Urea metabolism in beef steers grazing Bermudagrass, Caucasian bluestem, or gamagrass pastures varying in plant morphology, protein content, and protein composition1,2,3,4

Abstract

Pastures of Bermudagrass (Cynodon dactylon, BG), Caucasian bluestem (Bothriochloa caucasica, CBS), and gamagrass (Tripsacum dactyloides, GG) were evaluated from the perspectives of forage composition, selection during grazing, and N metabolism in beef steers. All pastures were fertilized with 78 kg/ha of N approximately 60 and 30 d before sample collection. In 2000 and 2001, 12 steers (250 kg of BW) were blocked based on BW and then assigned randomly to a replicated, randomized complete block design, with 2 pastures of each forage and 2 steers per pasture. Three other steers with esophageal fistulas were used to collect masticate samples to represent intake preferences. Herbage mass was >1,900 kg/ha. After at least 14 d of adaptation, urine and blood samples were collected for determination of serum urea N and percentage of urinary N in the form of urea. One steer per pasture (6 steers per year) was infused i.v. with (15,15)N urea for 50 h before collecting urine for 6 h to measure urea N enrichment, urea entry rate, urinary urea excretion, gut urea recycling, and return of urea N to the ornithine cycle. The canopy leaf:stem DM ratio differed (P = 0.01) among BG (0.50), CBS (1.01), and GG (4.00). Caucasian bluestem had less CP (% of DM) than GG or BG in the canopy (9.6 vs. 12.0 or 12.3, P = 0.07) and in the masticate (9.8 vs. 14.7 or 13.9, P = 0.04). Bermudagrass had less true protein (% of CP) than CBS or GG in the canopy (72.9 vs. 83.3 or 83.0, P = 0.07) and in the masticate (73.7 vs. 85.8 or 88.0, P = 0.04). Compared with GG and BG, CBS had less serum urea N (10.1 or 12.2 vs. 2.5 mM, P = 0.01), urea entry rate (353 or 391 vs. 209 mmol of N/h, P = 0.07), and urinary urea excretion (105 or 95 vs. 18 mmol of N/h, P = 0.04), and a greater return of urea N to the ornithine cycle as a proportion of gut urea recycling (0.109 or 0.118 vs. 0.231, P = 0.02). Urea production and recycling in these steers responded more to the N concentration in the grasses than to differences in plant protein fractions. There was no evidence of improved N capture by the steers due to changes in the leaf:stem ratio among the grasses at the herbage mass evaluated.

Methodology for concurrent determination of urea kinetics and the capture of recycled urea nitrogen by ruminal microbes in cattle

“…Additionally, it allows observation of the effect of frequency of nutrient provision (e.g., frequency of protein supplementation). Our observation of urea kinetics over a 24-h period is in contrast to previous observations in cattle where samples for determination of urea kinetics were collected over a 6 to 8 h time period in beef cattle (Archibeque et al, 2001 andHuntington et al, 2007), in dairy cattle (Marini and Van Amburgh, 2003) and in sheep (Sunny et al, 2007). In work by Sunny et al (2007) and Marini and Van Amburgh (2003), animals were fed every 2 h to ensure the animals were at steady state.…”

“…In contrast, Archibeque et al (2001 and offered hay twice daily and provided the supplement once daily, and they made no attempt to account for the discontinuous supply of nutrients when collecting samples for the determination of urea kinetics. Additionally, Huntington et al (2007) used grazing cattle and also made no attempt to correct for the discontinuous supply of nutrients in their work. Presently, we are unaware of any research documenting the measure of urea kinetics over a 24-h period in using the continuous infusion of […”

Feed restriction followed by realimentation in prepubescent Zebu females

“…The increase in the urea concentration (P<0.05) observed from period to the compensatory gain period reflected an increase in hepatic synthesis, which resulted from an increase in microbial nitrogen due to enhanced ruminal production because of the greater amino acid input in the liver and the feed (Huntington et al 2004). The increase in the glucose concentration observed after realimentation occurred as a result of greater ruminal propionic acid production, which was associated with a greater concentration of nutrient energy.…”

Urea metabolism in beef steers fed tall fescue, orchardgrass, or gamagrass hays1,2

“…Urea content of plasma and urine was determined using the diacetyl monoxime method of Marsh et al (1957) adapted to a Technicon Auto Analyzer (Technicon Instrument Corp.. Tarrytown, NY). Preparation and analysis of urine samples, and calculation of urea entry rate, urinary urea-N excretion, return of urea-N to the gut, and return of urea-N to the ornithine cycle have been described (Marini arid Attene-Ranios, 2006;Huntington et al, 2007).…”

“…Data from the current experiments were combined with unpublished data from other experiments with steers fed high-or all-forage diets (Archibeque et, al., 2001(Archibeque et, al., , 2002Huntington et al, 2007) to assess relationships between the proportion of urea-N returning to the ornithine cycle rno1ar percent excess of 29N2/(molar percent excess of 2. N2 + 30N2 )] and urea-N entry rate or recycling of urea-N to the gut.…”