Regional variation in milk production and lactation curves in response to heat stress in US dairy systems

The objective of this study was to quantify state-level differences in milk yield, milk component yields, and lactation curve characteristics across 8 major US dairy states, and to determine whether temperature-humidity index (THI) category contributed to differences in milk production. Milk production records from the Dairy Herd Information Association were obtained for California, Florida, Michigan, Minnesota, New York, Pennsylvania, Texas, and Wisconsin from January 1-December 31, 2021. Biologically reasonable records (4.54-90.72 kg/d milk yield, DIM <305 d, THI <101, fat 2-6.5%, protein 2.5-4.5%) were retained (n = 1,524,403). Daily ambient temperature and relative humidity were used to calculate THI, and test-day THI was categorized as either NO HS (THI <68), MOD HS (68 ≤ THI <80), or HIGH HS (THI ≥80). Data were summarized by state using PROC SUMMARY, and production variability was quantified using the interquartile range. State and THI category effects were evaluated using PROC GLM with significance declared at P ≤ 0.05. Lactation curves were modeled for each state × THI category using the linearized Wood's model. Parameters were estimated using PROC REG, back-transformed, and used to calculate peak milk, DIM peak, and persistency. Daily THI differed marginally among states, and the percentage of days spent in each THI category did not differ significantly by state or THI category. Milk yield differed among states; Michigan (44.1 ± 0.8 kg/d) and Wisconsin (43.0 ± 0.8 kg/d) produced the highest yields, whereas Minnesota had the lowest (34.7 ± 0.8 kg/d). Moreover, ECM followed the same pattern. Fat and protein yield also differed significantly among states, and fat yield tended to differ by THI category. Production variability differed by state, and THI category did not affect interquartile range. Lactation curve parameters differed among states for peak milk, rise rate, and decline rate. THI category showed tendencies for differences in curve rise rate and curve decline rate but did not affect predicted peak milk, DIM peak, or persistency. Regional differences in milk yield, components, variability, and lactation curve structure were observed. Overall, THI category did not alter peak milk production or persistency, highlighting the importance of regional context when evaluating milk production responses to heat load.