- Varying fibre production potentials under veld conditions.
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VARYING FIBRE PRODUCTION POTENTIALS UNDER VELD CONDITIONS
M.J. Herselman, J.J. Olivier and D. Wentzel
Grootfontein Agricultural Development Institute, Middelburg CP, 5900
It is important for the financial survival of Karoo sheepfarmers to farm with well-adapted animals to ensure maximum production with minimum input costs. Several of the current research projects at Grootfontein are aimed at investigating the aspect of hardiness in small stock (the ability of animals to survive, produce and reproduce under adverse environmental conditions). From practical experience it is evident that breeds with a high fibre production function are not as hardy and well adapted to harsh environmental conditions as most mutton breeds. Whether this negative correlation between wool production and hardiness observed in South-African sheep breeds is caused by wool production per se, rather than the origin of each breed, has yet to be clarified. In this respect, different breeds of sheep with major differences in hardiness are at present used at Grootfontein in an attempt to identify possible physiological regulators of hardiness. Once these specific regulatory mechanisms are understood, the physiological significance of some selection practices may be elicited. The aim of the present study was to investigate the negative correlation between fibre production potential and adaptability to semi-arid environments in South African woolled sheep types.
MATERIAL AND METHODS
In order to obtain sheep with varying potentials for fibre production, Merino ewes were mated to rams of 5 different breeds, namely Merino (M), Dohne Merino (D), Mutton Merino (Y), Merino (A) and Ronderib-Afrikaner (R). The Fl-progeny, born from these matings, namely MM, MD, MV, MA and MR, was raised on irrigated pastures up to the age of 8 months, and thereafter kept on Karoo veld without any supplementary feeding. Wool production potential was calculated as wool produced per unit of body mass (Herselman & King, 1993).
Fl-ewes were tested for sexual maturity at the age of 12 months by recording the number of ewes marked by teaser rams. This procedure was repeated at the age of 18 months but in this case ewes of different types were mated separately with the corresponding type of ram. Following mating, all ewes were run together on Karoo veld where they also lambed and weaned their lambs. Ewes were mated again at the age of 30 months. No supplementary feeding was provided at any stage. Data of the two lamb crops were pooled for analysis.
RESULTS AND DISCUSSION
Changes in body mass of ewes are presented in Fig. 1. From this figure the effect of favourable feeding conditions on growth rate up to the age of 8 months is evident. On moving ewes to the veld they displayed an initial loss of mass, followed by a period of approximately 10 months during which no growth occurred. According to Fig. 1, ewes had already reached 81 to 92 % of their final mass at the age of 8 months on the pastures. Fluctuation in body mass thereafter was related mainly to seasonal variation, gestation, lactation and shearing.
Wool production between 6 and 18 months, 18- month body mass and wool production potential of the various types of sheep are shown in Table 1. From these results it is evident that relatively small differences occurred between the amount of wool produced by MM, MD, MV and MA. The observation that MD produced more wool than MM was rather surprising because it is well known that among wool breeds in the RSA the Merino has the highest genetic potential for fibre production. This finding was probably caused by an interaction between genotype and the environment. This interaction, in turn, was brought about by the adverse environmental conditions which most probably had a more severe inhibitory effect on sheep with a higher genetical potential for fibre production. The negative effect of the specific conditions on MM ewes is also reflected by the relatively low 18-month body mass of only 35,9 kg. Using the definition of wool production potential (wool production expressed as a percentage of body mass) to compare differences in potential for fibre production between breeds, more realistic values were obtained to describe expected differences in wool production between breeds when kept under more favourable conditions (Table 1).
According to Table 2 neither the MM nor the MD ewes had reached puberty at the age of 12 months while 25, 35, and 100 % of MV, MA: and MR ewes were already sexually active at this age. At 18 months, all but 16 % of MM ewes were sexually active. These results are in accordance with practical experience which indicates that breeds with a low fibre production potential usually reach sexual maturity at an earlier age.
Furthermore, from the results recorded for body mass in this trial it is also evident that the age at which sexual maturity is reached, is influenced to a greater extent by the type of animal than by body mass.
Reproduction data of experimental animals for 2 successive years are summarised in Table 3. Weaning percentages (number of lambs weaned/100 ewes mated) were 30, 58, 71, 95 and 95 % for MM, MD, MV, MA and MR ewes respectively. Weaning mass for the respective groups were 15,4, 19,8, 23,2, 23,6 and 23,7 kg while values recorded for ewe productivity (kg of lambs weaned/ewe/year) were 4,7, 11,6, 16,5, 22,3 and 22,4 kg respectively. From these results it is evident that wool production potential and reproduction are inversely correlated. Apparently the specific environmental conditions in this study restricted reproduction in some types of ewes, especially those with a high fibre production potential. However, under more favourable conditions where environmental constraints are less, it may be expected that the difference in reproduction between high and low fibre producing breeds will be reduced. Under favourable conditions it is thus also possible that growth and reproduction of high fibre producing animals may equal that of mutton breeds.
Although an apparently negative correlation was obtained between wool production potential and hardiness in sheep used in the present experiment, low production and reproduction should not only be ascribed to high genetic potential for fibre production. Most mutton breeds in South Africa originate from indigenous breeds whereas mutton-wool breeds originate from crosses between these indigenous breeds and the Merino from the Mediterranean. Whether these differences in origin are linked to genes controlling the specific physiological mechanisms underlying hardiness, remains to be established.
The results of the present study confirm observations made in practice, namely, that high wool-producing sheep have lower reproduction and growth rates and reach sexual maturity at a later stage when kept in sub-optimum environments.
HERSELMAN, M.J. & KING, B.R., 1993. The effect of plane of nutrition on wool production, body mass and pleat development in adult Merino rams. Karoo Agric (in press).
Karoo Agric, Vol. 5, No 1, 1993 (8-10)