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In many countries, forage production varies seasonally. Depending on climatic conditions, this can lead to a deficient supply of forages over short or long periods of time. To overcome the negative effect of such variations, forages are harvested in the growing season and preserved as feed for periods in which forage is temporarily unavailable. In intensive dairy production systems, ensiling is the main method of forage conservation.

Ensiling is a forage preservation method based on fermentation by lactic acid bacteria under anaerobic conditions (Oude Elferink et al., 2001). Lactic acid bacteria (LAB) ferment the water-soluble carbohydrates (WSC) into lactic acid. The increase in lactic acid concentration reduces the pH of the ensiled forage, which in turn inhibits the activities of enzymes and microorganisms. Lactic acid bacteria are ubiquitous, and forages may hold 102 to 105 colony forming units (CFU) of LAB per g.

Due to respiration and fermentation of substrates during the ensiling process, forage preservation results in dry matter (DM) loss and consequently in the loss of energy and other nutrients (e.g., proteins and WSC). A rapid decline in pH not only reduces these losses in nutritive value, but also reduces the risks of LAB being crowded out by other detrimental microorganisms like clostridia, yeasts and moulds. Therefore, silage additives have been introduced to facilitate the ensiling process and/or to reduce silage pH.

In Western Europe, two main trends in silage making can be observed: 1) an increase in the proportion of grass ensiled, and 2) an increase in the use of silage additives. Both trends will be discussed as well as the effect of ensiling and silage additives on the nutritive quality of forage.


Silage production

Originally, preserved forages were used during periods of reduced availability of homegrown feeds. In intensive dairy production systems involving cows in year-round confinement, the use of preserved forages throughout the year has become common practice. Preserved forages can be sampled and analyzed before being fed and hence guarantee a better and more balanced nutrient supply for high-yielding dairy cows.

The desire to improve animal performance, which requires a more balanced energy and nutrient supply, legislation for maximum mineral output, use of milking robots, and desire for easier feed management are the main reasons for a reduction in grazing and an increased proportion of forage preservation over the last decade. In the Netherlands, for example, the production of grass silage has increased from 3.5 billion to more than 5.0 billion kg of dry matter over the last 10 years (Figure 1), despite a 7% reduction in total grassland area over this period (CBS, 2006).




Figure 1. Annual amount of grass silage dry matter harvested in the Netherlands (CBS, 2006).



With a higher proportion of ensiled forages in rations for dairy cattle, more attention must be paid to their nutritive value and consequently to ensiling technology. Although various technologies have been developed to reduce losses during ensiling, it should be realized that to date there are no ensiling techniques that can improve feeding values compared with the original, fresh product. Therefore, the main prerequisite for high quality silage is a high quality crop. Besides agronomic factors, like grass species, grass variety, fertilizing and irrigation, harvesting and wilting techniques contribute to the quality of crop to be ensiled.

Usually, the dry matter concentration of the forage is increased by drying (wilting) the crop in the open field for a period of time. Higher dry matter content inhibits bacterial growth, thus reducing the rate and extent of fermentation, and consequently, lactic acid production. The higher dry matter content inhibits growth of enterobacteria and other detrimental species such as clostridia. High dry matter concentration also prevents leaching of nutrients with the effluent. However, efforts to achieve high dry matter content should be in balance with the length of the field period, because during this period, valuable nutrients can be lost due to respiration and leaching.

Various wilting techniques have been developed to increase water evaporation and to shorten the field period. Techniques vary between many passes over the grass swath with a mower conditioner tedding the grass, followed by windrowing, and cutting the grass and spreading it thinly over almost the whole field area in one operation (Bosma, 1995). Such techniques are aimed at cutting the grass in the morning and ensiling it in the afternoon of the same day during dry weather conditions. Laceration of grass leaves is yet another method to increase water evaporation, but also increases the risk of detrimental microorganisms, especially at low WSC concentrations (Knický, 2005).

Wilting to increase dry matter concentrations can impede adequate compaction of the silo, resulting in less anaerobic conditions.


Use of silage additives

The ensiling process can be divided in four phases (Oude Elferink et al., 2001):

- Phase 1, the aerobic phase, in which oxygen is still available and plant enzymes are active. It is important to shorten this phase, because the hydrolytic plant enzymes remain active, resulting in carbohydrate dissimilation and proteolysis. Proteolysis results in the formation of ammonia, which will inhibit reduction in pH.

- Phase 2 is the phase in which anaerobic fermentation takes place. Lactic acid is produced and the pH drops. At the end of this phase, growth of microorganisms is inhibited as well as the activity of plant and microbial enzymes.

- Phase 3 is the stable phase in which no further degradation of organic matter takes place.

- Phase 4 is the phase when the silage is opened for feeding (feedout phase).

To obtain a high nutritive value product it is essential to shorten phases 1 and 2, because during these phases organic matter is hydrolyzed by plant and microbial enzymes. Although a certain amount of organic matter fermentation is required for the formation of lactic acid, the resulting degradation of protein will reduce the protein quality of the ensiled forage.

Until recently, no additives were used when forage crops were harvested under good weather conditions, resulting in a dry matter content of at least 35%. However, during the last decade, the use of silage additives has increased. Several reasons underlie their increased use, such as better quality additives, more information provided about additives by their producers to the farmers, a reduction in dry matter concentrations to shorten the field period and to improve compaction, and the need to minimize risks in silage making.

Silage additives can shorten phases 1 and 2, especially in silages with relatively high dry matter. In a lab-silo experiment, we tested the effect of Sil-All^®4x4 (Alltech Inc.) on the fermentation process in grass silages. Sil-All^®4x4 is a silage additive containing four strains of bacteria and four hydrolytic enzymes.

In May and September, good quality grass was harvested and wilted to dry matter concentrations of 20 to 50%. The wilted grass was ensiled in 1 L Weck jars, inoculated with or without Sil-All^®4x4 (10 mg of inoculant per kg of grass), and stored at 18 to 22°C for 3 months. At day 90, the inoculated silages had the highest lactic acid concentration (60 to 140 g/kg of DM and 60 to 120 g/kg of DM in May and September, respectively).

In the control silages, the concentration of lactic acid varied from 40 to 130 and from 50 to 110 g/kg of DM in May and September, respectively. The inoculated silages showed the fastest and largest pH drop. The pH difference between the control and inoculated silages was the largest for the high dry matter silages (Figure 2). In general the inoculated silages reached a stable pH value of 3.8 to 4.2 between days 14 and 28. The control silages reached a stable pH value of 4.2 to 4.6 after 28 days.

The number of enterobacteria on the grass at ensiling varied from about 104 to 106 CFU per g. The number of enterobacteria dropped during phase 2, with silages inoculated with Sil-All^®4x4 always showing the fastest drop, and reaching the detection level (<102 CFU per g) at day 14, whereas the control silages reached the detection level at day 28. Thus, it can be concluded that even at high dry matter concentrations, Sil-All^®4x4 had a positive effect on the fermentation process.




Figure 2. Effect of Sil-All^®4x4 on pH reduction in grass silage.



In general, ensiling has a relatively small impact on the energy value of the forage. This is because lactic acid and other volatile fatty acids have a high energy value for the animal. Although some dry matter is degraded to water and CO2, the energy concentration (per kg of DM) does not drop dramatically (around 5%). However, it should be realized that unlike carbohydrates, lactic acid is not a substantial energy source for rumen microorganisms. Because rumen microorganisms are an important protein source for the dairy cow, high concentrations of fermentation products may have a negative effect on the protein supply for ruminants.

Besides microbial protein, rumen undegraded feed protein (RUP) is a protein source for ruminants. However, during ensiling a variable proportion of protein is hydrolyzed into non-protein nitrogen. Non-protein nitrogen components are ammonia, amines, amino acids and peptides. The ammonia concentration of silages is often used as a parameter for protein degradation and the risk for detrimental microorganisms. Petit and Tremblay (1992) observed that the soluble (non-protein) nitrogen components increased from less than 30% of total crude protein in wilted grass to more than 70% in grass silage.

In protein evaluation systems it is assumed that these soluble (non-protein) nitrogen components are rapidly and totally degraded in the rumen and hence do not contribute to RUP. In our experiment with Sil-All^®4x4, the proportion of ammonia was substantially reduced (-2% to -4% of total N) and that of insoluble nitrogen was slightly increased, which indicates an increased concentration of true protein. However, the effect of ensiling on the estimated amount of RUP reaching the small intestine was usually less than expected from the changes in soluble protein. In the study of Petit and Tremblay (1992), RUP dropped from 20% to 10% of total crude protein due to ensiling. This smaller decrease was due to changes in the rate of degradation of the insoluble protein fraction, which partly compensated for the reduction of this fraction.

A higher proportion of true protein in grass silages may also have an impact on microbial growth. Although rumen microorganisms can grow on non-amino nitrogen sources, some bacteria grow better in the presence of amino acids or peptides. Thus, an increase in insoluble protein may also have a positive effect on the efficiency of rumen microbial protein synthesis. We studied this in vitro, using the gas production technique of Cone et al. (1996).

The silages from our study with Sil-All^®4x4 were incubated in vitro to determine the time at which the rate of fermentation of insoluble organic matter was at its maximum. At this time, the concentration of purines (expressed as RNA equivalents) was analyzed as a parameter for the microbial mass. In the incubation vials with inoculated silages, the concentration of RNA equivalents was higher (P<0.05) than in the control, being 18.8 vs. 17.7, respectively. In addition, dry matter concentration had a positive effect on the amount of RNA equivalents in the incubation vial. No interactions between dry matter concentration and additive or between date of harvest and additive or between dry matter concentration and date of harvest could be detected.

From the results we concluded that Sil-All^®4x4 had a positive effect on microbial growth in vitro. From the absence of interactions, we concluded that the positive effect was similar for different dry matter levels and for grass harvested in May and September.

Silages treated with Sil-All^®4x4 gave a 6% higher amount of RNA equivalents.

Extrapolating this relative increase to an in vivo scenario suggests that, per kg of ingested (fermentable) organic matter, 6% more microbial protein will be synthesized. At a silage intake level of 16 kg of dry matter (about 8.9 kg of fermentable organic matter), this would amount to 53 g of DVE (intestinal digestible protein), being the protein requirement of an extra 1 kg of fat- and-protein-corrected milk (Tamminga et al., 1994).

Better utilization of feed protein enables farmers to improve the precision of protein feeding, which will reduce the excretion of excess nitrogen and subsequently contribute to a lower emission of nitrogenous compounds in our environment.

References

Bosma, A.H. 1995. New systems for wilting grass. In: Grassland into the 21st Century: Challenges and Opportunities – Proceedings of the 50th Anniversary Meeeting of the British Grassland Society (G.E. Pollott, ed). BGS Occasional Symposium S., Harrogate, UK, pp. 247-249.

CBS. 2006. http://statline.cbs.nl/StatWeb/.

Cone, J.W., A.H. van Gelder, G.J.W. Visscher and L. Oudshoorn. 1996. Influence of rumen fluid and substrate concentration on fermentation kinetics measured with a fully automated time related gas production apparatus. Anim. Feed Sci. Tech. 61:113- 128.

Knický, M. 2005. Possibilities to improve silage conservation. Effects of crop, ensiling technology and additives. Doctoral Thesis, Swedish University of Agricultural Sciences, Uppsala, Sweden.

Oude Elferink, S.J.H.W, F. Driehuis, J.C. Gottschal and S.F. Spoelstra. 2001. Silage fermentation processes and their manipulation. In: Silage Making in the Tropics with Particular Emphasis on Smallholders. FAO Plant Production and Protection Papers 161, FAO, Rome, pp. 196.

Petit, H.V. and G.F. Tremblay. 1992. In situ degradability of fresh grass and grass conserved under different harvesting methods. J. Dairy Sci. 75:774-781.

Tamminga, S., W.M. van Straalen, A.P.J. Subnel, R.G.M. Meijer, A. Steg, C.J.G. Wever and M.C. Blok. 1994. The Dutch protein evaluation system: the DVE/OEB-system. Livest. Prod. Sci. 40:139-155.

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