Evaluation of Slow-Release Urea for Winter Supplementation of Lactating Range Cows2

Abstract

A new slow-release urea compound (SRU), previously shown to release ammonia slowly in the rumen was evaluated under range conditions. Eighty-five lactating Hereford cows, grazing dormant native range forage, were individually fed five different supplements in a 92-day trial. Supplements contained: 15 or 40% natural protein (negative and positive control, respectively), 1.22 kg/head/day; 40% protein (62.5% of crude protein equivalent from SRU), 1.22 kg/head/day; 40% protein (62.5% of crude protein from urea), 1.22 kg/head/day; and 20% protein (62.5% of crude protein equivalent from urea), 2.44 kg/head/day. Cow daily weight changes were −.98, −.39, −.74, −.85 and −.85 kg, respectively. The positive control was different (P<.05) from all other treatments. Cows fed SRU tended to lose less weight than urea-fed cows, while both SRU-and urea-fed cows lost less (P<.05) weight than negative control cows. Calf daily gains were: .39, .50, .44, .37 and .36 kg, respectively. Calves of positive control cows gained faster (P<.05) than from all other treatments. Calves of SRU cows gained faster (P<.05) than calves of urea or negative control cows. Cow rebreeding performance was poorest for the negative control and highest for the positive control, with SRU and urea treatment being intermediate. No supplement refusals were observed in cows fed natural protein or SRU supplements, whereas cows fed urea supplements consumed only about 60% of the amount offered. Rumen fluid samples taken by stomach tube at 1 and 4 hr after supplement feeding showed that SRU produced rumen ammonia patterns similar to those seen with soybean meal. Forage intake, estimated by use of markers, was 7.0, 9.3, 8.4, 8.5 and 8.0 kg/day for the negative and positive controls, 40% SRU, 40% urea and 20% urea supplements, respectively. Although SRU improved cow and calf performance over that seen with urea, performance of SRU-fed cows was still unacceptable. It is probable that some of the improved performance with SRU resulted from improved supplement (and energy) intake.