Kirby begins with a description of the mechanism of amylase-enhanced corn. The amylase is located in the kernel and, once activated by temperature change, works to increase the digestibility of the starch. A small amount of activation is thought to occur during silage fermentation, with further activation once it reaches the rumen. This paper evaluated digestibility and milk production in cows fed corn silage made from a hybrid with the amylase-enhanced gene compared to the same hybrid without the genomic enhancement. (4:51)
The experiment was designed as a factorial with four treatments combining the two different types of silage with either 25% or 30% starch in the total diet. Only the silage was amylase-enhanced, not the corn grain that was fed. Kirby expected the amylase-enhanced silage group at 25% starch to perform best because he expected some subclinical rumen acidosis and potentially some feed intake issues at the higher dietary starch concentration. (9:09)
The experiment was eight weeks long, consisting of a two-week covariate and then a six-week feeding period with 11 cows on each of the four treatments. Blood and milk samples were collected weekly. Total tract digestibility was evaluated twice over those six weeks, once soon after silage harvest (approximately 40 days) and again six weeks later to evaluate whether the impact or efficacy of the enhanced starch enzyme changed over time. (13:29)
One surprising result was that the two silages had different in vitro NDF digestibility during week one of the feeding period. The amylase-enhanced silage had higher fiber digestibility even though the genomic enhancement is for starch digestibility. Kirby is unsure of the mechanism but hypothesizes that the amylolytic enzyme may free up some simple sugars or polysaccharides that allow microbes to have greater action and more energy available to digest fiber. By week six, the in vitro NDF digestibility of the two silages was essentially the same (15:09)
Kirby mentions that if he could do this experiment again, he would do a longer-term study for 12 or 18 weeks and start feeding the silage as green chop right away to evaluate if ensiling takes away some of the benefits of the amylase-enhancement. (19:02)
From the production data, the alpha-amylase enhancement didn't provide a benefit, but a fairly consistent benefit of additional dietary starch was observed, including increased feed efficiency, increased energy-corrected milk, and increased milk protein yield with few to no interactions in these results.
Kirby also would like to have some data looking at the impacts of these types of diets on fresh cows since the cows in this experiment averaged 160 days in milk at the start of the feeding period. (24:11)
The alpha-amylase-enhanced silage did not impact body weight, body condition, or feed intake. Kirby anticipated that the higher starch-fed cows would experience greater body weight gain in the later lactation period, but he observed the opposite. At the end of the study, an interaction was observed for feed intake where the high starch cows ate a little less - around three pounds. This resulted in a difference in feed efficiency for the high starch cows, where their intake decreased, but they maintained milk production. (25:29)
Bill asks if the feed efficiency data was adjusted for the difference in body weight change, but Kirby responds that it was just gross feed efficiency, milk over feed. Bill wonders if that adjustment would make the two groups’ feed efficiencies closer together, where it’s more of a difference in how nutrients are being partitioned rather than a difference in feed efficiency (27:26)
Another follow-up experiment Kirby would like to conduct is another factorial with the enhanced silage variety and the non-enhanced combined with a higher and lower rumen degradable protein concentration. (35:16)
Bill wonders if this experiment was conducted with silage at a later maturity, say 40-42% dry matter, would the amylase have a bigger effect? Kirby thinks there is a chance that as the kernel dries down, the amylase may have a greater impact. (38:53)
Kirby’s take-home messages for the audience are to consider the amylase-enhanced gene as an approach to bridging an inventory challenge gap from year to year and not to avoid dietary starch due to worries about subclinical inflammation.
Kirby’s paper can be found here: https://www.journalofdairyscience.org/article/S0022-0302(23)00309-0/fulltext
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