Factors affecting in sacco degradation of dry matter and crude protein in grass silage

The degradability of dry matter and crude protein was studied in 96 grass silages, which were collected from practical farms in different parts of Finland. The degradabilities were determined by the nylon bag technique in sheep on a grass silage and hay (50 : 50 on DM basis) -based diet. Among chemical components the N-freeextracts increased, and the crude fibre decreased the dry matter degradation in the rumen. The correlation between the end-products from silage fermentation and the dry matter degradability was generally negative. The level of the crude protein degradability was significantly increased when the crude protein content in the silage DM was increased. The amount of N03 in the silage DM had a similar effect. The rate of crude protein degradation was regulated mainly by the proteolysis in the silage, e.g. the amounts of NH, and especially water soluble N in the total N of silage. Crude fibre tended to protect crude protein against ruminal digestion.


Introduction
Grass is the most potential feed with reasonably high energy and protein contents, which can successfully be grown and harvest- ed for silage also in the northern countries.Factors affecting the energy and protein value of grass silage have intensively been studied in many countries.Some of the experimental results in Finland have shortly been reviewed by StiAi a (1984).
The increase in the knowledge concerning the protein metabolism and requirements of a ruminant has drawn attention to the importance of the feed energy and protein de- gradability in the rumen.This regards also grass silage.The aim of the present study was to investigate factors affecting ruminal de- gradability of the dry matter and crude protein of the silage.In order to receive the most realistic data, experimental material was col- lected from practical farms.

Material and methods
The material included 96 grass silage sam- ples which were collected from practical farms in 1981 in different parts of Finland.The si- lages were unwilted and preserved mainly with the AIV-solutions (Valio, Finnish Co-operative Dairies' Association), which contain either formic acid (80 %) and orthophosphoric acid (2 %) as AIV 11, or formic acid (27 %) and HCI (22 °/o) as AIV I. AIV II and AIV I were used in 76.5 % and 9.2 % of the silages, res- pectively.A formaldehyde containing additi- ve (Viher-solution, 55 °/o formalin, 30 % ace- tic acid, Farmos Group Ltd) was used in 14.3 % of the silages.The silage samples we- re immediately frozen on a farm and they were sent to the laboratory in insulated boxes.
The DM content of the silage was calculated after drying at + 80 °C overnight.For chem- ical analyses the samples were dried in a vacuum at + 50 °C for 24 hours, and the dried sample for the analyses was milled through a 1.0 mm o screen.Water soluble N was ana- lyzed after the fresh sample had been extracted in distilled water as described by Huida   (1973).
The chemical composition of the silages was analyzed by the standard methods.The silage pH and NH 3 were measured in the effluent pressed from the silage (Heikonen et al.  1979).The water soluble N (WSN) was ana- lyzed by the Kjeldahl method.The reducing sugars were determined according to Somogyi   (1945) and the lactic acid and volatile fatty acids using enzymatic (Anon 1980) and gas chromatographic methods, respectively.The analyses of NG 3 were made by the ionselective electrode.
The degradabilities of dry matter (DM) and crude protein (CP) were determined by the nylon bag technique (Mehrez and orskov 1977)   as explained by Setälä (1983).The tests were made with two sheep on a hay and grass silage diet (1:1 on DM basis).Fresh silage was chopped to the length of less than 0.5 cm, and 5 grams of silage dry matter was placed in each bag.Five bags were incubated in the rumen at the same time.There was a grass silage sample in four bags and a standard hay sam- ple in one bag.Only one replicate for silage/ incubation period was used and for each silage the incubation was performed during one day.
Grass silage samples were incubated for 2,5, 18, and 24 hours.A standard hay sample was always incubated for 24 hours and the degra- dability of dry matter in the hay was used for controlling ruminal fermentations during the incubations.
The degradability of dry matter without crude protein (N-free DM) was also calculated in order to exclude the effect of the crude pro- tein degradation on the DM degradability.The degradability of N-free DM was calcu- lated as follows: DM inc = amount of DM incubated (correspondingly CP inc ) DM res = amount of DM left in the bag after incubation (correspondingly CP res )

Results and discussion
Chemical composition and quality of the silages The average quality data of the silages showed that the quality of the silages accord- ing to e.g.Heikonen et ai. (1979)was relatively good (Table 1).There were, however, great variations between silages if the contents of the reducing sugars and butyric acid are considered.The average crude protein content was lower and the crude fibre content higher than the corresponding values in all the silages analyzed in Finland 1981.The correlations between different chemical components (Table 2) should be regarded as quite typical of grass silages.A high ash content (soil contamination) increased the pH, NHj (deamination), and butyric acid in the si- lage.Moreover, high pH increased proteoly- sis, deamination (NH 3 , WSN) and butyrate fermentation.The latter was closely connected to the yields of propionic acid.If it is assumed that a low sugar content of the silage is a sign of a vigorous fermentation in the silo, it could be concluded that a vigorous fermentation increased proteolysis and deamination in the si- lages.Part of these processes could be explained by a Clostridia fermentation (Ohshima and McDonald 1978) but it must be em- phasized that lactic acid fermentation also cor- related negatively with the content of the re- ducing sugars in the present study.

Comparison between the degradability of dry matter and crude protein
The dry matter and crude protein in the si- lages degraded at different rates in the rumen   3, Fig. 1).Especially during the first five hours the difference was very clear.The degradabilities for DM varied from 10 to 42 °7o and for crude protein from 15 to 75 °7o, respectively (Fig. 1).The variation in the crude protein degradability was much more exten-Table 3. Degradability-% of silage DM, N-free DM, and crude protein in sacco (96 silages).
sive than the variation in the DM degradability.This can have a great effect on the utili- zation of ruminally degradable silage N. If it is assumed that the organic matter of silage is fermented at a similar rate as the silage DM, and the degradability of DM and crude pro-  tein are correspondingly 42 °/o and 75 %, the ratio of the ruminally degradable N (RDN) and fermentable organic matter (OMF) would be 4.6 g RDN/100 g OMF in the present ma- terial.If it is assumed that the efficiency of the utilization of RDN for microbial protein synthesis is 100 %, this value is almost twice as great as suggested for an appropriate value according to the average microbial protein synthesis (2.5 g microbial N/100 g OMF) in grass silage -based diets (Miller 1982, Tho- mas 1982).

Factors affecting the degradability of dry matter
The most important factors affecting the DM and N-free DM degradability were the contents of the crude fibre and N-free extracts in the silage DM (Table 4).The N-free extracts include hemicellulose and sugars which can more rapidly and easily be degraded in the ru- men than cellulose fraction in crude fibre.On the other hand, crude fibre including cellulose and lignin seemed to protect the silage DM against ruminal digestion.
Regarding the correlations it must be em- phasized, that although they were significant (df 94), they accounted only for a very small proportion of the variance.However, the negative correlations between the degradability of N-free DM and pH (and NH 3 ) or the degradability of DM and lactic + acetic acids might indicate that the highest degradability of DM is obtained when the silage is well pre- served without vigorous fermentation and made from a relatively young grass having a low crude fibre content in DM.A decrease in the organic matter degradability in an inten- sively fermented silage was also demonstrated by Cation et al. (1982).
The content of crude protein in the DM af- fected significantly the ruminal degradation rate and level of N-free DM and DM, because crude protein was rapidly and to a great extent degraded in the rumen (see Table 3).The crude protein content affected also clearly the degradation of crude protein.The effect was especially clear if the level of the degradability (after 18-24 hours) was con- sidered (also Pekkarinen et al. 1983).As similar results were obtained with the N0 3 content of the silages it could be suggested that the crude protein content of the silages on the farm was increased by N-fertilization which increases the level of the crude protein degra- dability (Pekkarinen et al. 1983).It seems obvious that, regarding protein degradability, more attention and research should be paid to the use of N-fertilizers for grass.
However, the rate of the degradation was significantly dependent on the proportions of the NH 3 and especially WSN in silage (see Table 5), e.g. on the extent of proteolysis and hence on the quality of silage (also Brett et  al. 1981, Catton et al. 1982).It is known that Clostridia (butyrate fermentation) cause an extensive proteolysis (deamination and de- carboxylation) in the silage (Ohshima and   McDonald 1978).However, proteolysis can also be caused by heterofermentative lactic acid bacteria, and although this may take place to a limited extend (McDonald 1982), silage which is almost continuously fermented cannot therefore be regarded as a good silage.
Moreover, the degradability of silage energy (DM, organic matter) is decreased in an intensively fermented silage.
Crude fibre tended to protect crude protein against digestive processes in the rumen.However, although this is most obviously true, it must be pointed out that while grass matures, the crude fibre content increases and the crude protein content decreases, and this interaction may affect correlation.Moreover, because the ADF-fraction was not determined, it is difficult to say how much ADF- bound and hence poorly degradable nitrogen was included in the crude fibre fraction.One can only speculate that the proportion of ADF-N was low because there were not many vigorously fermented silages which may con- tain larger amounts of ADF-N (Goering et  al. 1972, 1973). 1 WSN 0-39.9 :y = 34.07x 02695WSN 40.0 59.9 : y = 39.76x 02207WSN 60.0-100.0: y = 47.59X 01730 (y = crude protein degradation, %, x = incubation period, hrs) In 3 silages the crude protein degradability vigorous and long-term fermentations lead to was less than 20 % after 2 hours' incubation (see Fig. 1).Any clear explanation for this was not found, as for instance the chemical com- position or the quality of these silages did not clearly differ.It is possible that there was an especially strong attack by rumen microbes on feed particles in the bag and in spite of care- ful washings some microbial material had re- mained in the bag thus contributing to the amount of nitrogen in the residue of silage.This would cause »lower disappearance» of crude protein from the bag during the incu- bations.
To conclude, our results indicate that a decreased ruminal degradation (fermentation) of the silage dry matter while the degradability of the crude protein was increased.
This was most evident when the degradabilities in the rumen with the first hours after feeding were considered.
Regarding the crude protein degradability, it was not possible to calculate the most appropriate crude protein content of the si- lage on the basis of the present data.How- ever, it seems relevant to pay attention to the N-fertilization of grass.The maturity of the grass and its importance in this connection should also be considered.

Fig
Fig. I.Comparison between the DM and crude protein degradabilities (from 2to 24 hours) of the silages.

Table 1 .
The average chemical composition of the si- lages in the present study and in Finland 1981.

Table 5 .
Degradability-% of DM, N-free DM, and crude protein in silages with different WSN contents (WSN = water soluble nitrogen).