Protein fractions of Tifton 85 and rye-ryegrass due to sward management practices

In the South, early weaned calves (Bostaurus) can be raised on pasture-bascd feeding programs, but to effectively meet their nutritional requirements, more information is needed on forage protein characteristics. Experiments were conducted from January to April and May to July 2003 and 2004 to evaluate the effects of N fertilization and regrowth interval on crude protein (CP) fraction concentrations of rye (Secale cereale L.)-annual ryegrass (Lolium multiflorum Lam.) mixtures and 'Tifton 85' bermudagrass (Cynodon sp.). Treatments were the factorial combinations of three N levels (0, 40, and 80 kg ha(-1) period(-1)) and two regrowth intervals, 3 and 6 wk for rye-ryegrass and 2 and 4 wk for bermudagrass. In situ digestibility methodology was used to describe CP fractions. For rye-ryegrass herbage CP, there was a linear increase in the concentration of Fraction A (rapidly degradable; 410-500 g kg(-1)) and a linear decrease in Fractions B (potentially degradable; 530-370 gkg(-1)) and C (undegradable in the rumen, 73-47 g kg(-1)) as N fertilization increased. For bermudagrass, Fraction B increased (330-455 g kg(-1)) and C decreased (283-208 g kg(-1)) linearly as N fertilization increased. High Fraction A concentration in rye-ryegrass CP favors use of supplements containing rumen-undegradable protein for early weaned calves. In contrast, large Fractions B plus C concentrations suggest that calves grazing bermudagrass may respond best to supplements containing rumen-degradable protein.