Lead factors for total mixed ration formulation

Lead factors are used in computerized ration formulation programs developed at Virginia Tech to increase milk production above a herd or group average for which total mixed rations are formulated for group feeding. These lead factors theoretically increase milk production for which the ration is formulated so 83% of the cows in a group will receive adequate or more than adequate nutrients from the ration. Two methods of calculating lead factors produced similar results. When test-day Dairy Herd Improvement records within a herd were not grouped by production, the average lead factor was 1.31 for one method [lead factor = (mean milk yield + one standard deviation)/mean milk yield] and 1.32 for the other [lead factor = (milk yield of 83rd percentile cow)/mean milk yield]. Grouping test-day records in each herd by milk production resulted in smaller lead factors for each group compared with lead factors for ungrouped herds. Changes of percentage of cows in each group in a herd resulted in different lead factors. Generally, groups with greater proportion of cows had larger lead factors. Season, herd size, and herd milk production slightly affected lead factors, but little improvement of estimation was gained by considering these variables.