Livestock plays an important role in the economy of Pakistan. There are about 72 million heads of large ruminants, playing a vital role in rural economy of country,providing meat and milk to the masses. Withthe increase in human population, meat and milk demands are increasing. Thereis a majorcontribution of beef in total meat productionand cattle have a major share in total beef production. The meager feed resources are major factor for compromised growth performance of our livestock. Feed additives are feed ingredients of non-nutritive nature that stimulate growth, improve efficiency of feed utilization, and also beneficial for health. Products that improve feed efficiency are particularly important because feed costs are a major expense in animal production.
Yeast is aunicellular eukaryotic organism much different from bacteria. Among yeast Saccharomyces cerevisiaespecies are important and currently well accepted in ruminant feed. Incorporation of yeast (Saccharomyces cervesiae) culture to animal feed has become a common practice in ruminant feed. Many yeast culture based products have been shown to affect dry matter intake, stabilizing rumen pH and digestibility of different nutrients (Callaway and Martin, 1997).
The main purpose of suchadditives in ruminant feed is to prevent rumen flora disorders and disturbances. The inclusion of yeast increases consumption of dry matter, utilization of fiber, increase in average daily gain and yeast cells, and also increases absorption of phosphorus, magnesium, calcium, copper, potassium, zinc and manganese.

Due to the greaterconcerns with antibiotics and other growth stimulants in the animal feed industry, research of other feed additives, such as direct-fed microbial (DFM), has increased.DFMs are naturally occurring living microorganisms which are supplemented to enhance microbial population of rumen, and to stabilize rumen PH. DFMare frequently used in milk replacers for gastrointestinal health, welfare, as well as improving average daily gain, daily dry matter intake, and feed efficiency. Practice of these supplements in calves as a preventative measure has increased from 13.1% to 20% from 1996 to 2007 in USA (USDA, 2008).
Basic rumen studies shows that yeast apparently does not affect the digestibility, it only alters the degradation curve causing reduction in lag phase before digestion commences, thereby increasing the rate of degradation (Williams et al. 1991). It was also observed that inclusion of yeast may increase cellulolytic microorganisms in rumen. Birkelo and Berg (1994) observed that a yeast culture additive improved performance of yearling cattle fed corn-based finishing diets containing less than 10% roughage.Similarly, yeast addition was found to improve rumen fermentation by enhancing the bacterial population, soultimately increase the growth of ruminants (Beauchemin et al. 2003).Yeast culture addition increasedaily dry matter intake and average daily gain (Cole et al. 1992; Mir and Mir, 1994).
According to Blake et al. (1993) and Girard et al. (1997) yeast culture noticeably increase the cellulolytic activities of rumen microorganisms in such a way that they increase their total numbers, improve fiber digestion, decrease lactate accumulation, lessen the concentration of oxygen in rumen fluid and improve utilization of starch supplied in the feeding ration. In this way they influence (inhibit) the rate of volatile fatty acids production and, thus, increase the permanency of rumen environment and improve the intensity of digestion. Yeast culture has also directly enhanced rumen fungi, which may improve fiber digestion (Chaucheryaset al. 1995).
Yeast culture addition impact on ruminal lactic acid metabolism; prevent the accumulation of lactic acid in the rumen when they are fed on yeast supplementation. Sullivan and Martin (1999) described that yeast culture supplementation improved the utilization of lactate and digestion of cellulose. Yeast culture may improve ruminal fermentation because they are able to diminish excess oxygen (Newbold, 1996).So they provide optimal environment for fermentation. They amplified the number of rumen protozoa and NDF digestion in calves fed straw-based diets (Plata et al. 1994). Yeast culture has also been revealed to encourage acetogenic bacteria in the occurrence of methanogens (Chaucheryaset al. 1995), which may result in more effective ruminal fermentation.
The effect of altered doses of yeast (S. cerevisiae) (0, 3, 6 and 12 g of yeast/day respectively) on the lactating performance of Holstein dairy cows was described by Nikkhah et al. (2004). They concluded that the live yeast(LY) had a favorable effect on the rumen health. Other available data indicated that in the rumen fluid of animals receiving supplements of LY the total content of volatile fatty acids , the percentage of propionic acid (Sullivan and Martin, 1999), acetic acid were enhanced (Nursoy and Baytok, 2003) and the total numbers of ruminal bacteria were also significantly increased (Sune, 1998; Alshaikh et al. 2002; Kamra et al. 2002).
The positive effect of yeast may be due to increased dry matter intake reported byWohlt et al. (1991) and Williams et al. (1991).The increase in average daily gain and other productive parameters can be the function of increased dry matter intake and outflow rate of digested material from rumen. The dietary supplementation of yeast culture showed significant increase in degradability of roughage in 6 h (P<0.05) after live yeast addition(Ando et al. (2004).Moreover it is stated that yeast also contribute in maintaining rumen pH, reducing the risk of acidosis, improve rumen metabolism by stabilizing anaerobic rumen environment. It also regulates the digestibility approach, a positive influence on feed efficiency. It also maintains balance of ruminal ammonia concentration,improve feed efficiency in young calves and ultimately increase daily gain in cattle calves. The yeast supplementation in ruminant diets is being practiced at commercial herds both beef and dairy, especially where high concentrate rations are in use.
Based on the existing knowledge of live yeast supplementation and the inadequateinformation on the supplemental use of yeast in calves feedand effects on growth performance, under local conditions, the objective of present study was to determine the effects of yeast supplementation in non-descript male cattle calves on growth.