The effect of grinding size and amylase enzyme supplementation on potential ruminal and total tract starch digestion of maize in dairy cows

Abstract

Although the market price of maize depends on environmental and political factors, thus deeming it variable, South Africa uses maize as the primary energy concentrate in ruminant diets, providing the energy for high-performance animals. Producers recently experienced the severity of the impact of these environmental factors, with maize prices reaching a record high during 2016 due to droughts from 2014 to 2016, decreasing the milk-to-feed price ratio in South Africa to a critical level. Since international milk prices still have not recovered after the 2014 global price crash, it is critical to formulate diets accurately and purposefully for optimal production. One factor that can significantly affect this goal is finding ways to reduce the wastage of nutrients in the digestive tract.

Many factors, including genetics, cultivar, variety, geographical location, year, climatic conditions, and agronomic practices, directly influence the nutrient value and digestibility of grains. In addition to these production parameters, the ratio of amylose to amylopectin, which makes up the type of starch contained in the endosperm of grains, and the particle size of milled grains have a substantial effect on the digestibility of the grain component in feed. The encapsulation of maize starch particles in a bed of hydrophobic proteins called prolamin primarily influences the digestibility of the starch in maize. The type of starch is the second factor that affects starch digestibility since amylose is less digestible than amylopectin because of its molecular structure. It is commonly accepted that finer ground maize leads to less starch in the manure, thus indicating improved starch digestion.

The digestibility of grains is directly altered by the amylose-to-amylopectin ratio of grains. In-vitro rumen digestibility increased as the amylose content of grains decreased. Amylose has tighter intermolecular bonding between starch molecules than amylopectin. Commercial feed enzyme development is a radical innovation in dairy cow nutrition. Ronozyme®. Rumistar is an α-amylase enzyme specifically developed to improve starch digestion in the rumen of ruminants. This study aimed to determine the efficiency and the interaction between maize processing and the addition of exogenous enzymes and to provide additional insights into these two most important factors that affect ruminal starch disappearance and the post-ruminal digestibility of starch in ruminants.

This trial was conducted in two phases, assessing total tract starch digestion in the first phase and ruminal digestion in the second. The first phase of the trial was conducted on a commercial dairy farm. One diet was fed throughout the trial period, with the maize component ground into three different milling sizes (coarse maize with a mean particle size of >3 mm, fine maize with a mean particle size of <3 mm, and micro-milled maize with a particle size of <1 mm). The cows were randomly allocated to two treatment groups. The feed of one group was supplemented with an additional exogenous amylase enzyme, while the other group served as the control. This study reported the impact of different particle sizes in the maize fraction of the feed and the addition of an amylase enzyme on milk yield, milk composition, and the manure nutrient profile. Phase Two of the trial was conducted as a cross-over design with four rumen cannulated cows at the University of Pretoria’s experimental farm to determine the rate of starch disappearance over time. Each cow served as a repetition of the trial, and the study was executed in two periods, with one sampling day per period. Before the start of the study, two of the cows were adapted to the enzyme treatment by adding the enzyme directly into the rumen via a cannula twice a day for 21 days before the 24-hour in-sacco trial to allow the cows to adjust to the diet and conditions. Ruminal pH and temperature were measured and recorded at every enzyme insertion. The treatments were inverted for the second round. A seven-hour, in-vitro starch digestibility assay was run with an adapted rumen fluid mixture for the different treatments. Ruminal volatile fatty acid production was measured for the different treatment runs.

In the first phase, which focused on total tract starch digestibility, no significant differences were found in milk yield when the diets with different maize particle sizes supplemented with amylase enzyme were compared. Milk fat was not affected by either the supplementation of the amylase enzyme or the particle size of the maize component in the feed. However, a significant interaction was found between treatment and particle size (P<0.05). The feeding of coarse maize resulted in higher milk fat production in the control group that received the base diet with no supplemental enzyme, but the effect was suppressed when the amylase enzyme was added. Particle size significantly affected milk protein, with a smaller particle size resulting in an increased milk protein percentage (P<0.05).

The particle size of the maize component in the diet significantly affected the faecal starch content, with less starch in the faeces of the cows eating the finer ground particles (P<0.05). Manure neutral detergent fibre was significantly affected by the maize particle size and enzyme interaction. Overprocessing effects seemed to reduce the neutral detergent fibre content of micro-milled maize in conjunction with the supplemental enzyme. Across the particle sizes, the mean protein content of the manure was significantly lower for the enzyme treatment, suggesting reduced hindgut fermentation as a result of improved rumen fermentation.

In the second phase, ruminal starch degradability was measured using a seven-hour in-vitro digestibility assay. Analysed digestibility of coarse maize was half that of fine maize, with the digestibility of micro-milled maize being an additional 25% higher than fine maize.

As expected, in the in-sacco digestibility assay, digestibility curves showed that the maize particle size had a significant impact on the soluble fraction A, which increased with the degree of processing (P<0.005). The particle size did not affect the insoluble fraction B. The fast digestible fraction was lower with the supplemental enzyme, but total tract digestibility was improved (P<0.05). This corresponded with the volatile fatty acid results, showing increased volatile fatty acid production with the supplemental enzyme across all maize particle sizes. Propionic acid production was raised, and the acetate-to-propionate ratio was reduced.

There is no doubt that the use of exogenous enzymes with amylolytic and proteolytic activity in diets with a high starch amylase content for ruminants is already under way although the precise mode of action and the limitations of the metabolic system in grains are not yet well understood. More research is necessary to understand all the factors that are influencing and being influenced by these enzymes in order to utilise them for the benefit of the animal and the producer.