Relationship between average daily gain of heifers at various stages of rearing and milk production in the first and second lactation

The study aimed to determine whether ADG of Holstein heifers at various stages of rearing affects 1st and 2nd lactation performance. Data on BW of heifers were obtained from 3 independent dairy operators (operator A, B and C) and used to calculate ADG from birth until weaning (all operators), from weaning until 180 d of age (all operators), from 180 until 360 d of age (operator A and C), and from 360 until 410 d of age (operator A). Data sets included: 1583 records for the 1st and 1290 records for the 2nd lactation for operator A; 886 records for the 1st and 664 records the 2nd lactation for operator B; and 1190 records for the 1st and 675 records for the 2nd lactation for operator C. Subsequently, BW records were used to calculate ADG for each phase of rearing and used in a 2-step data analysis. In the first step, ADG was regressed against the 1st and 2nd 305-d lactation milk yield, 1st and 2nd 305-d lactation protein yield and 1st and 2nd 305-d lactation fat yield to test for a linear and quadratic relationship between those variables. In the second step, a statistical model was built which evaluated linear and quadratic impact of ADG, year and season of birth, year and season of calving, calving age, and dam parity (primi- or multiparous) on 1st and 2nd 305-d lactation milk, protein and fat yield. In both steps of analysis, pedigree data were used to correct phenotypic values for genetic effect. With increasing ADG preweaning (step 1), the 305-d milk yield in 1st lactation increased linearly for operator A and C but not for operator B, whereas 305-d milk yield in the 2nd lactation increased quadratically for operator A, linearly for operator C but no association was found for operator B. With increasing ADG from weaning until 180 d of age, the 305-d milk yield in 1st lactation increased linearly for all 3 operators, but the 305-d milk yield in 2nd lactation increased linearly for operator B and C, but not for operator A. With increasing ADG from 180 until 360 d of age, the 305-d milk yield in the 1st lactation decreased linearly for operator A but no association was found for operator C. With increasing ADG from 360 until 410 d of age (operator A), the 305-d milk yield in the 1st lactation decreased linearly. When other effects were included in the statistical model (step 2), the association of ADG with 305-d milk yield in the 1st and 2nd lactation found in step 1 of analysis oftentimes changed. This include direction or shape of the relationship, or its significance. Although some associations remained statistically significant, large variability around estimates suggests a substantial influence of unaccounted effects. Moreover, while with increasing ADG the 305-d milk yield increased, milk component yield in some instances decreased, or no association could be found. In summary, the relationship between ADG of heifers at different stages of growth and future milk and milk component yield differed substantially between operators, being either linear (both positive and negative), curvilinear or not significant, which makes clear conclusions difficult.