Effects of slow-intake urea supplementation on goat kids pasturing natural Mexican rangeland

Eighty-six Alpine kids, 18.765 (±0.510) kg BW, plus four cannulated adult goats were allocated to two treatments in a production trial lasting 150 days. They also were evaluated for in situ DM disappearance, VDMI, OMI, rumen degradation rate of passage, NH3 and VFA concentrations, apparent digestibility, pH, total fermentable carbohydrates, and weight gains. The first diet (n=43 kids, 19.120 (±0.700) kg BW plus two cannulated adult goats) were pastured daily on rangeland (RM) plus 200 g per day of a slow-intake urea supplement (SIUS) of previously established composition. A second group (n=43 kids, 18.410 (±0.600) kg BW plus two cannulated adult goats) were equally pastured and supplemented with 300 g per day of a balanced concentrate (BC) with also previously established composition. Both groups were managed on pasture divided by an electrical fence. Kid growth averaged 101 (±18) g per day for RM/SIUS and 83 (±26) g per day for RM/BC (P<0.05). Total DMI was 1010±293 g per day for RM/SIUS and 747±107 for RM/BC (P<0.05). Ammonia concentration and degradation of potentially digestible and indigestible fractions were augmented by RM/SIUS (P<0.05). Rumen pH rose slightly post-feeding for RM/SIUS goats and remained higher compared to RM/BC or RM 12 h after feed offer. Nitrogen intake was similar from RM/SIUS and RM/BC (118.60 g per day versus 122.53). In vivo N-digestibility was higher (P<0.05) in the RM/SIUS diet (79.12%) than for RM/BC (56.14%); fiber digestibility was also higher (P<0.05) for RM/SIUS. In situ DM disappearance did not show differences between diets at 9, 12, 24, 48, 72 and 92 h of incubation but was lower for RM. Digestion rate of NDF constant (kd, h-1) favored the RM/SIUS diet (P<0.05). Digestion rate for cellulose and hemicellulose was similar between diets. Passage rate (kp, h-1) for NDF differed between diets, 0.059 h-1 for RM/BC versus 0.080 h-1 for RM/SIUS (P<0.05). True digestibility was higher in RM/SIUS, 48.33% compared to RM/BC 34.11% (P<0.05). In situ degradation of potentially digestible fiber for cellulose was higher in RM/SIUS 67.14% compared to RM/BC 53.14% (P<0.05). Indigestible fiber was similar for RM/BC (51.42%) and RM/SIUS (66.27%). Time of disappearance of cellulose in RM/BC (17.54 h) was less (P<0.05) than in RM/SIUS (30.34 h). Hemicellulose in situ digestion was similar between diets. Passage rate was different (P<0.05) between RM/SIUS (0.080 h-1) and RM/BC (0.059 h-1). The half-time (t1/2) disappearance for hemicellulose was higher for RM/SIUS (31.14 h) as compared to RM/BC (22.14 h) (P<0.05). Propionic acid increased with time of sampling in the RM/BC diet (P<0.05). Butyric acid production did not differ between diets (P<0.05). Total amounts of VFA differed between BC and SIUS diets, allowing higher energy to RM/SIUS (P<0.05). Supplementation of high fiber diets with NPN did improve fermentation with better ammonia and VFA production. SIUS intake was consumed in 8-10 h after offer but the BC in 30 min. It was concluded that SIUS supplementation offered critical nutrients to the rumen, improved DM intake, rate of passage, increased ruminal pH and resulted in better weight gains than BC. Present results showed that high fiber forages could be used efficiently by ruminants (up to 70-80% DMI) when ruminal fermentation is improved with a continuous N supplementation.