COMPARISON BETWEEN VISUAL ASSESSMENT AND MEASUREMENT OF HEART GIRTH OR HIP-WIDTH TO ESTIMATE LIVE WEIGHT IN CROSSBRED BEEF HEIFERS

Background. Estimation of animal body weight (BW) is a fundamental tool in herd management for the development of reproductive, nutritional and health programmes. Objective. To compare the actual BW and its estimation by visual method and measurement of the heart girth (HG) or hip width (HW) for estimating BW in crossbred beef heifers reared under humid tropical conditions in Mexico. Methodology. Data on GH, HW, BW estimated by the visual method (MV) and actual BW were recorded in 105 crossbred replacement heifers (Bos taurus × Bos indicus) Swiss American, Beef Master, Simmental, and Brahman with different degrees of crossbreeding. Heifers ranged in age from three to 20 months and were grazed on star grass (Cynodon nlemfuensis) and humidicola grass (Brachiaria humidicola) pastures without supplementation. BW was recorded using a digital scale, HG was measured using a flexible fibreglass tape and HW was measured using a 65 cm forceps. BW by visual estimation was considered as the average of three observations made by three observers, HG and HW methods using the formula 1) BW (kg): 202.68 - 4.39 × HG + 0.03 × HG²; 2) BW (kg): 0.26 × HW^1.90. The Pearson coefficient and the distribution (density) of each variable, were assessed using a correlation matrix graph. Also, a comparison the measures obtained by each of the different estimation methods with the observed weights, by inspecting the paired Bland-Altman plots, prior to logarithmic transformation. Results. Correlations between observed BW and predicted BW for the HG, HW and visual methods showed a positive and significant relationship (P<0.001), with r values of 0.95, 0.89 and 0.92, respectively. The HG method tended to overestimate the real BW, whereas, in the visual and HW methods, the values are evenly and randomly distributed around the line, indicating that these methods neither underestimated nor overestimated the BW. Implications. Although the HG method showed the highest correlation coefficient between observed and predicted BW, the result of the present study showed that visual assessment highlights the ability of observers to visually estimate the BW of growing heifers. Conclusion. Livestock handlers have a very accurate estimation of BW. The use of alternative methods to scales, such as the HG measuring method, can be useful and practical tools to improve the accuracy of the assessment.