# Long-Term Production and Reproductive Outcomes in Dairy Calves Following Early-Life Ultrasonographic Lung Consolidation: A Longitudinal Follow-Up Study

## TL;DR

This study examines how early-life lung issues in dairy calves affect their long-term milk production and reproductive performance.

## Contribution

The study provides new insights into the long-term effects of subclinical lung consolidation in calves using longitudinal data from an Iranian dairy farm.

## Key findings

- Early-life lung consolidation did not significantly affect culling risk or mature milk yield.
- Numerical trends were observed in corrected milk yield and reproductive parameters like conception rate.
- Calves with consolidation tended to calve earlier, suggesting potential subtle long-term effects.

## Abstract

Bovine respiratory disease is known to reduce short-term growth in dairy calves, but its longer-term effects on adult milk production and reproductive performance are less clear, particularly for subclinical pneumonia detected by thoracic ultrasonography. This longitudinal follow-up study investigated 221 female Holstein-Friesian calves from an Iranian dairy farm, whose early-life lung consolidation was identified weekly via TUS until weaning, and subsequently tracked their adult production and reproductive data for two years. Our findings revealed no statistically significant associations between early-life lung consolidation and later culling risk or mature equivalent milk yield. However, some numerical trends appeared for corrected milk yield and reproductive parameters like services per conception and conception rate. An unexpected numerical finding was that calves with consolidation tended to calve earlier. This study highlights the complexity of quantifying the precise long-term economic and biological consequences of subclinical lung consolidation, suggesting that larger, multi-farm studies are crucial to more definitively evaluate long term effects of TUS.

Bovine respiratory disease is a significant health concern in dairy calves, impacting short-term growth and potentially long-term productivity. While previous studies have linked early-life lung consolidation, often subclinical and diagnosed by lung ultrasonography, to reduced preweaning average daily gain, its extended effects on production and reproductive parameters remain less studied, particularly in specific geographical contexts. This study presents a follow-up analysis on a cohort of dairy calves originally monitored weekly from birth to weaning for lung consolidation via TUS in an Iranian dairy herd. Two years post-weaning, comprehensive production and reproduction data were collected and analyzed for these same animals. Our objective was to investigate the long-term associations between early-life lung consolidation and subsequent outcomes, including mature equivalent milk yield, corrected milk yield, culling risk before and during lactation, age at first breeding, age at first calving, and reproduction parameters such as services per conception and conception rate (the percentage of cows or heifers that become pregnant after a single insemination or breeding attempt). Data analysis, employing descriptive statistics, survival analysis, and non-parametric tests, revealed that while early-life lung consolidation (defined by ≥1 cm or ≥3 cm depth, and number of episodes) did not show significant associations with culling probability or mature equivalent milk yield, interesting numerical trends were observed. The study highlights the complexities of establishing clear long-term links, suggesting that while subclinical BRD can have immediate growth impacts, its chronic effects on later production and reproduction may be subtle and require larger cohorts or more targeted analysis to achieve statistical significance. The findings reinforce the need for cautious interpretation of p-values in the context of multiple comparisons and underscore the challenges in quantifying long-term economic consequences of early-life respiratory health.