Effect of supplementary feeding on growth performance and ruminal fermentation in winter season grazing sheep

The experiment was conducted with twenty-four grazing sheep divided into 3 experimental groups on the basis of body weights, i.e control, optimal and non-optimal groups to study the effect of optimized supplementation on growth performance and rumen fermentation characteristics of grazing sheep in winter. The sheep in control were grazing only, whereas the grazing sheep in the optimal were supplemented with 250 g of an optimal forage mix and 404 g of concentrate (1) the animals in non-optimal were supplemented with 465 g of concentrate (2). Weight gain rate in the optimal increased by 1.2 and 15.3% as compared to that of the non-optimal and control group, respectively. Ruminal fermentation parameters were not signifi cantly different between the optimal and non-optimal groups, however, they were signifi cantly improved compared to the control. These results indicated that optimized supplementation strategies in winter were benefi cial for markedly improving growth performance and rumen fermentation characteristics of grazing sheep .


INTRODUCTION
In northern China, there are vast areas of grasslands in both arid and alpine environments.These are used for grazing animals and support more than 55% of the sheep population in China.Due to seasonal changes, green pasture is usually available for only fi ve months (approximately mid May to mid October) and dry pasture residues and low quality hays are the main sources of fodder for the remaining seven months of each year.Therefore, the nutrient intake is not able to meet nutrient requirements of grazing animals solely from grazing in winter and spring.There is considerable opportunity for improving the supply of available nutrients, especially in winter and spring, through supplementation strategies that stimulates the intake of poorer quality feeds and /or increases the rate and level of rumen digestion (Lu, 1992).
The primary objective of this experiment was to study the effect of an optimized supplementation strategy on growth performance and rumen fermentation characteristics of grazing sheep in winter.

Study site
The study was conducted in semi-desert grassland in Siziwang Banner of Inner Mongolia (41º10'N longitude, 110º20'E latitude, about 1430-1506 m in elevation), where the long-term averages of annual mean air temperature and annual rainfall are 2.9ºC and 200 mm, respectively.According to our spot fi eld analysis the total number of plant species recorded was about twenty species.Stipa brevifl ora Griseb, Artemisia frigida Willd and Cleistogenes songorica Ohwi were the dominant plant species in this area.

Experimental animals and management
Twenty-four sheep were selected and randomly assigned to one of 3 treatment groups on the basis of weight: i.e the control, the optimal, and the non-optimal groups.All groups were grazed on natural grassland from 08.00 to 17.00 h followed by shed feeding at night.Fresh and clean drinking water was provided twice daily.Sheep in control group were grazed only in the whole period while sheep in optimal and non-optimal groups were fed by grazing plus supplementary feeding.From the start of experiment, sheep in the optimal group were supplemented with 250 g of an optimized forage mix (lucerne hay: millet straw: maize straw = 40:10:50) and 404 g the concentrate (1).after coming back from grazing at night.The optimized forage mix was designed and selected by the forage grading index (GI) technique proposed by Lu De-xun (Wang, 2005) in which the optimized forage mixture formula must have the maximum GI value with lowest cost.An equation for calculation of GI was as follows: Then a further selection of the concentrates matched with the forage mixture by incubation in vitro technique as described by Wang (2005) was carried out.Sheep in non-optimal group were supplemented with 465 g of concentrate (2).The experimental diets were formulated to meet nutrient requirements of a 43 kg sheep by NRC (1985) (Table 1).Experimental periods lasted for 45 days.The experimental animals were weighed before feeding and watering in morning by the beginning and end of the supplementation period.

Collection of samples and chemical analyses
Six sheep from each treatment groups were randomly selected.Rumen fl uid samples were collected for six consecutive days by plastic tubes through mouth at 08.00 a.m. then ruminal pH was measured immediately with a portable pH meter (Horiba Cardy Compact pH meter model C-1; Spectrum Technologies, IL). Rumen fl uid samples were strained through four layers of cheesecloth, and fi ltrate were mixed and pooled for each individual sheep and stored (-20ºC) for later analysis.VFA concentrations were detected by using crotonic acid as the internal standard with gas chromatograph (GC-2014, Sh Shzmadzu Janan) and fl ame-ionization detection, NH 3 -N concentration was determined by the colorimetry method (Gao and Feng, 1993).

Statistical analyses
The data obtained were analysed under one way analysis of variance (ANOVA) for comparing the means and signifi cant differences between the experimental SUPPLEMENTARY FEEDING OF GRAZING SHEEP groups were assessed using Duncan's Multiple Range test using SPSS (version 11) statistical package.There was a signifi cant (P<0.05)difference in daily weight gain (DWG) in three groups; DWG both optimized and non-optimized groups were 66.7 g and 54 g, respectively.Whereas daily weight loss in control group was 88.3 g.Body weight loss in case of control group was a clear refl ection of their poor nutritional status as the nutrient intake was not able to meet their nutrient requirements solely from grazing.As shown in Table 2, weight gain rate increased by 15.3 and 14.1 percentile in the optimized and non-optimized groups as compared to that of the control group, respectively.and weight gain rate increased by 1.2% in the optimized group as compared to that of nonoptimized group.These results indicated that nutrients of pasture in winter were very low and not suffi cient to meet the requirements of grazing sheep, which would prevent the achievement of high levels of sheep production, so appropriate supplementation strategies in winter could improve production performance, which was similar to earlier reports in ewes (Chaturvedi et al., 2001(Chaturvedi et al., , 2003)).As seen from Table 2, weight gain rate of animals in the optimized group was higher than those in the other groups.Present study also showed that the optimized supplementation strategies improved markedly growth performance of sheep grazing in winter.

RESULTS AND DISCUSSION
Effect of different treatments on rumen fermentation characteristics in grazing sheep were shown in Table 3.There were no signifi cant differences in rumen pH values between different groups.Rumen pH can be a useful tool for the optimization of ruminal digestion and nutrient supply to sheep.Ammonia-N (NH 3 -N) concentration and total volatile fatty acids (TVFA) concentration were not signifi cantly difference between the optimized and non-optimized groups, however they were higher signifi cantly than that of the control (Table 3).The HU H.L. ET AL. associative effects of feedstuffs may occur when forages with different types, species and qualities are mixed together.The results from the present experiment showed that supplementation of concentrates plus forage mixture or concentrate only to grazing sheep enhanced NH 3 -N and TVFA production, which indicated associated effects between different forages or between forage and concentrate might existed in the optimized and non-optimized groups, especially, the positive associative effects occurred might bigger in the optimized than that of the non-optimized.These results showed that supplementation strategies were benefi cial for improving fermentation characteristics of grazing sheep, and the optimized supplementation was an more benefi cial strategy.

CONCLUSIONS
The results of the present study confi rmed optimization supplementation strategies in winter could improve markedly growth performance and rumen fermentation characteristics of grazing sheep.

Table 1 .
Ingredient and chemical composition of supplementary concentrates for different groups, %DM basis

Table 2
presented the effect of different treatments on growth performance of grazing sheep, during the supplementation period.The body weight change of grazing sheep ranged from 43.40 to 46.4 kg in optimized group, 42.86 to 45.29 kg in non-optimized group, 44.75 to 41.00 kg in control group, respectively.

Table 2 .
Changes of weight gain of grazing sheep in different groups

Table 3 .
Effect of different treatments on rumen fermentation parameters of grazing sheep