Development and evaluation of equations to predict growth of Holstein dairy heifers in a tropical climate

The objective of this study was to develop and validate equations for estimating growth of dairy heifers using measures of withers height, body weight (BW), and age. Measures of BW and withers height of 207 Holstein heifers raised in a tropical climate were taken from birth to calving, totaling to 2,047 observations. To be included in the database, the heifer had to have at least 4 measures recorded. After that, 4 models were built and evaluated as follows: (1) a linear model of BW as function of age (BW similar to A), (2) a linear model of the BW-to-height ratio as function of age (BW:H similar to A), (3) a quadratic model, adjusted for a defined plateau, to describe height as function of age (H similar to A), and (4) an exponential growth model of BW as function of height (BW similar to H). A cross-validation procedure was performed to evaluate accuracy and precision of the models. The linear relationship of BW similar to A and BW:H similar to A were estimated, respectively, by the equations: BW = 42.65 + 0.62 x A and BW:H = 0.70 + 0.0041 x A, where BW is in kilograms, BW:H = BW-to-height ratio (kg/cm), and A = age (d). Using the quadratic plateau for the model H similar to A, a critical "x" value of similar to 806 d and a height plateau of 138.6 cm were identified. Therefore, the following equations for estimating the height of animals younger and older than 806 d, respectively, were developed: H = 78.15 A - 0.150 x A - 0.00009 x A(2) and H = 78.15 + 0.150 x cvx - 0.00009 x cvx(2), where H = height (cm) and cvx = 806 (critical "x" value; given in days). Additionally, the exponential model of W similar to H was estimated by the following equation: BW = 4.25 x exp ((0)(.034 x H)), where BW is in kilograms and H = height (cm). A cross validation demonstrated that all equations had very high accuracy and precision. Overall, these models demonstrated that BW and BW-to-height ratio increase linearly as a function of age, while BW follows an exponential growth pattern as a function of height. Additionally, the H similar to A model predicted that heifers achieve a maximum height of 138.6 cm at 806 d of age.