2020-07-29T14:42:45Z (GMT) by Tabitha S Steckler

The objective of this study was to evaluate the long-term effects that early life events have on heifer growth up to 400 d of age, heifer conception rate, survivability through first lactation, and first lactation milk production of calves raised in automatic calf feeders. Chapter one evaluates possible early life variables that would affect heifer growth and lifetime production as well as research that has been done to predict future growth. The major points discussed include pre-weaning feeding strategies, automatic calf feeding systems, respiratory disease and ways to diagnose cattle with this disease, and the impact of early life growth on the future productivity of the dairy cow.

The second chapter discusses in detail the process of creating a predictive equation using significant early life variables that affect Holstein heifer growth up to 400 d of age. Variables collected for the growth analysis included sixty d cumulative milk consumption (MC), serum total protein values, respiratory disease and scours incidences, genetic body size, birthweights, and incremental body weight variables on a commercial dairy farm from October 1, 2015 to January 1, 2019. Calves were fed pasteurized whole milk through an automated calf feeding system (feeders = 8) for 60 d (range: 48 – 126d), with a 30% Crude Protein (CP) and 5% Crude Fat enhancer added at 20 g/L of milk. Calves were weighed at birth and several other times prior to calving. Average birth weight of calves was 40.6 ± 4.9 kg (mean ± SD), serum total protein was 6.7 ± 0.63 g/dL, and cumulative 60 d MC was 508.1 ± 67.3 L with a range of 179.9 to 785.1 L. Daily body weights were predicted for individual animals using a third order orthogonal polynomial to model growth curves. The linear and quadratic effects of cumulative 60 d milk consumption, birthweight, feeder, yr born, season born, respiratory incidence, and genetic body size score were significant (P<0.0001) when predicting heifer body weight at 400 d (pBW400) of age (R2=0.31). There was up to a 263 kg difference in pBW400 between the heaviest and lightest animal. Birthweight had a significant effect on predicted weights up to 400 d (P<0.0001), and for every 1 kg increase in birthweight, there was a 2.5 kg increase in pBW400. The quadratic effect of cumulative 60 d MC was significant for pBW400 (P<0.0001). When 60 d MC was divided into quartiles, heifers had the highest pBW400 in the third quartile, when 60 d MC was between 507.8 and 552.5 L. Body size composite (genomic index) showed a 21.5 kg difference in pBW400 between the top and bottom 25th percentile of heifers. Heifers were 4.2 kg lighter at 400 d if treated for respiratory disease 3+ times during the first 60 d of life, compared to heifers not treated for respiratory disease.

The third chapter utilizes the data described in chapter two and followed those heifers through breeding and first lactation. Heifer conception age and 280 d first lactation milk production (280M) were collected. Average age at conception was 437.5 ± 45.0 d; range of 308 to 631 d (n=5,193), and average 280M was 9,305 ± 1,371.8 kg; range of 712-13,358 kg (n=1,324). Heifer conception age was impacted by season, yr, and the quadratic effects of predicted bodyweight at 300 d of age (pBW300) and ADG (0-400; all P < 0.05; total model R2 = 0.08). Season born, ADG (0 - 400 d), genomic milk, and the linear effect of heifer conception age had a significant impact on 280M (all P < 0.05; R2 = 0.28). For every 1 kg increase in genomic milk value there is 1.42 kg increase in first lactation 280M. Calves not diagnosed with bovine respiratory disease (BRD) from 60-120 d old had a significantly higher chance for survival to first lactation than animals treated three or more times for BRD (hazard ratio = 0.71, 95% CI = 0.574 to 0.886, P = 0.0023, Table 3.3). Heifers treated twice or more for BRD had reduced likelihood to become pregnant than heifers not treated for BRD from 60-120 d (twice P = 0.02; three or more P = 0.05).

In conclusion, the results from this thesis support that early life events in Holstein heifers continue to influence future growth and productivity. Future research aims to validate the predictive equation generated in chapter two on farm as well as adapt the equation to other farms allowing them to utilize it as well. The goal is to have farms utilize this tool to aid in their replacement heifer management decisions and to select the most productive heifers for the future of their herds.