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1 USDA-ARS, US Meat Animal Research Center, Clay Center, NE 68933-0166
2 USDA-ARS, Conservation and Production Research Laboratory, Bushland, TX 79106
* To whom correspondence should be addressed. E-mail: ferrell{at}email.marc.usda.gov.
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Abstract |
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We hypothesized that oscillating dietary CP concentrations would improve efficiency of N use and reduce N loss to the environment. Charolais-cross steers (n = 8; 315 ± 21 kg BW) were used in a replicated 4 x 4 Latin square design. The steers were allowed ad libitum access to the following diets: 1) 9.1% CP (LOW), 2) 11.8% CP (MED), 3) 13.9% CP (HIGH), or 4) LOW and HIGH oscillated with a 48-h interval on each diet (OSC). Dry matter intake did not differ among treatments (P = 0.46), but N intake differed (P < 0.01) from 94 (LOW) to 131 (MED), 142 (HIGH), and 133 g/d (OSC), as designed. Dry matter digestibility increased (P < 0.01) from 71.8% (LOW) to 75.8% (MED), 77.7% (HIGH), and 77.5% (OSC). Nitrogen digestibility increased (P < 0.01) from 62.2% (LOW) to 67.2% (MED) to 70.1% (HIGH) and 70.9% (OSC). Nitrogen retention was greater (P < 0.01) in steers fed OSC (55.0 g/d) than in either the steers fed LOW (34.8 g/d) or HIGH (40.2 g/d), but N retention of steers fed MED (49.8 g/d) differed (P = 0.02) only from that of steers fed LOW. Urinary urea N did not differ between steers fed either MED (19.5 g/d) or OSC (21.3 g/d), but was lowest (P < 0.01) for those fed LOW (8.2 g/d) and greatest for those fed HIGH (39.2 g/d). Daily heat production (kcal/BW0.75) tended (P = 0.09) to be less for the steers fed LOW (177) than those fed MED (189), HIGH (188), or OSC (182). Cumulative in vitro ammonia volatilization from the manure of steers fed OSC was lower (P < 0.01) for the initial 5 d of incubation than from manure of those fed MED, but there was no difference after 11 d of incubation. Additionally, there was a decrease (P < 0.01) in in vitro ammonia volatilization as protein concentration in the diet decreased from HIGH to MED to LOW. These data indicate that oscillating dietary protein improved N retention of finishing steers compared to those in both high and low N diets and that these changes were great enough to correspondingly alter ammonia volatilization from manure.
Key Words: beef cattle, heat production, nutrient balance, protein oscillation
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