Substitution of lucerne hay by ammoniated wheat straw in growth diets for lambs

Abstract

Lucerne hay (LH) was substituted by urea-ammoniated wheat straw (AWS) in four lamb-growth diets, all containing 60% roughage. The ratio of LH to AWS was 60: 0, 40: 20, 20: 40 and 0: 60 in the respective diets, which were composed on an iso-nitrogenous basis. However, cell wall constituents (CWC) increased with increased AWS levels. Results from an 84-day growth study indicated that the voluntary dry-matter intake (DMI) of the lambs declined by 164 g/ d per 20% increase in AWS inclusion (P ≤ 0,01). Growth rate (ADG) similarly declined by 64,5 g/d with higher inclusion levels of AWS (P ≤ 0,01). Feed conversion ratio (FCR) was also adversely affected, with an increase of 1,14 kg DMI required per 20% increase in AWS inclusion to give an increase of 1 kg in live mass (P ≤ 0,01). The apparent digestibility of dry matter (DM) and organic matter (OM), as well as the nitrogen balance, tended to decline with increased AWS levels, but no significant differences were recorded. The lower energy intake on the diets including increased levels of AWS was reflected by the decreased deposition of fat by the end of the growth study (P ≤ 0,01), as measured by tritium water space. The economic advantage of the substitution of a high quality roughage like LH with a cheaper source (AWS) must be balanced against the reduction in DMI and ADG, and the adverse effect of increased AWS levels on FCR. Inherent limitations of AWS, like a high CWC and resultant bulkiness, must be recognized and considered in the formulation of diets including AWS as a roughage source.

Lusernhooi (LH) is verplaas met ureum-geammoniseerde koringstrooi (AKS) in lamgroeidiëte wat 60% ruvoer bevat. Die verhouding van LH tot AKS in die onderskeie diëte was 60: 0, 40: 20, 20: 40 en 0: 60. Die diëte is op 'n iso-stikstofbasis saamgestel, maar die selwandinhoud het met toenemende AKS-insluitingspeile toegeneem. Resultate van 'n 84-dae-groeiproef dui op 'n verlaging van 164 g/ d in die vrywillige droëmateriaalinname (DMI) per 20% toename in AKS-insluitingspeil (P ≤ 0,01). Groeitempo (GDT) het ooreenstemmend verswak met 64,5 g/d met toenemende AKS-insluitingspeile (P ≤ 0,01). Voeromsettingsverhouding (VOV) is ook nadelig beïnvloed, met 'n toename van 1,14 kg DMI benodig per 20% toename in AKS-insluitingspeil vir'n toename van 1 kg in lewende massa (P ≤ 0,01). Die skynbare verteerbaarheid van droëmateriaal en organiese materiaal, sowel as stikstofbalans, het geneig om af te neem met toenemende AKS-insluitingspeile, maar geen betekenisvolle verskille is waargeneem nie. Die laer energie-inname op diëte hoog in AKS is weerspieël in laer liggaamsvetpeile aan die einde van die groeiproef (P ≤ 0,01), soos bepaal met tritiumwaterspasie. Die ekonomiese voordeel van die verplasing van 'n hoë kwaliteit ruvoer, soos LH, met'n goedkoper bron (AKS), moet opgeweeg word teen die laer DMI en GDT, sowel as die nadelige effek van toenemende AKS-insluitingspeile op VOV. Inherente beperkings van AKS, soos 'n hoë selwandinhoud en gepaardgaande lywigheid, moet erken en in ag geneem word by die formulering van groeidiëte wat AKS as 'n ruvoerbron bevat.

Keywords: Apparent digestibility, body composition, lamb growth, lucerne hay, urea-ammoniated wheat straw, voluntary intake.