title: Effects of Combining Feed Grade Urea and a Slow-release Urea Product on Performance, Dietary Energetics and Carcass Characteristics of Feedlot Lambs Fed Finishing Diets with Different Starch to Acid Detergent Fiber Ratios.
abstract: Recent findings have shown that microbial nitrogen flow and digestible energy of diets are increased when urea is combined with a slow-release urea (SRU) in diets with a starch to acid detergent fibre ratio (S:F) 4:1. This affect is attributable to enhanced synchrony between ruminal N availability for microbial growth and carbohydrate degradation. To verify the magnitude of this effects on lamb performance, an experiment was conducted to evaluate the effects of combining urea and a SRU in diets containing S:F ratios of 3:1, 4:1, or 5:1 on performance, dietary energetics and carcass characteristics of finishing lambs. For that, 40 Pelibuey×Katahdin lambs (36.65±3 kg) were assigned to one of five weight groupings in 20 pens (5 repetition/treatments). The S:F ratio in the diet was manipulated by partially replacing the corn grain and dried distiller's grain with solubles by forage (wheat straw) and soybean meal to reach S:F ratios of 3:1, 4:1 or 5:1. An additional treatment of 4:1 S:F ratio with 0.8% urea as the sole source of non-protein nitrogen was used as a reference for comparing the effect of urea combinationvs.conventional urea at the same S:F ratio. There were no treatment effects on dry matter intake (DMI). Compared the urea combination vs urea at the same S:F ratio, urea combination increased (p<0.01) average daily gain (ADG, 18.3%), gain for feed (G:F, 9.5%), and apparent energy retention per unit DMI (8.2%). Irrespective of the S:F ratio, the urea combination improved the observed-to-expected dietary ratio and apparent retention per unit DMI was maximal (quadratic effect, p≤0.03) at an S:F ratio of 4:1, while the conventional urea treatment did not modify the observed-to-expected net energy ratio nor the apparent retention per unit DMI at 4:1 S:F ratio. Urea combination group tended (3.8%, p = 0.08) to have heavier carcasses with no effects on the rest of carcass characteristics. As S:F ratio increased, ADG, G:F, dietary net energy, carcass weight, dressing percentage andlongissimus thoracis(LM) area increased linearly (p≤0.02). Combining urea and a slow-release urea product results in positive effects on growth performance and dietary energetics, but the best responses are apparently observed when there is a certain proportion (S:F ratio = 4:1) of starch to acid detergent fibre in the diet.
authors.affiliation: Veterinary and Animal Science School, University Autonomous of Sinaloa, Culiacán 1084, Sinaloa, México.
authors.forename: A
authors.initials: A
authors.lastname: Estrada-Angulo
authors.affiliation: Research Institute of Veterinary Sciences, University Autonomous of Baja California, Tijuana 21100, México.
authors.forename: M A
authors.initials: MA
authors.lastname: López-Soto
authors.affiliation: Research Institute of Veterinary Sciences, University Autonomous of Baja California, Tijuana 21100, México.
authors.forename: C R
authors.initials: CR
authors.lastname: Rivera-Méndez
authors.affiliation: Veterinary and Animal Science School, University Autonomous of Sinaloa, Culiacán 1084, Sinaloa, México.
authors.forename: B I
authors.initials: BI
authors.lastname: Castro
authors.affiliation: Veterinary and Animal Science School, University Autonomous of Sinaloa, Culiacán 1084, Sinaloa, México.
authors.forename: F G
authors.initials: FG
authors.lastname: Ríos
authors.affiliation: Veterinary and Animal Science School, University Autonomous of Sinaloa, Culiacán 1084, Sinaloa, México.
authors.forename: H
authors.initials: H
authors.lastname: Dávila-Ramos
authors.affiliation: Research Institute of Veterinary Sciences, University Autonomous of Baja California, Tijuana 21100, México.
authors.forename: A
authors.initials: A
authors.lastname: Barreras
authors.affiliation: Research Institute of Veterinary Sciences, University Autonomous of Baja California, Tijuana 21100, México.
authors.forename: J D
authors.initials: JD
authors.lastname: Urías-Estrada
authors.affiliation: Department of Animal Science, University of California, Davis, CA 95616, USA.
authors.forename: R A
authors.initials: RA
authors.lastname: Zinn
authors.affiliation: Research Institute of Veterinary Sciences, University Autonomous of Baja California, Tijuana 21100, México.
authors.forename: A
authors.initials: A
authors.lastname: Plascencia
keywords: Carcass; Dietary Energetics; Finishing Lambs; Growth Performance; Slow-release Urea