Feedlot performance of Dorper lambs on Opunti: based diets with different nitrogen sources

[摘要] Three feedlot diets were evaluated with Dorper wether lambs at Bergvlug ExperimentalFarm, Khomas Region, Namibia. Bergvlug is located about 35 km east of Windhoek. Thethree treatment diets consisted of a conventional feedlot diet (treatment diet T0) and twoOpuntia-based treatment diets (T1 and T2) containing different additional nitrogen sources,namely a non-protein nitrogen (NPN; feed grade urea) or natural protein (sunflower oilcakemeal). Treatment diet T0 was based on coarsely ground lucerne hay, yellow maize meal, feedgrade urea and molasses meal. The Opuntia-based treatment diets (T1 and T2) werereformulated and part of the lucerne was replaced by sun-dried and coarsely ground Opuntiacladodes at levels of 330 or 300 g/kg. In treatment diet T1 additional nitrogen was includedas feed grade urea (non-protein nitrogen; NPN) and for treatment diet T2 the additionalnitrogen was included as sunflower oilcake meal (a natural protein).The feed intake and water intake, the growth performance and carcass characteristics of theDorper wether lambs were evaluated in the feedlot. Forty-five newly weaned Dorper wether lambs, weighing on average about 22 kg, were randomly allocated to the three treatmentdiets. The 15 Dorper wether lambs per treatment diet were further subdivided into threesubgroups or replicates of five lambs each. For the duration of the trial the lambs were kept insmall pens in a shaded area (open-sided roofed shed). The Dorper wether lambs were fed thetreatment diets until a target average slaughter weight of 35 kg per treatment diet wasreached.During the feeding period in the feedlot, one replicate of five Dorper wether lambs pertreatment diet was moved from the feedlot pens to metabolism cages for a week every thirdweek to determine their individual daily feed and water intake and apparent digestibility ofthe three treatment diets. The daily urine and faecal excretions were also monitored.Chemical analysis of the three treatment diets used in this study showed that acid-detergentfibre (ADF), neutral-detergent fibre (NDF), organic matter (OM) and gross energy (GE) havedecreased with inclusion of sun-dried and coarsely ground Opuntia cladodes, which isascribed to the lower ADF, NDF, OM and GE content of the Opuntia cladodes. On the otherhand, the ash and lipids increased with inclusion of sun-dried and coarsely ground Opuntiacladodes in the treatment diets.The inclusion of sun-dried and coarsely ground Opuntia cladodes at 330 or 300 g/kg in thetreatment diets in general had little or no effect on the feed intake and digestibility of thetreatment diets by Dorper wether lambs. Exceptions were observed for the intake andapparent digestibility of ADF and NDF as a result of the difference in fibre content of thetreatment diets. Similar water intake and urine excretion were observed for Dorper wetherlambs fed any one of the treatment diets during the three cage feeding periods.The results of the study confirmed that the feed intake and apparent digestibility of thetreatment diets for Dorper wether lambs were not affected by: (1) the inclusion of sun-driedand coarsely ground Opuntia cladodes at 330 and 300 g/kg; or (2) two nitrogen sources used(NPN or natural protein) in the Opuntia-based diets.Considering the results of the Cage Periods 1 to 3, it can be summarised that the daily intakeand apparent digestibility of DM and other chemical constituents of the Dorper wether lambsincreased as the trial progressed, regardless of the treatment diets. It suggests that the reticulo-rumen of Dorper wether lambs were getting better adapted over time to the diets andconsequently digestibility improved.The average daily gain and feed conversion efficiency of Dorper wether lambs fed theOpuntia-based diet supplemented with natural protein were comparable to those fed theconventional feedlot diet. The Dorper wether lambs fed the Opuntia-based diet supplementedwith NPN had a lower growth rate than those fed the conventional feedlot diet and theOpuntia-based diet supplemented with natural protein. Thus, although feed conversionefficiency was not significantly (P>0.05) different among treatment diets, Dorper wetherlambs fed the conventional diet and the Opuntia-based diet supplemented with natural proteinrequired less feed to gain weight than those fed the Opuntia-based diet supplemented withNPN. This suggests that supplementing an Opuntia-based feedlot diet with a natural proteinsource will markedly improve feed efficiency and average daily gain of lambs. This mayreduce the feeding period required to reach the target slaughter weight and increase theeconomic benefit associated with the use of sun-dried and coarsely ground Opuntia cladodesin feedlot diets.Most of the carcass characteristics considered were not significantly different (P>0.05)among treatments. It suggests that carcass quality or grading is not markedly affected byinclusion of sun-dried and coarsely ground Opuntia cladodes in feedlot diets (up to 330 or300 g/kg) for Dorper wether lambs or by the nitrogen source used to balance the diets. Thecarcasses of the Dorper wether lambs fed the three different treatment diets fetched verysimilar prices per kg. However, the Dorper wether lambs fed treatment diet T1, the Opuntiabaseddiet with the inclusion of feed grade urea (an NPN source), did not reach the averagetarget slaughter weight of 35 kg, even after 91 days in the feedlot. Therefore, their lightercarcasses and poorer carcass grading at slaughter fetched a lower total price per carcass.The results of this study, the fourth study under the auspices of the UFS, opened the prospectof formulating affordable Opuntia-based diets for specific application to ruminant species ofdifferent ages and production classes. However, more research is needed to evaluate thegrowth performance, carcass characteristics and profitability of other small stock breeds andruminant species fed sun-dried and coarsely ground Opuntia cladodes in feedlot diets,balanced with different nitrogen sources.

[发布机构] University of the Free State