D.P. Gnad

Effect of rumen-degradable intake protein supplementation on urea kinetics and microbial use of recycled urea in steers consuming low-quality forage

We evaluated the effect of increasing amounts of rumen-degradable intake protein (DIP) on urea kinetics in steers consuming prairie hay. Ruminally and duodenally fistulated steers (278 kg of BW) were used in a 4 x 4 Latin square and provided ad libitum access to low-quality prairie hay (4.9% CP). The DIP was provided as casein dosed ruminally once daily in amounts of 0, 59, 118, and 177 mg of N/kg of BW daily. Periods were 13 d long, with 7 d for adaptation and 6 d for collection. Steers were in metabolism crates for total collection of urine and feces. Jugular infusion of (15)N(15)N-urea, followed by determination of urinary enrichment of (15)N(15)N-urea and (14)N(15)N-urea was used to determine urea kinetics. Forage and N intake increased (linear, P < 0.001) with increasing DIP. Retention of N was negative (-2.7 g/d) for steers receiving no DIP and increased linearly (P < 0.001; 11.7, 23.0, and 35.2 g/d for 59, 118, and 177 mg of N/kg of BW daily) with DIP. Urea synthesis was 19.9, 24.8, 42.9, and 50.9 g of urea-N/d for 0, 59, 118, and 177 mg of N/kg of BW daily (linear, P = 0.004). Entry of urea into the gut was 98.9, 98.8, 98.6, and 95.9% of production for 0, 59, 118, and 177 mg of N/kg of BW daily, respectively (quadratic, P = 0.003). The amount of urea-N entering the gastrointestinal tract was greatest for 177 mg of N/kg of BW daily (48.6 g of urea-N/d) and decreased (linear, P = 0.005) to 42.4, 24.5, and 19.8 g of urea-N/d for 118, 59, and 0 mg of N/kg of BW daily. Microbial incorporation of recycled urea-N increased linearly (P = 0.02) from 12.3 g of N/d for 0 mg of N/kg of BW daily to 28.9 g of N/d for 177 mg of N/kg of BW daily. Provision of DIP produced the desired and previously observed increase in forage intake while also increasing N retention. The large percentage of urea synthesis that was recycled to the gut (95.9% even when steers received the greatest amount of DIP) points to the remarkable ability of cattle to conserve N when fed a low-protein diet.

Supplementation with degradable intake protein increases low-quality forage utilization and microbial use of recycled urea

A common production practice throughout the United States is to supplement protein to cattle consuming low-quality forage (forage with a crude protein content of less than 7%) in order to improve animal performance (i.e., maintain body condition score and body weight) during the winter. Protein supplementation increases forage utilization (intake and digestion) and cow performance by supplying ruminal microbes with protein that is essential for microbial growth. Increased microbial activity in turn provides sources of both protein and energy to the cow. In addition to the protein that is fed and degraded in the rumen, ruminants have the ability to recycle urea—the same compound found in fertilizer and cattle feed—to the rumen, where microbes can use the urea to fulfill a portion of their nitrogen requirement. Although nutritionists know that recycling occurs, we have inadequate data to describe this process and, subsequently, the contribution from recycled urea is not adequately included in our present cattle feeding systems. Previous research at Kansas State University has clearly demonstrated that the greatest response to supplemental protein occurs when the supplemental protein is highly degraded within the rumen, as the degradable fraction of protein is directly available to ruminal microbes. The current project’s objective was to measure how much recycled urea is used to meet the microbial nitrogen requirement when increasing amounts of degradable intake protein were provided to steers consuming low-quality forage. Researchers hoped to generate data useful in refining supplementation recommendations for cattle consuming low-quality forage.

Effects of energy level on methionine utilization by growing steers

We evaluated the effect of energy supplementation on Met use in growing steers. Six ruminally cannulated Holstein steers (228 +/- 8 kg of BW) were used in a 6 x 6 Latin square and fed 2.8 kg of DM/d of a diet based on soybean hulls. Treatments were abomasal infusion of 2 amounts of Met (0 or 3 g/d) and supplementation with 3 amounts of energy (0, 1.3, or 2.6 Mcal of GE/d) in a 2 x 3 factorial arrangement. The 1.3 Mcal/d treatment was supplied through ruminal infusion of 90 g/d of acetate, 90 g/d of propionate, and 30 g/d of butyrate, and abomasal infusion of 30 g/d of glucose and 30 g/d of fat. The 2.6 Mcal/d treatment supplied twice these amounts. All steers received basal infusions of 400 g/d of acetate into the rumen and a mixture (125 g/d) containing all essential AA except Met into the abomasum. No interactions between Met and energy levels were observed. Nitrogen balance was increased (P < 0.05) by Met supplementation from 23.6 to 27.8 g/d, indicating that protein deposition was limited by Met. Nitrogen retention increased linearly (P < 0.05) from 23.6 to 27.7 g/d with increased energy supply. Increased energy supply also linearly reduced (P < 0.05) urinary N excretion from 44.6 to 39.7 g/d and reduced plasma urea concentrations from 2.8 to 2.1 mM. Total tract apparent OM and NDF digestibilities were reduced linearly (P < 0.05) by energy supplementation, from 78.2 and 78.7% to 74.3 and 74.5%, respectively. Whole-body protein synthesis and degradation were not affected significantly by energy supplementation. Energy supplementation linearly increased (P < 0.05) serum IGF-I from 694 to 818 ng/mL and quadratically increased (P < 0.05) serum insulin (0.38, 0.47, and 0.42 ng/mL for 0, 1.3, and 2.6 Mcal/d, respectively). In growing steers, N retention was improved by energy supplementation, even when Met limited protein deposition, suggesting that energy supplementation affects the efficiency of AA use.

Effects of ammonia load on amino acidutilization by growing steers

Ruminally cannulated steers were used in two experiments to study effects of rumen ammonia load on methionine and leucine utilization. All steers were limit-fed a diet based on soybean hulls, received ruminal infusions of volatile fatty acids and abomasal infusions of glucose to provide energy, and received an abomasal infusion containing a mixture of all essential amino acids except methionine in Exp. 1 or leucine in Exp. 2. Treatments were arranged as 3 × 2 factorials and included urea (0, 40, or 80 g/day) infused ruminally and methionine (2 or 5 g/day) infused abomasally in Exp. 1 and leucine (0, 4, or 8 g/day) infused abomasally and urea (0 or 80 g/day) infused ruminally in Exp. 2. In Exp. 1, supplementation with the greater amount of methionine improved retained nitrogen, but urea infusions did not alter nitrogen retention. In Exp. 2, leucine linearly increased retained nitrogen, and urea infusions also increased nitrogen retention. The efficiency of deposition of supplemental methionine ranged between 18 and 27%, whereas that for leucine ranged from 24 to 43%. Increasing ammonia load did not negatively impact whole-body protein deposition in growing steers when either methionine or leucine was limiting.

Ruminal ammonia load does not affect histidine utilization in growing steers

Fermentation of dietary protein in the rumen leads to ammonia absorption, which could impair amino acid utilization in cattle. Our study was conducted to determine the effects of rumen ammonia load on histidine utilization. Six ruminally cannulated Holstein steers (318 lb) housed in metabolism crates were used in a 6 × 6 Latin square design. Treatments were arranged as a 3 × 2 factorial and included: 0, 1.5, or 3 grams/day Lhistidine infused abomasally; and 0 or 80 grams/day urea infused ruminally to supply a metabolic ammonia load. As expected, urea infusions increased rumen ammonia and plasma urea concentrations. No change in nitrogen retention, a measure of lean tissue growth, occurred in response to urea. Retained nitrogen increased with histidine supply, and the maximal response occurred with 1.5 grams/day of histidine, suggesting that this amount was near the supplemental requirement. Our research revealed that increases in ammonia load did not demonstrate a metabolic cost in terms of whole body protein deposition, regardless of whether histidine was limiting. Thus, although an excess protein supply may not be economically efficient or environmentally friendly, it does not appear to directly penalize animal performance.

Relative value of ruminally degradable and undegradable protein on the utilization of low-quality prairie hay by steers

An experiment was performed to investigate the impact of providing six levels of ruminally degradable protein (RDP; protein that is available to ruminal microbes) in combination with two levels of ruminally undegradable protein (RUP; protein that is not available to the ruminal microbes, but can be digested directly by cattle) on the intake and digestion of low-quality prairie hay. Twelve steers were provided unlimited access to low-quality prairie hay (5.3% crude protein and 71.7% neutral detergent fiber) throughout the trial. To simulate dietary RUP, casein was infused abomasally once daily at either 0 or 0.087% of body weight. To simulate dietary RDP, casein was infused ruminally once daily at 0, 0.029, 0.058, 0.087, 0.116, or 0.145% of body weight. As provision of RDP increased, forage intake and fiber digestion increased. Supplementing with RUP alone increased forage intake but not fiber digestion, although the intake response was not as large as providing the same amount of RDP. In conclusion, RUP is less efficient than RDP in stimulating forage intake and digestion.