The growth of replacement heifers in dairy farms in Costa Rica. 2. The effect of growth rate and age at first calving on subsequent milk production

Abstract

A study was carried out with 727 replacement heifers (434 Holstein and 293 Jersey) from 24 dairy farms in two areas of Costa Rica to quantify the effect of prepubertal (GDPPR, between 3 and 10 months) and postpubertal (GDPPO, between 11 and 18 months) and the age at first parturition (EDPP) on milk production during the first lactation (P305). The parameters of the individual heifer growth curves were generated by fitting them to Brody's mathematical model. From the equation, the instantaneous daily weight gains were derived for each month of age from 1 to 18 and the average daily gains were calculated for each stage. In order to determine the significant effects and the least squares means, an analysis of variance of a linear model with fixed effects, a covariate (EDPP) and relevant interactions was performed. The effects of the GDPPR and GDPPO variables were analyzed, both as fixed effects in categories of daily weight gain (cGDPPR and cGDPPO) and for their regression effect (covGDPPR and covGDPPO), on P305. The analyzes were carried out independently for each breed. The results showed that the global fit of the individual curves to the mathematical model was efficient. The analysis of covariance showed a positive relationship (P <0.01) between EDPP and P305 in both the Jersey breed (b = 50.61) and the Holstein breed (b = 75.03). The effects of cGDPPR and cGDPPO on P305 were significant (P <0.009 and P <0.003) only in the Holstein breed, observing a positive relationship between both. The regression coefficients covGDPPR and covGDPPO were b = 2.06 and b = 1.76, respectively. Effects (P <0.01) of life zone, farm nested in zone and year of calving were detected in both Holstein and Jersey, showing an increase in production during the first lactation through the years studied.