The effect of dietary protein degradability on the performance of Saanen dairy goats

[摘要] ENGLISH ABSTRACT:The goat is a significant domestic animal throughout the world today. With an estimated world goatpopulation of 590 million goats in 1991 (FAO, 1991 as citied by Haenlein, 1996) it is impossible toconsider the goat as insignificant. The need for milk, and it seems particularly goat's milk, is obviousif one considers the increase in dairy goat populations over the past 20 years. Across the globe thedairy goat population has increased by 52% while in developing and developed countries, there hasbeen an increase of 56% and 17%, respectively (Haenlein, 2000).The goat dairy industry in South Africa is still very underdeveloped, yet it holds tremendous potentialfor the entrepreneur willing to take the risk and do the job correctly. With the present South Africanfinancial situation the opportunities that exist for exporting value added products to countries withstronger currencies is a market with extraordinary potential. In New Zealand, the national herdconsists of approximately 16000 dairy goats and 90% of the milk produced is turned to powdered milkand then exported to the East, a valuable source of foreign currency. In South Africa, the samepotential exists and with some vision and hard work the dairy goat industry can make a valuablecontribution to generating foreign currency.Research into the protein requirements and particularly protein degradability requirements of dairygoats is scarce, yet in recent years there has been an increased interest in the effect of proteinsupplementation to lactating animals (Mishra & Rai, 1996). In the work of Mishra & Rai (1996) therewere benefits obtained from the use of different rumen degradable proteins for lactating dairy goatdoes. The does on the highly degradable protein diet had a better feed intake while the does on thelow degradable protein diet gave a higher milk production. Other research on this field of study hasalso delivered positive results with more than one species of lactating animal that had increasedlevels of UDP in the diet (Robinson et al., 1991 and Christensen et al., 1993).Loerch et al. (1995) suggested that improved production by making use of rumen undegradableproteins would have no effect if crude protein were not a limiting factor in production. Pailan & Kaur(1995) and Mishra & Rai (1996) did research on lowered CP levels with increased UDP levels inlactating dairy does. They used of three diets, with the one having a 20% lower CP value but an increased level of UDP (40-45% of total CP). From this work it was concluded that a decreased CPlevel and an increased level of UDP is able to sustain production when compared with diets with ahigher CP value.The current study consists of two trials. In the first trial the effect of weaning age and dietary proteindegradability on the growth of Saanen kids was investigated. In the second trial the effect of dietaryprotein degradability on the production of lactating Saanen does was investigated.Fifty-eight Saanen kids were divided into groups to determine the effect of weaning age (42 vs. 70days) on animal performance. Within the weaning day treatments, the kids were again divided intotwo dietary treatments. One group received a low UDP creep diet (LC) and the other a high UDPcreep diet (HC). The two creep diets were formulated with rumen degradable: undegradable protein(RDP : UDP) ratios of 70:30 and 60: 40, referred to as LC and HC, respectively. However, the resultsfrom the degradability trial indicated no difference in RDP: UDP ratios for the low and high creep(72:28 and 73:27 respectively) diets. At 15.66 ± 3.09 kg the kids were taken off the creep diet andput on the growth diet. At this transition, the kids in each of the 4 established treatments were againrandomly divided into two dietary treatments, a high or a low UDP growth diet, resulting in a total ofeight treatments for the trial. The two growth diets were formulated with RDP: UDP ratios of 70:30and 60:40, referred to as low growth (LG) and high growth (HG) respectively. Results from thedegradability trial indicated RDP: UDP ratios for the LG and HG of 73:27 and 68:32 respectively. Thegrowth trial was conducted over 140 days and feed intake, bodyweight change and feed conversionefficiency were compared for each of the 8 treatments.From the trial with the Saanen kids it was concluded that weaning dairy goat kids at 42 days of agewhen feed intake was 240 g/day resulted in similar growth rates when compared with weaning at 70days. The two creep diets did not differ in RDP: UDP ratios and thus no conclusion can be maderegarding the influence of the creep diets on the growth of Saanen kids from 20 to 80 days of age.The two growth diets did in fact differ from one another, in terms of RDP: UDP however, proteindegradability had no influence on the performance of the Saanen kids from 80 to 140 days of age.Twenty-one lactating Saanen does were randomly assigned to one of three experimental diets. Thetreatments had two RDP: UDP ratios and two crude protein (CP) levels. Treatments were formulatedto be 1) RDP: UDP = 70:30, CP = 20 % 2) RDP: UDP = 62:38, CP = 20% and 3) RDP: UDP = 62:38,CP = 18.3%. In the production trial the does were milked for 120 days, during which milk yield, milkcomposition, bodyweight change, feed intake and feed conversion efficiency were compared betweenthe treatments. In the digestibility and nitrogen metabolism trial, 18 does varying from 84 to 110 days in lactation, were used to compare the experimental diets. Furthermore, the experimental diets werecompared in a degradability and rate of passage trial using cannulated Dohne merino wethers.Results from the degradability trial indicated that the low UDP, low protein high UDP and high UDPdiets had RDP: UDP ratios of 82:18, 78:22 and 79:21 respectively, and that the dietary proteindegradability did not differ significantly between diets. Results from the production trial indicated thatthere was a significant difference in feed intake, dry matter (DM) intake and bodyweight. The does onthe low UDP diet had significantly higher feed intakes and DM intakes and were significantly heavierat the end of the trial period. As the diets didn't differ in protein degradability other factors must haveinfluenced the intakes between diets. Palatability may have influenced feed and DM intake, as thelow protein high UDP and high UDP diets both contained higher levels of fishmeal. No significantdifferences in milk production, milk composition or milk production efficiency were observed. Besidesthe fact that the diets did not differ in effective protein degradability, large variations in milk productionbetween animals and low numbers of animals per treatment limited the ability to measure a differencebetween the treatments. Results from the digestibility trial varied between diets with the low UDP diethaving a significantly lower digestibility overall than the other two diets. Reasons for the difference indigestibility could be due to the difference in rate of passage (low UDP = 0.064/hour versus the 0.044-0.045/hour of the low protein and high UDP diets respectively) and the high ADF value of the lowUDP diet. Because no difference in effective protein degradability existed between the diets it is notpossible to make an accurate conclusion on whether or not the dietary protein degradability had aninfluence on production parameters tested in this trial.

[发布机构] Stellenbosch University