Effects of Different Supplementations During the Rainy Season on Nitrogen Metabolism of Beef Cattle Steers on Tropical Pasture

Abstract Phytogenic additives are used as food additives in ruminant nutrition due to their potential to alter the nitrogen (N) excretion route and minimize N emissions into the environment. This study aimed to evaluate the effect of two levels of supplementation plus a mixture of phytogenic additives (essential oils, saponins and spices) on the efficiency of N utilization by beef cattle in tropical pastures. The experiment was conducted at the Beef Cattle Sector of UNESP, Jaboticabal-BR, and lasted 84 days. Nine castrated Nelore steers (body weight 262 ± 31.2 kg) cannulated in the rumen were distributed in 3 simultaneous 3 × 3 Latin square designs. The animals were kept in pasture of Urochloa brizantha cv. Marandu during the rainy season. Treatments included ad libitum mineral supplementation (MS) as a control group, mineral supplementation added 0.1% of body weight per day plus Cargill phytogenic mixture (AMSP) and energy-protein supplementation, 0.3% of body weight per day plus Cargill phytogenic mixture (EPSP). Point urine and fecal samples were collected in the last three days of each experimental period. The concentration of total nitrogen in the urine was analyzed by the Kjeldahl procedure. The nitrogen composition of forage, supplements and feces was estimated by near-infrared spectroscopy (NIRS) using a modular FT-NIR spectrometer (NIRFlex N500, Büchi Labortechnik AG, Flawil, Switzerland). Nitrogen retention was calculated using the following equation: NR = N intake (g/d) – [N excretion in feces (g/d) + N excretion in urine (g/d)]. The nitrogen metabolism data were analyzed as a 3 × 3 Latin square with 3 replications, balanced for residual effects. Considering as fixed effect the treatment, and as random effect the Latin square, period, steer, steer × period interaction, and residues corresponding to the model. The data obtained were submitted to ANOVA and Tukey's test (P ≤ 0.05), using the statistical program R, version 3.4.4. The addition of phytogenic additives in the supplements led to decreased (P < 0.036) urinary excretion of N (g/day) from 41.16 g/day to 28.38 g/day and urinary excretion of N (% of N intake) from 30.66% to 23.21%, but did not differ between the different levels of phytogenic supplementation (Table 1). In addition, retained N (% of N intake) was greater (P < 0.01) in animals supplemented with AMSP and EPSP compared with MS, increasing N retention from 26.78% to 33.88%. This study indicates that the different levels of supplementation plus the addition of the phytogenic mixture improved the efficiency of N utilization by decreasing the excretion of N in the urine and increasing the retained N compared with MS supplementation.