Kinetics of alfalfa nitrogen and cell wall disappearance from ruminally-incubated dacron bags

Farquhar, Andrew
Major Professor
Committee Member
Journal Title
Journal ISSN
Volume Title
Research Projects
Organizational Units
Animal Science
Organizational Unit
Journal Issue
Animal Science

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.