Kinetics of alfalfa nitrogen and cell wall disappearance from ruminally-incubated dacron bags
Kinetics of the disappearance of alfalfa N and cell wall (CW) from ruminally-incubated dacron bags were determined and compared in two separate studies. Alfalfa harvested at three maturity stages (early bud, first bloom, and seed pod) and treated with either 10% NaOH or heated at 150 C for 4 h was utilized in trial 1. In trial 2, alfalfa leaves, stems, and a 50:50 leaf/stem mixture were ruminally-incubated in steers fed either hay or an 85% concentrate diet. Parameters determined were rates of disappearance (k) of the potentially digestible (RD) N and CW fractions in bag residues, proportions of RDN and RDCW, and estimates of the discrete lag period prior to initiation of digestion;Rate constants (-k/h) generally averaged .16 for CW and .36 for N in first bloom, whole plant alfalfa incubated in hay-fed animals. Rates tended to be inversely related to advancing maturity and stem content; declines of a similar degree were noted in both CW and N estimates. RDCW averaged approximately 50% of the total CW in first bloom and mixed forages. Approximately 40-60% of initial plant N was lost when the bags were washed after incubation; RDN, consequently, ranged from 34% (seed pod stage forage) to 46% (leaves). Lag times for N and CW in untreated forage were variable and generally less than 1.5 h. Heat treatment produced marked declines in disappearance rates; k values ranged from -.04 to -.06/h for both N and CW. RDN and RDCW were also reduced whereas lag times were increased two- to three-fold. Alkali treatment caused little change in CW rate constants but reduced by nearly half those derived for N in untreated forages. RDN and RDCW were increased by alkali treatment, particularly in later maturity forages; lag times also tended to be increased. Incubation of forage substrates in animals fed high-grain diets caused declines of 40-60% in N and CW rate constants. Diet, however, had little effect on RD fractions or lag times;Through regressions of N and CW rate constants on the fractional contribution by stems to each of the substrates examined in the second trial, the existence of a quantitative relationship between N and CW disappearance was established. Slopes for N and CW regression lines, whether obtained with k values derived from incubations in hay- or grain-fed animals, were identical. Effects on such an interaction on the efficiency of animal utilization of forage N are discussed.