sections are dependent on energy and angle, a more refined type of experiment is desirable. In principle one would like to have two well-15 10 v \--

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Faecal egg counts (FECs) are commonly promulgated as a means of determining the status and impact of parasitic gastroenteritis (PGE) in cattle, yet there is little scientific evidence for consistent, quantitative associations between FECs, worm burdens or the effects of PGE on the hosts. Examples are provided of applications of FEC in cattle that do have an evidence base and also of alternative aids to anthelmintic treatment decisions, such as growth rates in young cattle. The potential value of FECs in veterinary/farmer interactions is also addressed as is the need to justify diagnostics through demonstration of favourable cost–benefit ratios.

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Treatment options for clinical parasitic bronchitis (PB) are largely based on anthelmintics, with supportive therapy including non-steroidal anti-inflammatories (NSAIDs) and antibiotics, when indicated. Macrocyclic lactones (MLs), benzimidazoles and levamisole are all effective against adult and larval lungworms, so the choice of product depends on other criteria, including duration of action and ease of administration, which can be important in dyspnoeic animals. Control of PB in youngstock can be accomplished through the use of anthelmintics in early-season strategies designed to limit pasture D. viviparus larval populations, but are not easy to implement in herds other than on autumn-calving dairy farms. Likewise, though vaccination provides predictable control of PB in youngstock, it is best suited to calves from autumn calving herds, however, it can also be used pre-turnout as a 'booster' in previously exposed, older cattle. Grazing management has a limited role to play in the control of PB both because of unreliability and because some of the recommendations run counter to recommended grassland management practices. Veterinary clinicians need to familiarise themselves with all possible options and outcomes for the control of PB and tailor their recommendations to the individual farm and farmer.

Farmers manage their pastures and grazing animals primarily to ensure that swards provide adequate, quality nutrients to support animal performance, but it is also possible to provide useful levels of parasite control without compromising production. On cattle only farms the easiest options for the control of parasitic gastroenteritis in young cattle revolve around the avoidance of high risk pastures from July onwards. On many farms, silage/hay aftermaths that have not been grazed by cattle for 12 months or more can provide low risk fields on which cattle can thrive with minimal reliance on anthelmintics.

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