María Villagrasa
Cereal grains are rich in starch and their content depends on plant variety, weather conditions, and agricultural practices. Adult cows have limited ability to chew cereal grains; it is essential then to break down the seed pericarp by means of chemical or physical treatments to improve its digestibility in the digestive tract.
Grain processing can be divided in two: non-thermal processes such as roller and hammer mills, and thermal processes including dry processing (roasting, etc.) and wet processing (autoclave, steam laminate etc.).
Thermal processing, however, has been associated with greater efficiency by enabling better microbial enzymatic digestion. Increasing the availability of starch is the main objective of grain processing. Processing reduces grain particle size and increases the surface area available for microbial colonization and attachment. Combined, these actions increase the speed and degree of starch digestion.
Processing degree has been widely used by nutritionists to manipulate rate and extent of digestion of grains in the rumen. An optimal grain processing aims to strike a balance between maximizing their digestibility and controlling the rate of digestion of starch in the rumen, to increase its utilization and prevent digestive and metabolic upsets.
Once the seed pericarp breaks after processing barley starch degrades in the rumen more easily than corn starch, since the protein matrix of barley is easily solubilized. However, rapid and extensive ruminal fermentation of carbohydrates increases the incidence of digestive disorders.
Thus, the degree of processing can affect feed value, and livestock performance and health. Over-processing grain increases the proportion of fine particles, reduces palatability, increases the grain’s rate of digestion in the rumen and therefore the incidence of digestive disorders such as ruminal acidosis, swelling, laminitis and liver abscesses. On the other hand, under-processing the grain reduces starch availability for fermentation by rumen microbes.
Although there are studies focused on steam-laminated barley feeding, few of them have evaluated the most adequate processing conditions for barley grain. Thus, the objective of a recent study (J. A. Johnson et al., 2020) was to determine how the different processing conditions for steam-flaking affected the availability of barley starch.
Five independent sources of barley were used to assess the effects on the availability of starch depending on the duration of the steam-flaking processing (5, 20 or 35 min) and the density of flakes (high, medium and low density, kg/L).
The composition (DM basis) of the unprocessed barley was starch 61.5%, crude protein 11.2%, and neutral detergent fiber 16.52%. Samples were tested to determine the in vitro digestibility of starch and starch reactive to amyloglucosidase.
The average density of the flakes was 0.47 kg/L (high), 0.37 kg/L (mean) and 0.32 kg/L (low). As expected, flaking modified DM content due to the added moisture in the process in relation to the untreated barley; DM was also affected by flake density, the low-density treatment had the highest DM content, medium density had intermediate, and the high had minimum density.
Steam-flaking had no effect on the starch or crude protein content of processed barley compared to unprocessed samples, but the NDF content increased. Unsurprisingly, the thickness of the flakes decreased by decreasing their density so that the high, medium and low-density treatments had thicknesses of 1.72, 1.34 and 1.11 mm, respectively.
Starch digestibility increased in steam-flaked barley
Duration of the steam-flaking process of more than 5 minutes tended to increase the reactive starch to amyloglucosydase, but the effect was small. Similarly, the digestibility of in vitro starch increased for flaked barley compared to untreated barley, but did not improve further by increasing processing time.
These data suggest that increasing flaking duration by more than 5 minutes provides minimal additional benefit to improve starch availability. Similarly, some authors suggest that the density of flakes rather than laminate time is the main factor affecting the digestibility of starch in barley.
The values of starch reactive to amyloglucosidase (21.4, 12.7, 9.2 and 5.91% for low, medium, high, and raw density, respectively), ruminal degradability (99.2, 97.3, 95.8, and 94.3%), and in vitro digestibility of starch at 7 hours (86.3, 83.8, 78.9, and 75.5%) were higher by decreasing the density of the flakes.
Compared to unprocessed barley, flaked barley and reduced flake density increased the in vitro digestibility of starch. Increasing the duration of the flaking process by more than 5 minutes is not recommended because it does not have an effect.
Reference
J. A. Johnson, K. Burakowska, L. A. Radke, and G. B. Penne. 2020. In vitro evaluation of the effects of steam-flaking conditions on starch availability of barley grain. Applied Animal Science 36:471-479.
© 2020 Dairy Knowledge Center. All Rights Reserved.
