Understanding Forages and Feedstuffs

Dr. Brady Campbell, Assistant Professor, OSU State Small Ruminant Extension Specialist

Understanding the quality and value of forages within your operation will greatly improve your bottom line. With the recent green up in fields that I’ve seen across the countryside, today we’ll focus a bit more heavily on forages as it is the primary feed source used in sheep production globally, as well as the one that is most commonly overlooked and underappreciated. Ruminant species in grazing situations need to maximize forage digestion in order to meet their daily energy and protein requirements. Forages with unknown nutritive values can be dangerous as a producer may unintentionally limit or reduce and animals’ ability to meet their daily energy and protein requirements. Additional factors to consider are forage species, maturity at harvest, and lignin concentration. Remember, forage quality, nutritive values, and species grown/available will differ based upon geographic location. This section will highlight factors that should be considered to improve your forage feeding system.

Because of their versatility, forages play an important role in modern sheep production systems because they can be grazed or harvested and stored as fermented or dry feeds for later use. Forages are unique because of their structural carbohydrates content in the form of cellulose that can only be digested by rumen bacteria. In return, the rumen bacteria provide ruminants with a large proportion of daily crude protein intake, therefore, diets that are greater in forages may result in less protein available on a per pound basis when compared with grain-based diets and thus require additional supplementation.

From an animal perspective, increased levels of forages in the diet result in the production of a greater proportion of acetate, a VFA produced and used in fatty acid synthesis. It is recommended that ewes be offered high quality forages during late gestation and early lactation so the acetate can be used to increase fat content and energy concentration of milk for newborn lambs. This is just one example on why it is so important to test your forages! Comparatively, diets containing greater proportions of grain during late gestation and early lactation would result in a greater proportion of propionate, another VFA, which supports the production of milk volume. However, during this critical period, milk volume is not needed as the small size of a newborn lamb will regulate the amount of milk that is consumed. Therefore, it’s more important that colostrum and milk produced during early lactation be energy dense because of limited intake.

Moreover, feeding or grazing forages tends to increase rumination time in our ruminant species. Rumination is an important process to generate saliva for buffering the acidic rumen environment. This buffering function allows the rumen to function properly as fiber-digesting ruminal bacteria may perish once the pH drops too low. Forages also provide fiber, which is essential to proper rumen function and health. Neutral detergent fiber (NDF) is used to describe the bulk density or gut fill of the diet. Diets with high NDF values will limit the intake of the diet. Alternatively, diets with low NDF values will result in a high passage rate. High passage rates are also undesirable as feedstuffs requiring more digestion pass through the system too quickly, thus losing access to nutrients in the process. Therefore, forages that provide a moderate NDF value are beneficial as this allows for a slowing in passage rate and potentially for the animal to better digest the feeds they are consuming (see table listed below).

Table 1. Forage quality parameter benchmarks to establish a quick quality rating using acid detergent fiber (ADF) and neutral detergent fiber (NDF).

Source: Dr. Robert J. Van Saun, DVM, M.S., Ph.D. – PennState Extension, Determining Forage Quality: Understanding Feed Analysis

In other words, forage dry matter intake, palatability, and overall digestibility is affected by plant maturity (concentration of lignin), NDF concentration, and plant structure. It is of importance to note that forages are digested from the inside out, further supporting the benefits of processing forages prior to feeding. As plant matures, lignin concentrations increase as well as a decrease in stem to leaf ratio. A greater proportion of stems and seed heads will further decrease the quality of feed on offer. This in turn will create forages that are ‘bulkier’, thus further decreasing intake overall dry matter intake.