Silage quality

The nutritive value of grass silage produced in Norway is highly variable (TINE, Statistical reports 2002-2008). This may be challenging when trying to maximize the use of grass silage in diets to productive animals rather than increasing the proportion of concentrate.

The quality of forages manipulated by the stage of grass maturity has been thoroughly studied throughout the history of animal science because of its importance to the performance of ruminant based production systems (Rinne, 2000). The primary goal in silage production is to close the gap between the feeding value of the original crop and that of the resulting silage. The parameters affecting silage quality can be divided into crop related factors and fermentation related factors (Charmley, 2001). Crop related factors are mainly related to maturity stage at harvest, and thereby the chemical and physical changes in plants. Digestibility of grass is to a large extent determined by the stage of plant maturity. Delayed harvest will decrease the content of digestible organic matter per kg dry matter (DM) in silage (value) (Thomas et al., 1981). D-value is probably more representative of the maturity stage at harvest than other chemical entities such as neutral detergent fiber (NDF) or crude protein (CP). The average daily decrease in silage D-value is reported to be 4.8 g/kg DM by Rinne et al. (1999), 5.4 g/kg DM by Randby (2003), and 5.0 g/kg DM by Kuoppala et al. (2008). Deinum et al. (1981) observed that D-value declined

3 the faster the further north the grass was growing when comparing the development of timothy at latitudes from 51 to 69°N. Rinne et al. (1997; 1999) found a curvilinear decrease in D-value when comparing silages harvested at four stages of maturity, and suggested that cumulative temperature explains D-value better than the date of harvest or the chemical composition of the grass. The general changes of chemical composition of the grass at delayed harvest are decreased content of CP and increased content of NDF. Lignification of the cell wall fraction increases with plant maturity and reduces digestibility, since lignin interacts with other cell wall components to provide structural integrity and is resistant to hydrolysis by rumen microorganisms (McDonald et al., 1991).

The proportion of total N as protein in fresh herbage is 75 to 90 %. In preserved silage, however, less than 50 % of total N is present as protein, mainly due to proteolysis by plant enzymes and microbial activity to non-protein N (NPN). The NPN fraction is mainly made up of peptides, free amino acids, amides and nitrates (McDonald et al., 1991). Non-protein N is highly degradable in the rumen and is rapidly converted to ammonia (Givens and Rulquin, 2004). The extent of proteolysis will increase during a long wilting period under humid conditions, and during ensiling if the temperature in the silo is high and the fall of pH during ensiling is slow

(McDonald et al., 1991). There will be an increased concentration of fiber in silage compared with the herbage, which may be due to proportionally larger losses of other chemical

components during ensilage by respiration, fermentation, and/or effluent losses of soluble nutrients (Rinne et al., 1997). This results in a lower digestibility of the ensilage than the fresh herbage (McDonald et al., 1991).

It is essential to have a good microbial fermentation process to produce high quality silage. A good fermentation process is dependent on the type and quality of the forage crop, and on the harvesting and ensiling technique. Sugars are the main substrates for both respiration and fermentation, whereof the water soluble carbohydrates (WSC) are more important than the structural carbohydrates. However, structural carbohydrates can be degraded during the ensiling period by acid hydrolysis or microbial breakdown (McDonald et al., 1991). Silage additives are used to ensure that the lactic acid bacteria dominate the fermentation and to inhibit microbial growth. The additives will lead to a rapid fermentation (quick lowering of pH), decreased

4 proteolysis and decreased content of acetic and butyric acid and ethanol (McDonald et al., 1991).

A purpose of using silage additives is also to improve the nutritional value of silage and to minimize ensiling losses (McDonald et al., 1991). The extent of fermentation is correlated to the DM content and especially at low DM content (<20-30 %) it is important to use silage additives that inhibits clostridial growth (McDonald et al., 1991). The fermentation quality criteria

presented by Saue and Breirem (1969) is used as an assessment of fermentation quality by the commercial feed analysis laboratory (Eurofins AS, Moss, Norway). Table 1 presents this fermentation quality criteria for low DM (<25 %) grass silage and the average DM content, pH, NH₃, lactic acid, acetic acid, and butyric acid for silage samples analyzed from 2002 to 2008 in Norway (TINE, Statistical reports 2002-2008). The concentration of fermentation parameters as a mean for all analyzed silage samples is within the criteria of fermentation quality, except of the content of NH3 of total N.

Table 1. Criteria defining good fermentation in low DM (<250 g DM/kg) grass silage, and average DM content, pH, NH3, lactic acid, acetic acid, and butyric acid for silage samples analyzed from 2002 to 2008 in Norway (TINE, Statistical reports 2002-2008).

DM

Silage fermentation characteristics may influence feed intake, and of the individual fermentation parameters the total acid concentration was found by Huhtanen et al. (2002) to be the best predictor of silage dry mater intake (SDMI). Feed intake and fermentation products can modify the profile of nutrients absorbed from the digestive tract and therefore affect milk yield and composition (Huhtanen, 1993). Both yields of milk, energy corrected milk (ECM), milk fat and protein are found to decrease with increasing extent of fermentation. Reduced milk fat content with increasing lactic acid or total acid in silage may be attributed to the reduced ratio between acetic and butyric (lipogenic) acid and propionic (glucogenic) acid in the rumen, as propionic

5 acid is the main end-product of ruminal lactate fermentation (Huhtanen, 1993). Milk protein content decreases as microbial protein synthesis in the rumen decreases. The metabolism of silage fermentation products in the rumen provides little or no ATP for microbial synthesis.

Therefore restricting fermentation would yield more energy for rumen microbes and support greater rates of microbial synthesis (Chamberlain, 1987).