359 Late-Breaking: Individual variation in free-choice mineral intake among beef cows.

Abstract Free-choice vitamin and mineral supplementation is a common practice in beef production, yet intake variation within a herd remains largely undocumented. Under typical production settings, producers can only evaluate mean herd intake, without knowing the extent of variation among individual animals. Understanding this variability will help optimize supplementation strategies to ensure that the majority of the herd meets nutritional requirements. Therefore, the objective of this study was to quantify the free choice mineral intake variation among cows during two distinct grazing periods to assess intake distribution and uniformity. For 105 days, a total of 174 five-year-old multiparous cows at the USDA Meat Animal Research Center were allowed ad libitum access to a vitamin and mineral mix (VMM; Table 1) using SmartFeed Pro trailers (C-Lock Inc., Rapid City, SD) to track free-choice intake. Period 1 (49 days) consisted of grazing corn residue, while Period 2 (56 days) involved dormant warm- and cool-season pasture supplemented with hay.. Mean daily VMM intake differed significantly (P < 0.01) between periods, averaging 219 ± 98 g/day (Period 1) and 47 ± 28 g/day (Period 2). Intake during Period 1 (corn residue) followed a normal distribution, whereas in Period 2 (hay/dormant pasture), intake was right-skewed due to a subset of high-intake cows consuming 160 ± 10 g/day. Despite this difference, overall intake distributions across both periods did not differ (Cramer-von Mises test; CM = 0.065), indicating cows consumed VMM in a similar pattern across forage systems. However, intake was more uniform in Period 2 (Levene’s test; P < 0.01) than Period 1, as evidenced by a narrower distribution. When the target intake range was defined as ±10% of the mean, only 20% and 17% (Chi-square; P = 0.49) of cows met this intake goal in Periods 1 and 2, respectively. When the target was set at the mean intake, 51% and 47% (Chi-square; P = 0.39) of cows consumed at or above this level in Periods 1 and 2. To ensure that at least 75% of the herd met vitamin and mineral requirements, a scaling factor of 0.65 was needed, meaning a supplement formulated for a 113 g (4 oz) target herd intake would need to meet requirements at 73 g (2.6 oz) to account for intake variation among individuals. Therefore, the percentage of cows meeting their requirements at 73 g/day in Periods 1 & 2 would be 92.5% and 83.9% respectively. These findings demonstrate that substantial individual variation in free choice vitamin and mineral mix intake exists within a herd, regardless of grazing system. Understanding this variability is critical for designing supplementation programs that better align with the nutritional needs of the majority of the herd. USDA is an equal opportunity provider and employer.