Visual weight estimation and the risk of underdosing dairy cattle

DRUG resistance is a major risk to human health and food security (WHO 2014). Worldwide, approximately 50 per cent of antimicrobials are used in animals (Teuber 2001) and zoonotic infections with resistant bacteria are rarely far from the news. Meanwhile, multiple-resistant helminths pose a direct threat to ruminant farming globally (Sutherland and Leathwick 2011). As attempts to secure food for a growing human population focus on further intensification of farming, drug resistance problems are likely to be compounded in future (van Dijk 2014). Prescription protocols, developed to encourage responsible use of veterinary medicines, generally address drug choice and application frequency (Ungemach and others 2006). Different routes to resistance include the deliverance of below-optimal doses of drugs, for example, sick animals not taking up enough medicated food (Love and others 2010) or calculated dose rates administered being too low (Silvestre and others 2001). Administrator-introduced risk factors for underdosing, such as underestimation of the weight of animals, drug decay during storage and non-calibrated drug delivery systems, have received very little attention. Drug volumes administered to cattle are routinely based on visual weight estimation, but it is not known how closely such estimates resemble true weights. The aims of this study were therefore to quantify the accuracy of visual weight estimation by veterinary surgeons and dairy farmers, identify associated risk factors for underdosing and investigate the effect of a simple intervention, namely allowing participants to compare the weight of cows with a single, weighed, ‘baseline’, animal. If having a measured reference point within the herd significantly improves weight estimation accuracy, this could provide the easiest achievable solution. The study was carried out at Wood Park Farm, University of Liverpool. …