A comprehensive study of respiration rates in dairy cattle in a Mediterranean climate

Respiration rate (RR) is often used to assess health and heat stress in cattle, influencing decisions that affect their welfare (e.g., medical treatment and cooling). Despite its importance, systematic information on how RR responds to various intrinsic and extrinsic factors is limited. This study recorded RR and behavior in 406 female Holstein and Jersey cattle (newborn to sixth-lactation cows) in California over a year to (1) describe RR across different life stages (calf, heifer, lactating, and dry), (2) evaluate how different weather or thermal load indexes predict RR using linear regression and mixed models, and (3) assess the effects of individual characteristics (life stage, breed, milk production, and lactation state) and behavior (posture and location) on RR in a Mediterranean climate. A total of 11,210 RR and behavior records were obtained over 39 d between October 2016 and August 2017. Each record was paired with individual characteristics and 25 different weather parameters within 5 min of collection. Data analysis involved descriptive statistics, mixed models, and linear and multinomial logistic regressions. Across all life stages, RR ranged from 16 to 185 breaths/min, with first and third quartiles at 37 and 59 breaths/min, respectively. The likelihood of upper normal RR values (30-50 breaths/min, according to some textbooks) was highest when air temperatures (AT) were below 20 to 25°C, depending on the life stage. During observations, AT ranged from 1.8 to 43.9°C and was the sole most reliable RR predictor, accounting for 35% of the observed variation. Although most individual characteristics and behaviors influenced RR, the biological significance was sometimes unclear. Calves showed the highest RR increase (+17 breaths/min per 10°C increase in AT), whereas dry cows showed the lowest (+11 breaths/min). Increases in RR for heifers and lactating cows were 13 and 14 breaths/min per 10°C increase in AT, respectively. We suspect differences in cooling provisions and metabolic rates across categories drove these results. In general, Jersey cattle had higher RR than Holsteins (∼3 breaths/min every 10°C), except in calves. Respiration rates were the lowest when heifers, lactating, and dry animals were near the feed bunk, likely due to soakers' provisions for mature cattle. Respiration rates averaged 52 and 49 ± 3 breaths/min for lying and standing postures, respectively, with <1 breath/min change per 10°C increase in AT. For lactating cows, RR increased by ∼2 breaths/min for every 10 kg of milk produced but decreased by ∼1 breath/min for every 50 d of pregnancy. No relationship of RR with DIM or lactation number was found. Our study systematically collected RR data from a relatively large number of dairy cattle across different life stages and weather conditions, suggesting that current textbook RR thresholds may underestimate upper values for cattle and demonstrating that AT was the most reliable predictor of RR changes in a Mediterranean climate. Respiration rates varied with life stage, breed, posture, location, milk yield, and gestation length, with more pronounced changes in calves, suggesting a need for further research to understand these variations and their welfare implications.