Intake, digestibility, and rumen and metabolic characteristics of cattle fed low-quality tropical forage and supplemented with nitrogen and different levels of starch.
OBJECTIVE
Effects of nitrogen supplementation associated with different levels of starch on voluntary intake, digestibility, and rumen and metabolic characteristics of cattle fed low-quality tropical forage (Brachiaria decumbenshay, 7.4% crude protein, CP) were evaluated using ruminal and abomasal cannulated steers.


METHODS
Five European×Zebu young bulls (186 kg body weight, BW) were distributed according to a 5×5 Latin square. The following treatments were evaluated: control, supplementation with 300 g CP/d (0:1), supplementation with 300 g starch/d and 300 g CP/d (1:1), supplementation with 600 g starch/d and 300 g CP/d (2:1), and supplementation with 900 g starch/d and 300 g CP/d (3:1). A mixture of nitrogenous compounds provided 1/3 from true protein (casein) and 2/3 from non-protein nitrogen (mixture of urea and ammonium sulphate, 9:1) was used as the nitrogen supplement. In order to supply energy a unique source of corn starch was used.


RESULTS
Supplements increased (p<0.05) dry matter intake, but did not affect (p>0.05) forage intake. There was a cubic effect (p<0.05) of starch on voluntary intake. This was attributed to the highest forage intake (g/kg BW) when using the 2:1 starch:CP ratio. Supplements increased (p<0.05) organic matter (OM) digestibility, but did not affect (p>0.05) neutral detergent fibre corrected for ash and protein (NDFap) digestibility. There was a positive linear effect (p<0.05) of the amount of starch supplemented on OM digestibility. Total NDFap digestibility was not affected (p>0.05) by the amount of supplemental starch. Ruminal ammonia nitrogen concentrations were higher (p<0.05) in supplemented animals, however, a negative linear effect (p<0.05) of amount of starch was observed. Supplements increased (p<0.05) the nitrogen balance (NB) and efficiency of nitrogen utilization. These effects were attributed to increased body anabolism, supported by higher (p<0.05) serum concentration of insulin-like growth factor 1. Increasing the amount of starch tended (p<0.06) to linearly increase the NB. In spite of this, there was a highest NB value for the 2:1 starch:CP ratio amongst the treatments with supplementation.


CONCLUSIONS
Nitrogen supplementation in cattle fed low-quality tropical forage increases nitrogen retention in the animal's body. An additional supply of starch increases nitrogen retention by increasing energy availability for both rumen and animal metabolism.
Department of Animal Science, Universidade Federal de Viçosa, Viçosa, MG 36570-000, Brazil.
Marcia
M
de Oliveira Franco
Department of Animal Science, Universidade Federal de Viçosa, Viçosa, MG 36570-000, Brazil.
Edenio
E
Detmann
Department of Animal Science, Universidade Federal de Viçosa, Viçosa, MG 36570-000, Brazil.
Sebastião
S
de Campos Valadares Filho
Department of Animal Science, Universidade Federal de Viçosa, Viçosa, MG 36570-000, Brazil.
Erick Darlisson
ED
Batista
Department of Animal Science, Universidade Federal de Viçosa, Viçosa, MG 36570-000, Brazil.
Luana Marta
LM
de Almeida Rufino
Department of Animal Science, Universidade Federal de Viçosa, Viçosa, MG 36570-000, Brazil.
Marcília Medrado
MM
Barbosa
Department of Animal Science, Universidade Federal de Viçosa, Viçosa, MG 36570-000, Brazil.
Alexandre Ribeiro
AR
Lopes
Brachiaria decumbens; Digestibility; Fibre; Nitrogen Balance; Zebu