Does drought stress improve fiber digestibility?

Gonzalo Ferreira

This is a frequent response from farmers when I ask about the effects of drought stress on corn silage quality. Further, some of the more prestigious ruminant nutrition textbooks state that drought stress improves digestibility. The issue is that increased digestibility is a vague concept that does not specify if greater digestibility refers to higher dry matter digestibility, improved fiber digestibility, or both. Throughout the last few years, I have been interested in learning whether improved corn fiber digestibility due to drought stress is a myth or a fact.

First, it is fair to say that most, if not all, published studies evaluating the effect of drought stress on forage quality used grass or legume pastures, and none of them evaluated corn harvested for silage. In 2018, in collaboration with colleagues from the University of Idaho, our research team performed a study in which five corn hybrids were subjected to abundant or restricted water supply while growing in the dry conditions of Idaho.

Opposite to the thought that drought stress makes fiber more digestible, we observed that fiber digestibility of stem internodes was slightly lower for water-restricted than for water-abundant corn (39.1% and 42.2% 30-hour in vitro neutral detergent fiber digestibility [IVNDFD], respectively). In the case of leaf blades, water supply had no effect on fiber digestibility as both treatments averaged 53.6% 30-hour IVNDFD.

To follow up, in 2019, we performed a greenhouse study evaluating the effect of water stress on fiber digestibility and observed that drought stress had minimal effects. Conversely, fiber digestibility was sensitive to genotype (for example, brown midrib versus conventional hybrids). These studies provided no evidence to confirm the concept that drought stress improves fiber digestibility.

Confounding factors

Given these results, it is fair to ask what facts support the claim from farmers that drought stress increases the quality of the silage and, hence, improves milk production. As a professional, I have learned to be humble enough to respond, “I don’t know.” However, I do have a few hypotheses that may explain this myth.

My first hypothesis centers on the forage-to-concentrate ratio of the diet. From my field experience as a dairy nutrition consultant, reducing the forage-to-concentrate ratio of the diet is a means to stretch corn silage inventories when going through a dry growing season. In this regard, a reduction of the forage-to-concentrate ratio of the diet is confounded with higher concentrate levels being fed.

If this is the case, then is the greater milk production attributed to the improved digestibility of the drought-stressed silage or to the greater inclusion of concentrates in the diet? Hopefully, you can perceive by now how easily these two factors can be confounded and, hence, lead us to erroneous conclusions.

My second hypothesis relies on the harvest “panic effect” caused by drought. Without judging, I have often seen farmers anxious to harvest their cornfields earlier than expected when going through a drought-stressed growing season. This is true especially when drought is accompanied by hot days of very low relative humidity. Under these conditions, the leaves of the corn plants start to dry and become brownish and brittle. This is a scenario that motivates farmers to harvest too early, thinking that forage quality is declining.

Harvesting at an earlier phenological stage results in the crop accumulating fewer growing degree units than the same crop harvested later and under normal conditions. This generally results in corn silage with higher fiber digestibility. As such, it’s fair to ask if the increased fiber digestibility is due to the drought conditions or the early harvesting time. Similar to my first hypothesis, these two factors are confounding and, hence, can easily lead to an erroneous conclusion.

More to learn

A follow-up thought to this discussion is why we know so little about the effects of drought stress on corn silage fiber digestibility. My best answer is that it is very difficult to perform controlled studies that induce drought stress. This has been the aim of our research work at Virginia Tech for the last few years, and we will still work on this in the near future. It is our conviction that the dairy industry needs this information, even if we go “against the current” and challenge the myth.

When going through a drought season, I want to leave some thoughts to consider. Overall, monitor the crop and evaluate alternatives when the drought occurs. A drought at vegetative stages, but with abundant precipitation around silking, may result in corn silage that’s still very good quality if grain development occurs.

A different scenario is when drought occurs around silking, which may result in poor kernel development. In this case, I recommend waiting 14 days after silking before any harvesting decision is made. Rainfall within two weeks after silking can make a huge difference in grain development.

Finally, if drought stress extends throughout the reproductive stages of the crop (before tasseling to three weeks after silking), then an earlier harvest might be the most appropriate decision, especially if drought is accompanied by hot days and low relative humidity.