Y i j = μ + β 1 X 1 i j + β 2 X 2 i j + U j + ∈ The aim of this study is to investigate the accuracy of DLWGs calculated from bodyweights estimated using HGT and weigh scales in pre‐weaned calves, as well as the level of inter‐observer agreement. The use of simulation models allows research questions to be answered without requiring large numbers of animals 15 and has been used in this study. None of these studies have investigated the reliability of estimating DLWG using HGT, which most likely reflects the large sample size required for ensuring the reliability of the results. 14 One of the limitations of these studies is that they have included heavier heifers which may have skewed the data. The relationship between bodyweights obtained via electronic scales and HGT has been investigated previously with some studies reporting a poor correlation between the HGT and weigh scales, 12, 13 and other studies reporting a good correlation. 11 Therefore, it is important to have a reliable and cheap method of estimating the bodyweight of pre‐weaned calves on‐farm. A recent study highlighted that many pre‐weaned dairy calves had very low growth rates, despite having a high feed conversion efficiency at this age and that monitoring heifer growth during the rearing period would help improve the efficiency of heifer rearing. This has led to the development of other more practical methods being used on farms, including HGT. The gold standard method of weighing calves involves the use of a calibrated weight scale or weighbridge, both of which involve considerable financial investment from the farmer and will often result in a non‐portable system. 8, 9 Other studies have reported that there is a tendency to overestimate the bodyweight of animals which are <150 kg when visual estimates were compared to heart girth tape (HGT) estimates, 10 which could result in inappropriate management, in terms of groupings and nutrition. Multiple studies have reported variation in the abilities of farmers and veterinary surgeons to accurately estimate the bodyweight of cattle visually, with the majority of people underestimating the bodyweight of the animals, 6, 7 which could lead to an increased risk of antibiotic and anthelmintic resistance developing. Estimates are commonly used on farms for management purposes, as well as for determining the dosage of medications, including antibiotics and anthelmintics. The accurate weighing of pre‐weaned calves is important for measuring the growth rate of calves. 2 Studies have also highlighted there are additional health and production benefits associated with growing at the target DLWG of 0.8 kg/day in the pre‐weaning period, for example, the average DLWG in the first 2 months of life has been linked to survival to the end of the second lactation and increased milk production. Factors affecting the DLWG of heifers and the use of different management decisions to maintain target DLWG while reducing the cost per kg of gain have been the subject of a growing amount of research, especially the pre‐weaning period where the cost is on average the highest at £3.14 per day on UK farms. 1 Feed costs represent a significant financial input, which may be affected by the desired target daily liveweight gain (DLWG) for the heifer. Heifer rearing represents a significant financial investment to dairy farmers, costing on average £1,391 on farms in the United Kingdom.
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