Effect of slow release urea supplementation on fattening of steers fed sugar cane tops ( Saccharum officinarum) and maize ( Zea mays): ruminal fermentation, feed intake and digestibility

Abstract

A total of 60 Zebu steers were fattened for 120 days with feed supplemented with slow release urea, sugar cane tops ( Saccharum officinarum) and maize ( Zea mays). The treatments were as follows: treatment 1 (T1), fed 100% sugar cane tops (SCT) ( n=20; 257±7 kg BW); treatment 2 (T2), diet of SCT supplemented with 1.8 kg dry matter (DM) of slow release urea supplement (SRUS) ( n=20; 254±4 kg BW); treatment 3 (T3), fed SCT:maize (40:60%) supplemented with 1.8 kg DM SRUS ( n=20; 255±5 kg BW). Voluntary dry matter intake (VDMI), organic matter intake (OMI), rumen digestion, rate of passage, rumen concentration of NH 3, volatile fatty acids (VFAs), apparent digestibility, ruminal pH, total fermentable carbohydrates and weight gains were measured. Ruminal kinetics were determined in four adult ruminal cannulated steers. At all times, fibrous forages were available and exceeded VDMI. BW gain by T1 was 125 g/day compared to 708 g/day for T2 and 641 g/day for T3 ( P<0.05). Total forage intake was similar for T1 and T3 but higher in T2 ( P<0.05). DMI varied from 5.831 kg/day for T1 to 8.240 for T2 and 7.874 for T3 ( P<0.001). VDMI and apparent digestibility of DM and organic matter (OM) was better, the acetate molar proportions were lower and the feed intake was greater in both SRUS diets. Rumen pH rose to 6.83 and 6.73 by 2 h and stayed above 6.5 during the 12 h of sampling when SRUS was the only supplement, and was 6.3 when maize was added. In SCT steers, ruminal pH decreased to 6.16 by 6 h and rose again to 6.42 after 12 h. Ammonia concentration and digestion of potentially digestible and indigestible fractions were significantly augmented by SRUS ( P<0.05). Ammonia was significantly lower in T1 ( P<0.05). N intake was increased ( P<0.05) in T2 (146 g/day) versus T1 (37 g/day), but declined to 118 g/day in T3. In vivo nitrogen digestibility was different ( P<0.05) among the three diets, being 46.63, 74.16, and 56.63% in T1, T2 and T3, respectively. In vivo DM and OM digestibility, for cellulose and hemicellulose, was similar in T2 and T3 groups but different in T1 ( P<0.05). NDF digestibility was higher ( P<0.05) for T3 (66.17%), similar in T2 (63.14%), but lower in T1 (52.41%) ( P<0.01). The amount of situ DM digested was similar in T2 and T3 diets at 9, 12, 24 and 48 h, but less was digested in T1. NDF digestion rate ( k d) was similar among SRUS (0.039 and 0.037) diets, but different in T1 (0.023) ( P<0.05). Digestion rate for cellulose and hemicellulose was similar to previous results, being similar for SRUS diets but different in SCT treatment ( P<0.01). NDF (Neutral Detergent Fibre) passage ( k p) was different ( P<0.05) between diets (0.061/h for T1; 0.081/h for T2 and 0.079/h for T3). True digestibility was higher ( P<0.05) in T2 (48.14%) and T3 (46.24%) compared with T1 (36.22). Digestion rate was higher ( P<0.05) for T2 (0.039) and T3 (0.037) as compared to T1 (0.023). True digestibility in T2 (48.14) and T3 (46.24) was higher ( P<0.05) than in T1 (36.22%). The half-time ( t 1/2) disappearance for hemicellulose was higher ( P<0.05) for T2 (21.14 h) and T3 (19.34 h) as compared to T1 (14.54 h). Fattening in 120 days averaged 125 g/day (±31) for T1 compared to 708 g/day (±150) for T2 and 641 g/day (±57) for T3 ( P<0.05). Acetic acid production increased significantly (mM/l) in the T1 diet compared with the T2 and T3 diets ( P<0.05). Propionic acid kinetics showed a significant increase ( P<0.05) at time of sampling in T3 diet. Supplementation of high fiber diets with NPN improved fermentation showing better ammonia and VFA production. Utilization of fibrous forages by steers was significantly higher ( P<0.05) for SRUS diets.