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 combination vs. 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 and longissimus 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.
Highlights
A topic of interest in recent years in feedlots has been the Submitted Jan. 6, 2016; Revised Apr. 3, 2016; Accepted May 18, 2016 search for strategies that optimise nutrient synchrony between N and carbohydrate compounds in the rumen in order to promote better nutrient utilisation and energy efficiency, and as a strategy for reducing the risk of environmental pollution (Hristov et al, 2011)
In growing–finishing diets, the few studies conducted in this field have been focused on evaluating the effect of SRU in direct substitution of high-protein ingredients (Pinos-Rodríguez et al, 2010; Bourg et al, 2012; Lascano et al, 2012) rather than as a strategy to promote synchrony, and no research has examined the role of the starch:fibre ratio of the finishing diets on the effects of the combination of both sources of urea on lambs growth performance and dietary energetics
Recent findings in a digestion trial showed that when conventional urea was combined with a SRU in diets with a ratio of starch-to-acid detergent fibre (S:F) of 4:1, the digestible energy (DE) was improved by 2% over the expected (p = 0.04) level; while, according to the expected DE values, the predicted DE was 1.00 time to the expected values with urea plus SRU in diets with lower (3:1) or greater (6:1) S:F ratios, and for those that were fed with only urea in diets with a similar ratio of 4:1 (López-Soto et al, 2014)
Summary
Submitted Jan. 6, 2016; Revised Apr. 3, 2016; Accepted May 18, 2016 search for strategies that optimise nutrient synchrony between N and carbohydrate compounds in the rumen in order to promote better nutrient utilisation and energy efficiency, and as a strategy for reducing the risk of environmental pollution (Hristov et al, 2011). Increases of 6% on net energy (NE) of diet was observed in feedlot cattle when were fed with a diet with a S:F ratio of 4.5 supplemented with a urea combination, while diets with conventional urea did not modify the observed-to-expected NE ratio when was included in diets with an identical S:F ratio (López-Soto et al, 2015) This is surprising, if is considers that different responses of animals can be caused by different sources of urea supplementation and by dietary variations (i.e. rumen undegradable intake protein level); the differences on observed-to-expected DE and dietary NE obtained by Lopez-Soto et al (2014; 2015) between SRU and for those that were fed with only urea in diets with a similar ratio of S:F justifies the need to confirm these results in a lamb performance trial. To test the findings of Lopez-Soto et al (2014; 2015) on the impact on the dietary energetics, the objectives of this experiment was to examine, in feedlot lambs, the magnitude of the responses on dietary energetics with combining urea and a SRU in diets containing different (3:1, 4:1, or 5:1) S:F ratios
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