Effects of protein sources and inclusion levels on nitrogen metabolism and urea kinetics of Nellore feedlot steers fed concentrate-based diets

Abstract

Urea recycling occurs in all mammalian species and represents an important source of ruminal nitrogen (N) for ruminants fed protein-restricted diets. However, its importance for cattle fed adequate amounts of protein and energy remains unclear. Six Nellore feedlot steers fed concentrate-based diets were used in a 6 × 6 Latin square design with a 3 × 2 factorial arrangement of treatments to evaluate ruminal fermentation, urea kinetics, and N excretion. Treatments consisted of 3 protein sources (PS: soybean meal plus urea [SU], corn gluten meal [CGM], and dry distillers grains [DDG]) and 2 inclusion levels (PL; 11 and 14%). Steers were adapted to the diets for 14 d followed by 8 d of sample collection. Feed intake, fecal output, and urine production were measured from d 18 to d 22 of each period. Blood samples were collected every 6 h on d 18. [ 15N- 15N]-urea was infused into the jugular vein for 82 h over d 19 to d 22, and measurement of 15N in background (d 18) and enriched feces and urine (d 21) were used to evaluate urea kinetics. To evaluate the incorporation of recycled urea N into microbial protein (MICP), ruminal and duodenal fluid were collected on d 22. Steers fed SU diets had lower (P < 0.05) nitrogen use efficiency (NUE), greater (P < 0.05) urea-N entry rate (UER), and tended (P < 0.10) to have greater gastrointestinal entry rate of urea-N (GER) compared to those fed CGM or DDG. In addition, steers fed SU had greater (P < 0.05) urea-N returned to ornithine cycle (ROC) compared to those fed CGM or DDG. Increasing PL tended (P < 0.10) to increase UER. The proportion of total microbial N from recycled urea-N was greater (P < 0.05) for steers fed CGM compared to those fed SU and also greater for steers fed diets with 11% CP than for those fed with 14% CP. Diets with 11% CP can be used for Nellore feedlot cattle fed concentrate-based diets without negatively affecting intake, digestibility, and ruminal fermentation. Moreover, diets containing rumen undegradable protein (RUP) feed sources (CGM or DDG) compared to diets with SU markedly increased NUE, while maintaining microbial protein (MICP) synthesis. Results from this study suggest that the equation adopted by NASEM (2016) was not accurate in estimating the urea-N used for anabolism (UUA) in Nellore feedlot cattle fed concentrate-based diets.

Models to predict nitrogen excretion from beef cattle fed a wide range of diets compiled from South America

“…Alternatively, the use of DMI may lead to lower cumulative error compared to NI, which requires the measurement of N content in feed samples ( Hristov et al., 2019 ). In contrast, urinary and manure N were best predicted from NI rather than DMI, which was expected as N intake is the major driver of urinary N excretion ( Dijkstra et al., 2013 ) and urine is the major route of excretion of surplus N in beef cattle ( Souza et al., 2021 ). It is well-documented that ruminal ammonia concentration increases in high CP diets ( Souza et al., 2021 ).…”

“…In contrast, urinary and manure N were best predicted from NI rather than DMI, which was expected as N intake is the major driver of urinary N excretion ( Dijkstra et al., 2013 ) and urine is the major route of excretion of surplus N in beef cattle ( Souza et al., 2021 ). It is well-documented that ruminal ammonia concentration increases in high CP diets ( Souza et al., 2021 ). The ammonia not incorporated into microbial CP can be absorbed across the ruminal wall, entering the portal vein, and transported to the liver, where it is largely converted to urea ( Batista et al., 2017 ).…”

“…In growing beef cattle, urine is the route of approximately 60% to 80% of total N excreted, while 20% to 40% is excreted in feces ( Dong et al., 2014 ). The proportion of N excreted in urine or feces is influenced by N intake ( NI ), apparent N digestibility, and dietary crude protein ( CP ) content ( Souza et al., 2021 ). The large variation observed in N use efficiency in beef cattle represents an opportunity to decrease N excretion through diet manipulation.…”

Comparisons of Hematological and Biochemical Profiles in Brahman and Yunling Cattle

“…Chikhou et al [ 31 ] showed that the level of BUN in plasma could reflect the protein metabolism of animals and could be used as an indicator of the body’s protein decomposition status. Under the condition of a low nitrogen diet, ruminants mainly increased nitrogen deposition through the BUN cycle; thus, the content of BUN increases to meet the needs of the body [ 32 ]. Compared with Brahman, Yunling cattle have higher BUN and lower URIC levels, indicating that Yunling cattle have a better roughage tolerance than Brahman, which is consistent with the actual production.…”