Ureumammonisering van koring-, hawer- en garsstrooi en hawerhooi. 2. Benutting deur skape

Abstract

Urea ammoniation of wheat, oat and barley straw and oat hay. 2. Utilization by sheep. Wheat, oat and barley straw, as well as oat hay were ammoniated, using 55 g urea and 400 g moisture kg^-1 roughage for a treatment period of 8 weeks. Nutritional values of the treated roughages were subsequently determined in metabolic studies on sheep, involving a voluntary intake and digestibility trial, nitrogen balance, blood and rumen parameters, a rate of passage and an in situ crude protein (CP) degradability study. The voluntary intake of treated oat hay was higher (P ≤ 0,05) than that of treated oat straw, and tended to be higher than that of treated wheat straw (P = 0,06) and barley straw (P = 0,08). Little variation regarding DM and OM digestibility existed between the diets. Cell wall and hemicellulose digestibility of treated oat hay was lower than that of the other roughages, possibly due to the substitution effect and/or the depression of cellulolytic activity in the rumen at the observed lower pH levels. The rate of passage of treated wheat, oat and barley straw was slower (P ≤ 0,05) than that of treated oat hay by 46%, 26% and 25% respectively. The rate of passage of treated wheat straw was slower (P ≤ 0,05) than that of barley and oat straw. The higher (P ≤ 0,01) CP content and apparent digestibility oftreated oat hay was also reflected in higher (P ≤ 0,05) rumen ammonia and plasma urea levels compared to the other roughages. CP digestibility of treated wheat straw was substantially lower (P ≤ 0,01) than that of oat or barley roughages, partially cancelling out the advantage of a higher apparent CP digestibility. This observation suggests that a lack of energy probably inhibited efficient protein synthesis, resulting in excess N being excreted in the urine. The N-balance obtained when feeding treated oat hay (1,4 g N day^-1) was nevertheless higher (P ≤ 0,01) than that of the negative N-balance values observed when treated wheat straw (-1,3 g N day^-1) and oat straw (-1,1 g N day^-1) were fed. CP-degradation studies suggested that the degradable CP content of treated wheat, oat and barley straw consisted almost entirely of water-soluble NPN compounds, highly susceptible to rumen degradation. The remaining CP was virtually undegradable in the rumen, and likely to be of very limited nutritional value.

Koring-, hawer- en garsstrooi, sowel as hawerhooi is met 55 g ureum en 400 g vog kg^-1 ruvoer vir 'n periode van 8 weke geammoniseer. Die voedingswaarde van die behandelde ruvoere is vervolgens in metabolismestudies met skape ondersoek, insluitende 'n vrywillige inname- en verteerbaarheidstudie, stikstofbalans, bloed- en rumenparameters, 'n spoed van deurgang, en 'n in situ ruproteïen(RP)-degradeerbaarheidsstudie. Die vrywillige inname van behandelde hawerhooi was hoër (P ≤ 0,05) as dié van behandelde hawerstrooi, en het geneig om hoër te wees as die inname van behandelde koringstrooi (P = 0,06) en garsstrooi (P = 0,08). Droë materiaal- en organiese materiaal-verteerbaarheid het relatief min tussen rantsoene gevarieer. Selwand- en hemiselluloseverteerbaarheid van behandelde hawerhooi was laer as by die ander ruvoere, moontlik as gevolg van die substitusie-effek, en/of die onderdrukking van sellulolitiese aktiwiteit in die rumen by die waargenome laer pH-peile. Die spoed van deurgang van behandelde koring-, hawer- en garsstrooi was onderskeidelik 46%, 26% en 25% stadiger (P ≤ 0,05) as by behandelde hawerhooi. Behandelde koringstrooi het ook stadiger (P ≤ 0,05) deur die spysverteringskanaal van die skape beweeg as behandelde hawer- en garsstrooi. Die hoër (P ≤ 0,01) skynbaar verteerbare RP-inhoud van behandelde hawerstrooi is weerspieël in hoër (P ≤ 0,05) rumen ammoniak- en plasma-ureumpeile as by die ander behandelde ruvoere. Behandelde koringstrooi het verder 'n heelwat laer (P ≤ 0,01) skynbare RP-verteerbaarheid gehad as behandelde hawer- en garsstrooi. Die waargenome urine-stikstof(N)-uitskeiding op behandelde hawerhooi was hoër (P ≤ 0,01) as by die ander behandelde ruvoere, met die gevolg dat die voordeel van 'n hoër skynbare RP-verteerbaarheid gedeeltelik uitgekanselleer is. Hierdie waarneming dui daarop dat 'n tekort aan energie moontlik steeds doeltreffende proteiensïntese inhibeer, en dat die oortollige N in die urine uitgeskei word. Die N-balans op behandelde hawerhooi (1,4 g N dag^-1) was nogtans hoër (P ≤ 0,01) as die negatiewe balanse op behandelde koringstrooi (-1,3 g N dag^-1) en hawerstrooi (-1,1 g N dag^-1). RP-degradeerbaarheidsstudies het daarop gedui dat die totale RP-inhoud van behandelde koring-, hawer- en garsstrooi grotendeels wateroplosbare NPN-verbindings was, wat grootliks aan rumendegradasie onderhewig is. Die oorblywende RP was prakties ondegradeerbaar in die rumen en waarskynlik van beperkte voedingswaarde.

Keywords: In vivo digestibility, nitrogen balance, rate of passage protein degradation, urea ammoniation, voluntary intake.