An evaluation of degradable protein and nonprotein nitrogen on intake and digestion by Dohne merino sheep fed wheat straw

[摘要] ENGLISH ABSTRACT: South Africa, like many other developing countries throughout the world, has a rapidlygrowing human population, resulting in a consequent increased demand for food. Ironically,this increased food supply has to be produced on a smaller area of available agricultural land,which means that agricultural production processes have to become more efficient.Furthermore, the majority of the people in these countries are unschooled and poverty is acommon phenomenon. Therefore, animal scientists are faced with the challenge and theresponsibility to provide affordable, high quality food to these people.One way of reaching this objective is to improve the utilisation of low-quality, high roughagefeedstuffs like crop residues and dry natural grass pastures. In the winter rainfall area ofSouth Africa alone, about 460 000 ha of wheat straw are annually available. The ruminantanimal has the ability to utilise the relatively unavailable energy (cellulose, hemi-celluloseand pectin) in the fibre component of these low-quality forages. Unfortunately, variousfactors, of which a N deficiency is the pre-dominant one, limit the utilisation of thesefeedstuffs. If the ability of the ruminant to utilise low-quality, fibrous energy sources isimproved, these abundantly available and relatively inexpensive crop residues and naturalpastures can be converted into high quality protein food for human consumption. Therefore, in order to rectify the N deficiency caused by these low-quality forages, thesupplemental N requirement to optimise the fermentation and digestive processes of theruminant animal has to be determined. The first study was conducted to determine thesupplemental rumen degradable protein (RDP) requirement, to maximise the digestibleorganic matter intake (DOMI) of Dohne Merino sheep fed wheat straw. Keeping the highcost of natural protein supplementation in mind, the purpose of the second study was todetermine the amount of true protein that can be replaced by nonprotein nitrogen (NPN) inRDP supplements fed to Dohne Merino sheep consuming wheat straw.In both trials animals had ad libitum access to low-quality wheat straw (3.2% CP;74.2% NDF) and water. In the first trial, RDP (calsium caseinate: 90% CP; 100% rumendegradable) was intraruminally administered at 07hOOand 19hOO,at the following levels: 0,40,80, 120 and 160 gld. Intake, fermentation and digestion were monitored to determine theRDP requirement to maximise DOM!. Digestible organic matter (OM) intake displayed aquadratic increase with elevated amounts of RDP (P < .01), and was maximised at anestimated 3.15 g RDPlkg BW·75 or 11.6% of DOM. Forage OM intake tended to increasequadratically (P = .15) with higher RDP levels. Microbial nitrogen (MN) flow to theduodenum and microbial efficiency increased quadratically (P ~ .04) and fluid dilution ratetended to increase in a quadratic manner (P = .15) with increased RDP supplementationlevels. Rumina! ammonia nitrogen (NH3-N) and total volatile fatty acid (VFA)concentrations increased linearly (P ~ .07), while rumen pH exhibited a variable response toincreased RDP levels (cubic; P = .08). Increasing RDP supplementation to Dohne Merinowethers consuming wheat straw, generally enhanced forage utilisation and DOM!.In the second trial, urea replaced different levels of casein N on an isonitrogenous basis,ranging from 0 - 100%. Since true protein is much more expensive than urea, the purpose ofthis study was to determine the maximum natural protein level that can be replaced by urea inRDP supplements, without adversely affecting intake and/or fermentation and digestiveprocesses. The control treatment provided all of the RDP in the form of calcium caseinate(90% CP; 100% rumen degradable). The percentages of supplemental RDP from urea in theother treatments were 25, 50, 75 and 100%. The 100% urea treatment was balanced withmaize starch to contain 40% CP and all other treatments received the same amount (150 gld)of starch. Intake of forage OM showed a weak decreasing trend (linear; P = .16) with increasing urea levels. Ruminal digestibilities of OM and NDF were not affected (P ;;::.:18) byurea level. Increasing urea levels resulted in linearly reduced total tract OM and NDFdigestibilities (P ~ .10). As a result, DOMI declined (linear; P < .01) with increasingproportions of urea. Effects of increasing urea proportions on duodenal N flow, microbialefficiency and fluid dilution rate were minimal. Ruminal NH3-N tended to increasequadratically with increasing urea levels (P = .l4). Total VFA concentration decreasedlinearly (P = .03), while rumen pH increased in a linear manner (P = .08) with increasingurea proportions. Branched-chain volatile fatty acids (BCVFA's) and valerate decreasedlinearly (P ~ .05) with increasing urea levels, while other VFA's and the acetate:propionateratio were generally not affected by treatment (P ;;:::.16). It appears as though ruminal andtotal tract OM and NDF digestibility criteria, as well as DOMI reached maximum values atsubstituting 25% of casein for urea. It is therefore concluded that replacing 25% of caseinwith urea in RDP supplements, will maintain effective utilisation of low-quality forages bysheep.

[发布机构] Stellenbosch University