E.R. Ørskov, The Rowett Research Institute, Bucksburn, Aberdeen, Scotland
Introduction
Effect of straw variety on animal performance
Causes of differences among varieties
Differences between years and variety × year interaction
Causes of year-to-year variability and variety × year interaction
Methods of routine analysis
Conclusion
References
Discussion
The recognition that straws from different cereal varieties vary considerably in nutritive value is relatively recent (Pearce et al, 1979; Kernan et al, 1984; Bainton et al, 1987; Tuah et al, 1986), although straws have been fed to ruminants for millenia. While farmers have observed differences between types of cereal straw, e.g. wheat and barley, there has been no clear recognition of the extent to which varieties differed. There is no doubt that this late recognition of very important differences is a result of analyses which were inadequate to describe differences, e.g. gross chemical analysis. Assessment of varieties by in vivo digestibility trials was too cumbersome. In recent years biological methods, such as in vitro digestion in rumen fluid or incubation of the substrate in nylon bags in the rumen, have revealed large differences in nutritive value in the absence of large differences in gross chemical composition (Ramazin et al, 1986).
A further improvement in biological methods to estimate nutritive value is reported by Ørskov et al (in press). They observed that by describing substrate disappearance from nylon bags over time, i.e. by withdrawing nylon bags at different time intervals, it was possible to assist also in predicting voluntary intake because both the disappearance rate and extent of digestion could be described using the exponential equation developed by Ørskov and McDonald (1979). The formula p=a+b (1-e-ct), where p is degradation at time (t), has been extensively used. With 10 straws varying widely in degradation characteristics it was possible, using a multiple correlation of a, b and c, to predict intake and growth rate very accurately and account for 90% or more in variability (Ørskov et al, in press). It can be seen that (a+b) is an expression of the potential extent of digestion while c is the rate constant of the disappearance of the insoluble but fermentable fraction (b).
It is a valid question to ask whether the effects of differences among varieties are large enough to be reflected significantly in animal performance. An example is given in Table 1. Three straws were fed to steers and their intake and growth rate recorded. Although there were only small differences in the extent (a+b) of digestion of Golden Promise and Corgi, the rate constant for Corgi was about 60% greater than that of Golden Promise. As a result the steers ate more and grew better on Corgi straw than on Golden Promise straw. Gerbel straw was inferior to the other varieties for both extent of digestion and rate constant. The data taken from Ørskov et al (in press) reflect that nutritive value cannot adequately be described by a single static measurement but that a combination of the rate constant and potential extent of digestion can account for most of the variability in intake of straws.
Table 1. Effect of barley variety on straw intake and performance by cattle and degradation characteristics of the straw.
|
Variety |
Extent |
Degradation rate constant |
Growth rate |
Intake |
|
(a+b) |
h-1 |
(g d-1) |
(kg DM d-1) |
|
|
Gerbel |
38.9 |
0.0337 |
106 |
3.43 |
|
Golden Promise |
55.5 |
0.0303 |
198 |
4.43 |
|
Corgi |
52.1 |
0.0483 |
400 |
5.16 |
The different morphological fractions, i.e. leaves and stems, vary considerably in nutritive value, particularly in temperate cereals such as wheat, oats, barley and rye in which the leaf and leaf sheath portion may be up to twice as digestible as stems. This is not so with rice straws (Walli et al, in press; Bainton et al, 1987), in which the leaves are generally slightly less digestible than stems. Some average values of morphological proportions are given in Table 2. It is clear that differences in the amount of leaf that adheres to the straw at harvesting can substantially change the nutritive value and some of the difference between varieties can be explained by differences in the leaf:stem ratio. However, Ramazin et al (1986) found that leaf-to-stem ratio only accounted for about 20% of the difference in nutritive value between two varieties. The largest differences were due to differences in degradability of both stems and leaves. They also showed that stems from straw benefit much more from chemical treatment than leaves, implying that stemmy varieties benefit most from chemical treatment.
Table 2. Average morphological fractions of various cereal straws.
|
Fraction |
Percentage of fraction in |
|||
|
Rice |
Wheat |
Oats |
Barley |
|
|
Leaf + sheath |
65.6 |
33.9 |
31.0 |
45.1 |
|
Internodes |
20.2 |
46.5 |
56.9 |
44.6 |
|
Chaff |
6.2 |
13.8 |
4.6 |
4.5 |
|
Nodes |
8.0 |
5.7 |
7.3 |
5.7 |
It is obviously of interest to plant breeders to know whether the ranking of straws according to quality is consistent between years and also, to some extent, whether there is a large between-year variation.
During the past 3 years the nutritive value of straw from several varieties of winter barley, spring barley, wheat and oats has been studied at the Rowett. In both wheat (Table 3) and spring barley (Table 4) there were significant differences among varieties and among years (P<0.001). The variety × year interaction was more significant in spring barley (P<0.01) than in wheat (P<0.05). However, ranking was generally similar among years. Two years' results from oats show a great variation between years but only small differences between the six varieties tested so far (Table 5).
Table 3. Differences in degradability of winter wheat straw among varieties and years.
|
Variety |
48-hour degradability (%) |
||
|
Year 1 |
Year 2 |
Year 3 |
|
|
Aquila |
35.0 |
42.4 |
45.2 |
|
Avalon |
36.9 |
47.7 |
48.3 |
|
Boxer |
37.0 |
38.9 |
50.5 |
|
Brigand |
38.7 |
41.9 |
54.9 |
|
Brimstone |
37.6 |
45.4 |
54.9 |
|
Brock |
37.8 |
46.4 |
56.4 |
|
Galahad |
37.7 |
46.1 |
55.2 |
|
Longbow |
38.4 |
43.5 |
52.0 |
|
Norman |
36.4 |
44.7 |
51.9 |
|
Renard |
37.0 |
42.1 |
47.8 |
|
Significance of difference: |
|
|
Between varieties |
P<0.001 |
|
Between years |
P<0.001 |
|
Variety × year interaction |
P<0.05 |
In winter barley there were highly significant differences between the varieties and between years but the variety × year interaction was not significant (Table 6). In comparison with Table 4 for spring barley it can be seen that the nutritive value of winter barley straw is consistently lower than that of spring barley straws.
Table 4. Differences in degradability of spring barley straw among varieties and years.
|
Variety |
48-hour degradability (%) |
||
|
Year 1 |
Year 2 |
Year 3 |
|
|
Celt |
46.4 |
39.6 |
45.5 |
|
Corgi |
58.9 |
46.2 |
52.7 |
|
Doublet |
61.1 |
45.9 |
57.9 |
|
Golden Promise |
40.3 |
34.4 |
41.5 |
|
Golf |
46.9 |
37.7 |
42.6 |
|
Heriot |
54.4 |
42.1 |
50.6 |
|
Klaxon |
48.8 |
34.3 |
39.9 |
|
Significance of difference: |
|
|
Between varieties |
P<0.001 |
|
Between years |
P<0.001 |
|
Variety × year interaction |
P<0.01 |
Table 5. Differences in degradability of oat straw among varieties and years.
|
Variety |
48-hour degradability (%) |
|
|
Year 1 |
Year 2 |
|
|
Ballard |
51.5 |
36.7 |
|
Cabana |
51.1 |
38.7 |
|
Dula |
46.5 |
38.3 |
|
Leanda |
47.6 |
36.7 |
|
Matra |
48.3 |
40.7 |
|
Trafalgar |
46.7 |
37.6 |
Table 6. Differences in degradability of winter barley straw among varieties and years.
|
Variety |
48-hour degradability (%) |
|
|
Year 1 |
Year 2 |
|
|
Gerbel |
32.7 |
43.4 |
|
Halcyon |
34.3 |
43.5 |
|
Igri |
35.6 |
44.4 |
|
Kaskade |
38.8 |
48.6 |
|
Magie |
41.0 |
49.5 |
|
Marinka |
37.5 |
44.6 |
|
Maris Otter |
32.9 |
48.1 |
|
Nevada |
36.3 |
47.9 |
|
Opera |
40.4 |
44.8 |
|
Panda |
37.7 |
46.9 |
|
Ripkin |
42.5 |
50.2 |
|
Pirate |
37.1 |
46.1 |
|
Significance of difference: |
|
|
Between varieties |
P<0.001 |
|
Between years |
P<0.001 |
|
Variety × year interaction |
NS |
Some of the variation between years can be accounted for by differences in the content of soluble nutrients. For instance the mean values for the 48-hour degradability of the wheat varieties were 37.2, 43.9 and 51.7% in year 1, 2 and 3, respectively. The respective content of soluble nutrients for the 3 years were 3.4, 11.4 and 16.1%. Thus it appears that most of the variability could be accounted for in this instance by differences in the content of water-soluble nutrients. For the spring barley straws in 1985 and 1986 the solubility cannot account for a large proportion of the differences except perhaps to make the ranking more consistent. In Table 7 the spring barley straws have been given where the water soluble component was subtracted. It is clear that the ranking here was very consistent for the 2 years.
Table 7. Effect of year on ranking order of spring barley varieties using the potential degradability less the water-soluble material.
|
1985 |
1986 | ||
|
Variety |
Potential (%) |
Variety |
Potential (%) |
|
Doublet |
39.9 |
Doublet |
51.1 |
|
Corgi |
37.1 |
Corgi |
45.3 |
|
Heriot |
36.7 |
Heriot |
44.3 |
|
Golf |
34.4 |
Celt |
37.5 |
|
Celt |
33.8 |
Golden Promise |
37.0 |
|
Golden Promise |
30.2 |
Golf |
36.8 |
|
Klaxson |
26.8 |
Klaxson |
32.1 |
Based on the results presented it is evident that there is a component of variety × year interactions the causes of which are not clear and which need further investigation. It is possible that variation in soluble materials or leaf-to-stem ratio could be responsible for this. It is abundantly clear however, that varieties vary and on the whole the ranking is such that it can with confidence be selected for. In the work carried out with more than 100 varieties a significant correlation between grain yield and straw quality has never been noted, nor has there been any significant relationship between N content and nutritive value. Thus it should be possible to select for improved nutritive value of straw without reducing grain yield. Whether environmental stresses such as drought will produce interactions with variety is not known with certainty but resources should be directed to solve these problems rapidly so that plant breeders can with confidence select for straw quality as well as grain yield and quality.
Although gross chemical analysis cannot adequately predict nutritive value, several biological measurements can (Table 8). The most promising purely laboratory method is cellulase digestion of the material after neutral-detergent extraction. However, it is difficult to standardize the mixtures of enzymes present in commercial preparations. Both in vitro measurement using rumen fluid and nylon-bag incubation can be used to provide reliable information for plant breeders on ranking of nutritive value.
While variety × year interaction exists, the ranking order is altered only little. The differences among varieties need to be exploited by plant breeders, particularly in areas where straw is a very large component of feeds for ruminants. The possibility of increasing digestibility of straws or stovers by 10 to 20% can have immense implications for animal production by small farmers in many regions of the world.
Table 8. Correlation between chemical and biological parameters of straw and intake and digestibility by steers.
|
Measurement |
Dry-matter intake |
Growth rate |
|
Crude fibre |
-0.70 |
-0.57 |
|
Neutral-detergent fibre |
-0.79 |
-0.77 |
|
Acid-detergent fibre |
-0.86 |
-0.79 |
|
Lignin |
-0.75 |
-0.72 |
|
Cellulase digestion of neutral-detergent fraction |
0.88 |
0.95 |
|
Near-infrared (calibrated to in vitro measurement) |
0.86 |
0.87 |
|
In vivo digestibility at maintenance in sheep |
0.70 |
0.77 |
|
In vitro digestibility |
0.86 |
0.90 |
|
Multiple correlations of a, b and c from exponential equation using nylon bags |
0.89 |
0.96 |
Bainton S J. Plumb V E and Capper B S. 1987. Botanical composition, chemical analysis and cellulase solubility of rice straw from different varieties. Animal Production 44:481 (Abstract).
Kernan J A, Coxworth E C, Crowle W L and Spurr D T. 1984. The nutritional value of crop residue components from several wheat cultivars grown at different fertilizer levels. Animal Feed Science and Technology 11:301-311.
Ørskov E R and McDonald I. 1979. The estimation of protein degradability in the rumen from incubation measurements weighted according to rate of passage. Journal of Agricultural Science (Cambridge) 92:499-503.
Ørskov E R. Reed G W and Kay M. (in press). Prediction of intake by cattle from degradation characteristics of roughages. Animal Production.
Pearce G R. Beard J and Hillard E P. 1979. Variability in the chemical composition of cereal straws and in vitro digestibility with and without sodium hydroxide treatment. Australian Journal of Experimental Agriculture and Animal Husbandry 19: 350-353.
Ramazin M, Ørskov E R and Tuah A K. 1986. Rumen degradation of straw. 2. Botanical fractions of straw from barley cultivars. Animal Production 43:271-278.
Tuah A K, Lufadeju E, Ørskov E R and Blacket G A. 1986. Rumen degradation of straw. 1. Untreated and ammonia treated barley, oat and wheat straw varieties and triticale straw. Animal Production 43:261-269.
Walli T K, Ørskov E R and Bhargava P K. (in press). Rumen degradation of straw. 3. Botanical fractions of two rice straw varieties from India. Animal Production.
Van Soest: Firstly, I want to correct a criticism made of the neutral-detergent system. We do not use cellulases; all our rates are done using a modified Tilley and Terry in vitro technique where the second stage is replaced by extraction with neutral detergent. Secondly, your rate constants and extents look low relative to our values. My worry is that nylon bags are very prone to microbial contamination, which leads to an underestimate of the real degradation.
Ørskov: The degradability of our straw is actually very high, usually between 40 and 60%.
Van Soest: We find in the average wheat straw that true digestibility is in the order of 60 to 65% when the apparent digestibility is in the forties, the difference being metabolic and microbial matter. The question arises, how did you wash your bags and wash the microbial contamination out?
Ørskov: We washed the bags in a washing machine by a rinsing process.
Van Soest: That is probably inadequate to remove the attached bacteria.
Ørskov: It may well be but if we could find a better method to predict intake and growth rate I would adopt it. If we can get correlations of 0.96 using the extent and rate constant that is really not too bad. If there was a laboratory method that would do it better I would adopt it.
Van Soest: I have to make the criticism of the rate constants. You have values as low as 2%, which we have never seen. Microbial attachment reaches a maximum at 12 to 18 hours, when contamination is greatest. Then you get lysis and cannibalism by micro-organisms and contamination declines. Contamination at times at the top of your logarithmic curves will bias your slopes and cause error.
Ørskov: I do not think we have shown rate constants as low as 2%. The rate constants are between 3 and 5% per hour, which is what one would expect for these sorts of materials. We have only found rates as low as 2% in stems.
Thomson: Did these straws come from a field station or farmers' fields.
Ørskov: The varieties we used in feeding trials were taken from big fields. The other materials were from small plot variety trials.
Thomson: Are you surprised that the nodes have such a high degradability?
Ørskov: Yes I am. I thought they were going to be the lowest, but they are always higher than stems.
Pearce: You stated that poor-quality straws always respond better to chemical treatment than high-quality straws. Is that because low-quality straws have a higher proportion of cell wall?
Ørskov: No, it's probably due to a higher stem content. One has to be careful of generalising, but on the whole the stems respond much better than leaf.
Pearce: If you have a higher proportion of stem you certainly have a higher proportion of cell wall, and chemicals react with cell walls rather than cell contents.
Schildkamp: You mentioned that it was difficult to make recommendations to breeders. In wheat straw you showed that leaves are more digestible than stems. Would you not make the recommendation to select for leafiness?
Ørskov: I would definitely recommend selection for leafiness, but it is not sufficient, because leaves also vary in value. If I were asked to recommend a technique for ranking varieties for nutritive value, I would recommend a relatively short incubation period, because you are not after a number that means something to animal nutritionists. If you use a short incubation period of maybe 24 hours you will get an idea of the trajectory of the curve, both the rate and extent.
