The annual or seasonal rhythm of milk yield and composition is important for dairy producers and it may represent an underlying adaptation of the cow to yearly changes. It is well appreciated that milk fat and protein concentration peak during the winter and reach a nadir in the summer. Summarized monthly production data from individual Federal Milk Marketing orders has suggested that the region of the US may impact the difference between mean and peak (amplitude) fat and protein concentration and the timing of peak production (acrophase). Less data is available on yields of milk, fat and protein. Our objective was to determine the seasonal rhythm of milk production and the effect of US region at the herd level. Monthly DHIA records of all herds in Pennsylvania, Minnesota, Texas and Florida from the years 2003 to 2016 were obtained from Dairy Records Managements Systems. Milk yield, fat and protein yield, and fat and protein concentration were fit to the linear form of the cosine function with a 12-mo period using a linear mixed effects model in ASreml. Model parameters included the fixed effects of state, cosine parameters, the interaction of state and cosine parameters, and breed and the random effects of herd and year. A zero-amplitude test was performed to determine the fit of the linear form of the cosine function. Milk yield and fat and protein yield and concentration fit a cosine function in all 4 states, indicating an annual rhythm (P < 0.001). The amplitude of the rhythm of milk yield varied by state, and was lower in PA (2.8 kg) and MN (2.4 kg) compared with TX (6.9 kg) and FL (8.1 kg; P < 0.05). Fat and protein yield similarly showed a greater amplitude in the southern versus northern states (P < 0.05). The concentrations of fat and protein was opposite, with greater amplitudes occurring in MN and PA than in TX and FL (P < 0.05). The acrophase of milk yield, fat and protein yield, and concentration also varied by state, but all peaked between October and March (P < 0.05). Results suggest that region of the US impacts annual production rhythms, with a greater yearly variation in milk, fat and protein yield occurring in the south.