Maximizing the use of forages suited for the local environment is both economical and sustainable. It’s not, however, without its challenges.
By Rick Lundquist, Ph.D.
Dairies in the southern states have come a long way maximizing the use of summer grasses. We used to think we had to buy expensive imported alfalfa hay to get good production. Digestibility of warm season grasses has improved with better plant genetics and more intense management. Maximizing the use of forages suited for the local environment is both economical and sustainable. However, it’s not without its challenges.
I only need to dust off my favorite graduate school textbook by Peter Van Soest to remind me of what influences the digestibility of forages, especially in tropical and subtropical regions. From the plant’s perspective, its survival depends on the nutrients it stores (which are highly digestible) and the structural components that protect it against the environment and predation, such as lignin (which is not digestible).
The age of the plant, or time after re-growth in the spring or interval between cuttings, is the most important variable influencing the digestibility of warm-season grasses. Several factors influence the relative maturity of the forage at a particular age. Since corn and sorghum are also C4 grasses, these factors can affect their digestibility as well.
1. Temperature. Higher temperatures promote more rapid growth, but reduce digestibility due to increased lignification of the stem and the leaf midrib in grasses.
2. Light. Longer days and more light intensity promote better digestibility due to photosynthetic production of sugars and conversion of nitrates to amino acids. The sun is more intense in the southern states, but the nights are longer. Plants use up digestible nutrients during the night. Angle of the sun and long days have much more influence on improved forage digestibility in northern latitudes during the growing season. Cloudy humid days, common during the rainy season in the South, generally lower forage quality.
3. Water. Lack of water retards the growth and maturity of forages, which tends to increase digestibility while decreasing yield. Alternatively, rain and humidity increase yield, but reduce digestibility.
4. Nitrogen. Nitrogen fertilization increases the yield and protein content of grasses. But the increase in protein content may displace soluble carbohydrates. So, other than what is needed for adequate growth, excessive nitrogen fertilization actually depresses forage digestibility.
Weather conditions can be a challenge during the summer. There’s not much you can do to control the environment. But you can try to balance yield and age of the forage, keeping in mind the factors that influence the growth rate and maturity of the plant. Adjusting cutting intervals will improve forage digestibility and milk production.