Effects of increasing dietary protein on nutrient utilization in heifers.
Four prepubertal Holstein heifers, average age 146.0 +/- 3.6 d and 152.8 +/- 8.6 kg of body weight (BW), fitted with rumen cannulae were used to evaluate increasing levels of dietary protein with a constant metabolizable energy intake on rumen ammonia N concentrations, N balance, total tract apparent digestibility, and urinary excretion of purine derivatives in a 4 x 4 Latin square design with 20 d periods. Heifers were fed dietary crude protein levels (CP) of 11.9, 16.7, 18.1, and 20.1% CP with similar amounts of metabolizable energy (ME) (2.6 Mcal per kg of dry matter) at 2.0% BW as dry matter intake. Resulting protein to ME ratios (CP:ME) were 45.0, 63.3, 69.4, and 77.3 g of dietary CP per Mcal of ME. Rumen ammonia N and plasma urea nitrogen increased as CP level increased. Rumen pH, volatile fatty acids, and the acetate-to-propionate ratio were not affected with increasing CP levels. Greater urinary N excretion resulted in an increase in total N excretion with increasing CP levels. Apparent dry matter digestibility was similar for all diets, while apparent total tract N digestibility was lowest for the 11.9% CP level. Microbial N calculated from urinary excretion of purine derivatives increased with increasing CP levels. Holstein heifers between 153 and 196 kg consuming a diet at 2.0% BW as dry matter intake containing a CP level of 16.7% achieved a better synergistic relationship of dietary protein to energy (CP:ME of 63.3 g of CP per Mcal of ME) than the diets lower or higher in CP that were studied.
Gabler
M T
MT
Department of Dairy and Animal Science, The Pennsylvania State University, University Park, PA 16802, USA.
Heinrichs
A J
AJ
D000085:Acetates / Q000032:analysis; D000641:Ammonia / Q000032:analysis; D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D001806:Blood Urea Nitrogen; D002417:Cattle / Q000502:physiology*; D004044:Dietary Proteins / Q000008:administration & dosage*; D004063:Digestion; D002149:Energy Intake; D005232:Fatty Acids, Volatile / Q000032:analysis; D005260:Female; D006863:Hydrogen-Ion Concentration; D009584:Nitrogen / Q000032:analysis / Q000378:metabolism; D011422:Propionates / Q000032:analysis; D012417:Rumen / Q000737:chemistry
D000085:Acetates; D004044:Dietary Proteins; D005232:Fatty Acids, Volatile; D011422:Propionates; D000641:Ammonia; C029658:propionic acid; D009584:Nitrogen