The objectives of this study were to quantify the variation in trace mineral (TM) concentrations in forages, evaluate the contribution of US geographical location and harvest season to the TM variation, and identify areas where high variation in TM concentrations would preclude using standard reference concentrations. Trace minerals of interest were Cu, Mn, Zn, and Fe. Data from Cumberland Valley Analytical Services covering the period from 2009 to 2014 with concentrations of major nutrients as well as mineral concentrations were used. Data were statistically filtered to remove outliers based on macronutrient concentrations and misidentified feeds before analyses of TM variation. As expected, TM concentrations for corn silage, legume hay, mixed mostly legume (MML) silage, and mixed mostly grass (MMG) silage displayed skewed distributions. TM values were log normalized before ANOVA with location, season, and their interaction as independent effects and total ash concentration as a covariate. Variation in TM concentrations due to location and total ash were significant (P < 0.0001) and the largest sources. Season was often non-significant (P > 0.20), while the interaction between location and season was significant (P < 0.05). Whereas forages in some US locations have consistently low TM concentrations, forages in other areas have both high concentrations and high variation. These results support sampling and analysis of forages and formulation of dairy rations for TM based on analytical results rather than reference values. Recommendations for sampling frequency will vary by location as a function of the observed variation in TM concentrations. Table 1. Lowest and highest variation in observed Cu and Zn concentrations among U.S. geographical locations, given as median and 5th (p5) to 95th (p95) percentiles in mg/kg

Item	Lowest variation				Highest variation
Median	p5	p95	Median		p5		p95
Cu
Corn silage	6.0	4.9	7.3		7.4		3.2		16.4
Legume hay	9.6	7.2	11.4		8.2		6.0		34.0
MML silage	7.8	5.7	9.6		10.9		5.7		30.1
MMG silage	8.8	6.1	9.4		7.8		4.7		37.9
Zn
Corn silage	23.2	21.4	28.5		26.6		15.3		60.7
Legume hay	24.0	21.1	29.5		27.8	20.9		49.2
MML silage	28.3	25.8	35.0		35.7	21.8		69.7
MMG silage	23.8	20.0	27.6		29.1	18.4		97.5