Effect of monensin withdrawal on intake, digestion, and ruminal fermentation parameters by Bos taurus indicus and Bos taurus taurus steers consuming bermudagrass hay

Metabolic studies reveal that ruminal microbes of adult steers do not degrade rumen-protected or unprotected L-citrulline

In vitro and in vivo experiments were conducted to determine the metabolism of rumen-protected or unprotected l-citrulline (Cit) plus l-glutamine (Gln) by ruminal microbes. In the in vitro experiment, whole ruminal fluid (3 mL, containing microorganisms) from steers was incubated at 37 ºC with 5 mM Cit plus 6 mM Gln (in a rumen-protected or unprotected form) for 0, 0.5, 2, or 4 h after which times 50 µL samples were collected for AA and ammonia analyses. In the in vivo experiment, at 0.5 h before and 0, 0.5, 1, 2, 4, and 6 h after cannulated adult steers consumed 0.56 kg dried-distillers' grain mixed with 70 g Cit plus 70 g Gln (in a rumen-protected or unprotected form), samples of ruminal fluid and jugular venous blood were obtained for AA analyses. Results from both in vitro and in vivo experiments demonstrated extensive hydrolysis of rumen-unprotected Gln into glutamate, but little degradation of the rumen-protected Gln or rumen-protected and unprotected Cit by ruminal microbes. Concentrations of Cit and arginine in the plasma of steers consuming rumen-protected or unprotected AA increased at 1 and 2 h after the meal, respectively, when compared with values at 0 h. Collectively, these novel findings indicate that ruminal microbes of adult steers do not degrade extracellular Cit in a rumen-protected or unprotected form. Our results refute the view that all dietary AAs are extensively catabolized by ruminal microorganisms and also have important implications for dietary supplementation with Cit to ruminants to enhance the concentration of arginine in their plasma and their productivity.

Effects of narasin supplementation on dry matter intake and rumen fermentation characteristics of Bos indicus steers fed a high-forage diet

This study evaluated the effects of narasin on intake and rumen fermentation characteristics of Bos indicus steers offered a high-forage diet for 140 d. On day 0 of the study, 30 rumen-fistulated Nellore steers [initial body weight (BW) = 281 ± 21 kg] were assigned to 30 individual pens in a randomized complete block design according to their initial BW. Animals were randomly assigned to 1 of the 3 treatments: 1) forage-based diet without narasin (CONT; n = 10), 2) CONT diet plus 13 ppm of narasin (13NAR; n = 10), and 3) CONT diet plus 20 ppm of narasin (20NAR; n = 10). The forage used was Tifton-85 (Cynodon dactylon spp.), whereas the carrier for narasin was a 50:50 mixture of soybean hull:corn. The experimental period was divided into 5 periods of 28 d each. Throughout the experimental period, total dry matter intake (DMI) was recorded daily, whereas mineral salt intake was recorded weekly. Blood and ruminal fluid samples were collected on day 0 (prior to treatment feeding), 28, 56, 84, 112, and 140 of the study. Moreover, total tract apparent nutrient digestibility was performed for a 5-d period every 28 d. No treatment effects were observed on forage, mineral, concentrate, or total DMI (P ≥ 0.22). Nonetheless, 13NAR tended to have a greater mineral intake vs. 20NAR cohorts (P = 0.08) Narasin-supplemented animals had reduced rumen acetate, Ac:Pr ratio, as well as greater (P ≤ 0.02) rumen propionate concentrations vs. CONT cohorts. Moreover, 13NAR increased rumen propionate and decreased butyrate, Ac:Pr vs. 20NAR cohorts (P ≤ 0.01). Throughout the experimental period, narasin-supplemented animals had reduced ammonia concentrations vs. CONT cohorts (P < 0.01), whereas no differences were observed between 13NAR and 20NAR (P = 0.80). No treatment or dose effects were observed (P ≥ 0.23) on DM, organic matter (OM), protein, neutral detergent fiber (NDF), acid detergent fiber (ADF), and mineral digestibility. Animals fed 13NAR had a reduced mean plasma urea concentration vs. CONT cohorts (P = 0.03), whereas no further differences were observed (P ≥ 0.12). In summary, narasin supplementation to beef steers offered a high-forage diet did not impact forage, mineral, and total DMI, as well as nutrient digestibility, whereas rumen fermentation characteristics, rumen ammonia, and plasma urea concentrations were positively impacted and lasted throughout the experimental period. Additionally, 13 ppm of narasin resulted in a reduced Ac:Pr ratio and rumen ammonia when compared to animals supplemented with 20 ppm.

Ruminal microbes of adult steers do not degrade extracellular L-citrulline and have a limited ability to metabolize extracellular L-glutamate1,2

The microbial population within the rumen has long been considered to have the capability of extensively degrading all dietary AA. Results from our feeding trials revealed that this dogma is not correct. In vitro studies were conducted to test the hypothesis that certain AA undergo little degradation by ruminal microbes. Whole ruminal fluid (3 mL, containing microorganisms) from cannulated adult steers (~500 kg, n = 6) was incubated at 37 °C with 5 mM l-glutamine, l-glutamate, l-arginine, or l-citrulline for 0, 0.5, 1, and 2 h to determine time-dependent changes in the metabolism of these AA. Additional ruminal fluid was incubated with 0, 0.5, 2 or 5 mM l-glutamine, l-glutamate, l-arginine, or l-citrulline for 2 h to determine dose-dependent changes in their metabolism. An aliquot (50 µL) of the incubation solution was collected at the predetermined time points for AA analyses. There was extensive hydrolysis of l-glutamine into l-glutamate and ammonia, and l-arginine into l-ornithine, l-proline, and ammonia, but the near absence of catabolism of extracellular l-glutamate and no degradation of extracellular l-citrulline by ruminal microbes. There was little uptake of 14C-labeled l-glutamate and no detectable uptake of 14C-labeled l-citrulline by the cells. These results indicate, for the first time, that ruminal microbes of adult steers do not degrade extracellular l-citrulline and that metabolism of extracellular l-glutamate is negligible compared with their ability to extensively catabolize extracellular l-arginine and l-glutamine.

Effects of feeding monensin to bred heifers fed in a drylot on nutrient and energy balance

The objective of this study was to determine if feeding monensin would improve diet digestion, energy and nitrogen balance in bred heifers receiving a limit-fed corn stalk-based diet. Sixteen pregnant Meat Animal Research Center (MARC) III composite heifers were used in a 161-d completely randomized design. Heifers were randomly assigned to one of two treatments, no monensin (CON) or 150 mg/d monensin (MON), with eight heifers in each treatment group. Heifers were limit-fed a corn stalk-based diet at 100% of MEm requirements. Effects of monensin on energy and nitrogen balance were determined via total fecal and urine collections and open-circuit respiration calorimetry. Total fecal and urine collection occurred on d 14, 42, and 161 of monensin feeding, and calorimetry measurements were made on d 0, 3, 14, 28, 42, and 161 of monensin feeding. DMI was not different (P = 0.94) for CON and MON heifers and, by design, increased (P < 0.01) from d 14 to d 161 of the trial to account for increasing fetal growth requirements. No differences (P = 0.91) in GE intake were observed between CON and MON heifers, and DE and ME intakes did not differ (P > 0.58) with monensin inclusion. DM, OM, NDF, and ADF digestion did not differ (P > 0.52) between treatments. Fecal, methane, urinary, and heat energy losses were not different (P > 0.16) for MON and CON heifers. Methane production was not different between treatments when expressed as daily liters of methane (P = 0.40); however, MON heifers produced 7% less (P = 0.03) methane per day than CON heifers when expressed as liters of methane produced on a metabolic body weight (MBW) basis. Furthermore, monensin had no effect (P = 0.36) on overall retained energy (RE). Nitrogen intake and excretion was not different (P > 0.13) between treatment groups. Results of this experiment indicate that adding monensin to limit-fed, corn stalk-based diets may not have a large effect on the energy and nitrogen balance of confined heifers.