Carbohydrates and protein metabolism in beef cattle grazing fertilized or intercropped tropical pasture

Abstract

This study was developed to determine the metabolism of nitrogen and carbohydrates of animals feeding on two types of pastures: brachiaria grass (monoculture) or intercropped with forage peanut, which contains condensed tannin. The experiment was carried out at the University of Lavras, in Lavras, MG. Two different areas were used in the experiment: 0.8 ha of Brachiaria brizantha cv. Marandu (brachiaria grass) intercropped with Arachis pintoi cv. Mandobi (forage peanut) and 0.6 ha of Brachiaria brizantha cv. Marandu fertilized with mineral nitrogen, applied at 150 kg N.ha-1 per year. Six pure breed zebu heifers cannulated on rumen with an initial body weight of 404 kg performed grazing. Those animals had a fistula located in the rumen. Experimental treatments had two types of pastures: monoculture brachiaria grass, fertilized with mineral nitrogen (FP), and intercropped with forage peanut (IP). The design used was double cross-over, with four experimental periods.Means with P≤0.10 was statistically different. DM and OM intakes were 11% greater at treatment FP in relation to IP. N intake was 15.0% greater at treatment IP in relation to FP. Besides no effects on NFC intake, IP tended to be 11.2 % greater than treatment FP. Condensed tannin intake was about 63.4 g.d-1. The N-NH3 on ruminal fluid was 28% greater at treatment FP in relation to IP. The VFA (Mm)was greater in IP than FP. The A: P ratio was greater at treatment IP in relation to FP. The microbial CP was 22.9% greater at treatment IP in relation to FP. Similarly, the efficiency of microbial protein supply and CP intake: digestible OM ratio were 47.8% and 32.5% greater at treatment IP in relation to FP.N ruminal passage rate was 12.3% greater at treatment IP in relation to FP. The DM and NDF ruminal digestion rates were 21.3% and 39.2% greater at treatment FP in relation to IP, respectively. The DM, OM, N and NDF apparent intestinal digestibility was33.8%, 41.9%, 23.3% and 36.5% greater at treatment IP compare to FP. Effects on N and non-ammonia nitrogen (NAN) flows only were observed between treatments, which were greater 14.1% and 15.7% on treatment IP in relation to FP. Fecal nitrogen output was 15.9% greater at treatment IP in relation to FP. However, urine nitrogen output did not differ between treatments. The nitrogen balance was greater at treatment IP in relation to FP.The protozoa population on FP treatment was greater in relation to IP, which averaged 7.05 (x104/ml) and 3.40 (x104/ml), respectively.The forage peanuts used in our study, which contain condensed tannin, altered the ruminal environment. Ruminal fiber degradation was lower and there was a decrease in apparent dry matter digestibility. The protein had lower ruminal degradation, as a result of binding with condensed tannin, generating an increase in the N flow to the post-rumen. There was higher production of microbial protein with the use of legumes. As a result of these facts, N balance was higher in the animals that fed on forage peanut, indicating that this plant provided more retained nitrogen. Lastly, CT was able to manipulate the ruminal environment in a way that decreased the protozoa population.