Effects of herd management factors on annual rhythms of milk synthesis, feed intake, and feed efficiency in the western United States

Milk yield and milk components follow repeatable yearly patterns that fit a cosine function with a period of 12 mo. Generally, milk yield peaks in mid-April and reaches a nadir in mid-October, whereas milk fat and protein concentration peak near January 1 and reach a nadir near July 1. We have previously demonstrated that these rhythms differ across regions of the US, with milk yield having greater amplitude in the Southeastern and Southwestern United States, and milk fat and protein concentration having greater amplitude in the Upper Midwest and Northeast. However, we have not characterized these rhythms in the Western US, and little is known about how annual rhythms vary among different management systems. This experiment used 129,530 monthly milk production and feed intake records collected from 2006 to 2019 from 332 herds throughout California, the Pacific Northwest, the Southwest and the Upper Midwest. We examined the effects of region, breed, housing system (freestall vs. loose housing), management system (conventional vs. organic) and bST use on annual rhythms of milk, milk fat and milk protein yield, milk fat, milk protein, and MUN concentration, and DMI and feed conversion efficiency (DMI/ECM). Using cosinor rhythmometry with linear mixed linear mixed effects models (R ver. 4.4.1), each explanatory variable was tested in its own independent model, which included the variable of interest, the linearized sine and cosine functions, and their interactions, with the random effects of herd and year and the covariate of average days in milk. The significance of the 12-mo rhythms was assessed via a zero-amplitude test, comparing the linearized cosine model against a reduced linear model excluding rhythmic effects. Dry matter intake fit a 12-mo rhythm (P < 0.0001) in all regions and peaked between Feb 1 and Feb 20. The greatest amplitude of the DMI rhythm occurred in the Southwest (1.07 kg). Milk yield fit an annual rhythm in all regions (P < 0.0001) and had a greater amplitude in the Southwest and California compared with the Pacific Northwest and Upper Midwest (P < 0.05). Milk yield fit a 12-mo rhythm regardless of breed (P < 0.0001), but the amplitude of the rhythm was greater for Holsteins than Jerseys or Crossbreds (P < 0.05). Neither housing system nor management system altered the annual of milk, milk fat, or milk protein yield, but organic herds had greater amplitudes of milk fat and protein concentration (P < 0.05). Use of rBST did not impact annual rhythms of milk, milk fat, or milk protein yield, but did increase the amplitude of feed conversion efficiency by 100% (P < 0.05). Results further confirm that geographic region and breed impact annual rhythms of milk synthesis, but suggest that the impacts of housing system, management system, and bST on these rhythms are minimal. Furthermore, this study establishes that both dry matter and feed conversion efficiency follow a seasonal pattern characteristic of an annual rhythm.