Miner Institute:Replace starch with nonforage fiber to beat heat?

During months of high heat stress, total dry matter intake for lactating dairy cows often decreases and meal frequency is reduced. However, the amount consumed per meal increases, resulting in what’s commonly called ‘slug-feeding’. Slug feeding negatively impacts ruminal fermentation. It results in an increased load of highly fermentable starch and a subsequent reduction in fiber digestion and rumen pH, potentially to the point of sub-acute ruminal acidosis.

Amid heat stress periods it’s a common practice to partially replace feeds containing high concentrations of starch such as grains with non-forage fiber sources (NFFS) such as beet pulp, citrus pulp, dried distillers grains plus solubles, soybean hulls, or wheat middlings. These NFFS contain less starch and more NDF compared with grains. A common belief is that replacing grains with NFFS in diets will lead to improved rumen function and an increase in rumen pH. However, does the inclusion of NFFS at the expense of grains actually increase rumen pH? And how does the inclusion of NFFS compare with the inclusion of forage at improving rumen pH?

Figure 1. Effect of dietary starch on rumen pH when starch was replaced by NFFS (panel A) or forage (panel B).

To gain a better understanding of the effect of replacing dietary starch with NFFS or forage on rumen pH from the available literature, I compiled a data set of 38 studies published from 1993 to 2014 where dietary starch was reduced by the partial replacement of starch with NFFS or forage. Figure 1 (panel A) compares the actual dietary starch content (% of DM) with rumen pH when starch was replaced with NFFS (66 treatment means). I anticipated that average rumen pH would decrease with increasing dietary starch, resulting in a negative slope. Surprisingly, average rumen pH was unaffected by dietary starch content. How does this data compare when starch is replaced with forage? Figure 1 (panel B) compares the actual dietary starch content (% of DM) with rumen pH when starch was replaced with forage (54 treatment means). As expected, average rumen pH decreased as dietary starch content increased.

Why is there a difference in rumen pH response when NFFS compared with forages replace starch in the diet? One reason is the difference in fiber digestibility; the rate of fiber digestibility in the rumen is greater for NFFS compared with forage. Also, due to small particle size, the physical effectiveness of fiber in NFFS is lower than the physical effectiveness of fiber in forage. This can result in less rumination per unit of DM, lower saliva production, and therefore reduced rumen buffering capacity for NFFS compared to forages. Furthermore, dry NFFS absorb more liquid compared with forages. Increased absorption of liquid by NFFS reduces the liquid passage rate out of the rumen. About 10-15% of volatile fatty acids produced in the rumen from fermentation leave with the liquid fraction, helping to reduce the total rumen acid load and increase pH. The absorption of rumen liquid by NFFS results in greater retention of volatile fatty acids in the rumen.

Therefore, is replacement of starch with a NFFS the best strategy during periods of heat stress? The available data suggests it may not be. Dairy producers and nutritionists that want to improve rumen function and increase rumen pH during times of high heat stress or incidences of slug-feeding should consider replacing dietary starch with forage, assuming that DMI will not be further limited due to rumen fill.

Fonte: dairyherd