Residual Feed Intake of Dairy Cows Estimated on Pasture and Compared to Previous Rfi in the Growsafe™ System.

Residual feed intake (RFI) is often used to classify feed use and efficiency in cattle. RFI is the difference between actual and predicted feed intake. Lately, RFI is more easily determined with concentrate/mixed rations in the GrowSafe™ System. However, little is known about RFI in grazing animals and few have investigated RFI of animals over time and in different production stages. We determined RFI of yearling dairy heifers using a forage-based ration in the GrowSafe™ system and again as primiparous lactating heifers while grazing pasture. The grazing research involved titanium dioxide and n-alkanes as external and internal forage digestibility markers, respectively. The first objective was to determine if postweaning RFI classification of dairy heifers remained consistent in the first lactation as cows. A secondary objective was to determine if individual DMI had a relationship to pasture DM disappearance estimated with an ultrasonic sensor mounted to an ATV. Two groups of dairy heifers were evaluated in 2011 and 2012, using the GrowSafe™ feed intake system; and RFI determination on pasture for the first lactation (305 days in lactation) was conducted on the same groups in 2012 and 2013 if they were not culled from the milking herd. We regressed RFI as yearlings against RFI during lactation to examine linear relationships. The data is currently being analyzed, but we expect there will be no difference between RFI determined in GrowSafe™ and on pasture. Additionally, we expect a robust relationship between the sums of individual DMI of heifers and pasture DM disappearance. However, it is likely the ultrasonic sensor will overestimate DMI and the discrepancy might be related to grazing efficiency/trampling of forage.