Studying the distribution characteristics of bioaerosols and their interaction with the environment is crucial for dairy farms. The distribution of aerosols differs in dairy farming from farming of other livestock, and their sensitivity to environmental factors varies across sites. Field experiments were conducted in an intensive commercial dairy farm in Northern China to investigate the horizontal and vertical distribution of culturable bacterial bioaerosols. Concentration levels and particle size ranges were analyzed, and the impact of multiple environmental factors on culturable bacterial bioaerosols was assessed. Significant variations in culturable bacterial bioaerosol concentrations were observed across eight functional zones, ranging from 1.14 × 10³ to 7.35 × 10³ CFU/m³. Culturable bacterial bioaerosols exhibited consistent carrier distribution patterns across six different size ranges. Vertical analysis revealed significantly higher culturable bacterial bioaerosol concentrations at a 1 m height compared to 4 m (p < 0.05), while similar size distributions were observed at different heights of the same sampling location. The top three environmental factors influencing culturable bacterial bioaerosol concentrations were PM100 concentration, wind direction, and air temperature. This study provides insights into the distribution characteristics of culturable bacterial bioaerosols on dairy farms and their response to environmental factors. The findings serve as a reference for evaluating bioaerosol emissions and establishing daily disinfection management measures on dairy farms.

At many modern dairy farms, calves raised in barns are kept in individual stalls separated by solid partitions, which act as barriers. Ventilation fans blowing air perpendicular to these stalls only provide the optimal airflow to the first few calves, while those further away receive a slower airflow. To ascertain whatever effects different airflow speeds may have on the health of animals kept in stalls located at increasing distances from ventilation fans, we divided a select group of 43 Holstein dairy calves into six subgroups based on age, and each subgroup was subjected to either a specified high-speed or low-speed airflow as follows: (1) Six 3-day-olds received high-speed airflow (D3-HA); (2) Six 3-day-olds received low-speed airflow (D3-LA); (3) Eight 19 (±3)-day-olds received high-speed airflow (D19-HA); (4) Eight 19 (± 3)-day-olds received low-speed airflow (D19-LA); (5) Eight 29 (±3)-day-olds received high-speed airflow (D29-HA); and (6) Seven 29 (±3)-day-olds received medium-speed airflow (D29-MA). These trials show that the rectal temperatures and respiratory rates of D19-LA (39.37 °C; 72.90 breaths/min) were significantly higher than those of D19-HA (39.14 °C; 61.57 breaths/min) (p ≤ 0.05), and those of D29-MA (39.40 °C; 75.52 breaths/min) were significantly higher than those of D29-HA (39.20 °C; 68.41 breaths/min) (p ≤ 0.05). At 33 (±3) days of age, those calves receiving high-speed airflow (p ≤ 0.05) registered significantly higher immunoglobulins A and M than calves receiving low-speed flow. Those calves subjected to a high-speed airflow also registered significantly lower tumor necrosis factor levels than those receiving low-speed flow (p ≤ 0.05). Among the 29 to 43-day-old calves, no significant differences in immunity parameters were found to exist between groups D29-HA and D29-MA. On the basis of these findings, we were able to conclude that in the warm season, when the calves were less than 0.5 months old, low-speed (0.17–0.18 m/s) airflows had no significant effect on calves; when the calves were 1 month old, low-speed airflow (0.20–0.21 m/s) may impair the immune functions; when the calves were 1 to 1.5 months old, the airflow velocity higher than 0.9 m/s can meet the needs of the calf without a negative impact on the calf.

On dairy farms, calves are typically raised inside barns (either in individual or group pens), or they are raised in outdoor hutches. To evaluate the effect of all three of these commonly used rearing practices on calves, an experiment was conducted. A group of 58 Holstein dairy female healthy calves (3 days of age) was randomly divided into three subgroups (outdoor hutches, individual-housed, and group-housed in a barn). The body weight, lying bouts, lying time, and immunity parameters of each calf were monitored, and the ambient temperature and relative humidity were measured. The average temperatures outside and in the barn and hutches were −16.67 °C, −15.26 °C, and −15.59 °C, respectively, from 22 November 2020 to 27 January 2021. All calves suffered from cold stress. Group-housed calves weighed significantly less than the other calves at the ages of 1 month and 2 month (p < 0.05). The lying time of the calves housed in individual pens and group pens was longer (p < 0.05) than that of the calves housed in hutches. The morbidity attributable to bovine respiratory disease was significantly lower among the calves housed in hutches than it was among the calves housed either individually or in group pens inside the barn (p < 0.05). No significant differences in the concentrations of TNF-α, IL-1β, and IL-6 were found between the three groups (p > 0.05). On the basis of these findings, we were able to conclude that calves housed in outdoor hutches were at a lower risk of developing a disease than were calves housed in barns without heating in winter. To optimize the management process, heating should be added to hutch systems. Moreover, more rigorous disease and environmental control management strategies should be applied when raising calves inside barns.