**2.1. Physical characteristics**

rations because of competitively priced wheat compared with other feed grains. Wheat is high in starch and protein, and low in fiber [1]. Similar to other cereal grains, feed wheat is primarily used a source of energy in the form of carbohydrates. Available energy expressed as either digestible energy or metabolizable energy per unit of dry matter is similar to corn, but higher than other major grains [2]. The majority of wheat has traditionally been used as feed for poultry and swine because the use of large quantities of feed wheat in ruminant rations has a number of concerns and problems. The primary problem appears that wheat starch is highly fermentable and its rate of digestion in the rumen is greater than that of corn and barley grains that increase the risk of rumen acidosis in animals fed high-grain diet [1, 3]. Furthermore, the physical characteristics and nutrient content of wheat can vary considerably due to different types of wheat: soft, hard and durum, and the growing conditions [4]. Therefore, nutrient contents of wheat, variety and growing conditions need to be considered when quality of feed wheat is assessed. In addition, kernel processing is another key factor affecting wheat quality. Whole wheat kernels are poorly digested due to the resistance of the seed coat to attack by rumen microbial or host enzymes. Therefore, wheat grains require processing to break seed coat. The digestibility of wheat can be increased from about 60% if fed whole to over 90% if properly processed. However, attention to processing is crucial for best results when feeding wheat since over processing can result in digestive upset and several factors (kernel hardness and uniformity, and processing methods) can significantly affect the processing results. Finally, level of deoxynivalenol in wheat grain may affect feed value. The deoxynivalenol, commonly referred to as vomitoxin, is a mycotoxin that may be produced in wheat and barley grain infected by Fusarium head blight or scab [5]. Ruminants are generally considered to have more tolerance to Fusarium toxins such as deoxynivalenol than poultry and swine because of the detoxifying potential of rumen microbes. However, little information is available on the effects of deoxynivalenol level in wheat on performance and health of ruminant animals. The levels of deoxynivalenol vary with type of wheat and the tolerance to deoxynivalenol also differs depending on type of animals (dairy versus beef cattle) or production levels [5]. The present review will be focusing on assessing physical and chemical characteristics of wheat, kernel processing and levels of deoxynivalenol related to

116 Global Wheat Production

feeding value for ruminant animals based on the published results.

**2. Physical and chemical characteristics of feed wheat**

Wheat quality is a complex term, and it depends upon the end-use. For feed wheat, the quality should be associated primarily to energy level and protein content as well as its digestibility in the digestive tract of animals. The feed wheat quality can be assessed with its nutrient content such as content of starch or protein, and physical characteristics like thousand kernel weight, test weight, and kernel hardness which are easily measured and commonly used by commercial feedlots and feed mills to assess the quality of wheat as animal feed. Wheat cultivars can be classified by planting season (Winter and Spring), hardness of the grain (soft and hard), and color (red and white). Winter wheats are winter hardy, so they are planted in the fall. In the spring they resume maturation and are harvested early in the summer. Spring wheats are planted in the spring and harvested late in Test weight, also referred to as volume weight or bulk density, is one of the criteria used to assess the quality and grade of cereal grains. Test weight is a measure of density and it is measured volume of grain expressed as kilograms per hectoliter (kg/hL; **Table 1**). The test weight is easily measured and commonly used by feed mills to assess the quality (energy value) of grain as animal feed. Heavy kernel typically has larger plumper kernels with greater starch and lower fiber concentrations than light weight grain, but there is no consistent relationship between test weight of grain and animal growth performance [6]. The test weight is influenced by genetics, agronomic management and environment conditions. Each kernel is composed of the bran (seed coat), the germ, and the endosperm. The endosperm is primarily comprised of carbohydrates (starch) with protein woven among the starch granules. Starch weighs more than protein, thus the tighter the starch molecules are woven within a kernel, the greater the test weight. The test weight in itself is not a good indicator of feeding value to ruminants. Generally, cattle fed wheat with test weights greater than 70 kg/hL will have similar feed efficiency. However, usually only wheat with low test weight (<70 kg/hL) will be used as feed, and its energy values will be lower and feed efficiency is poorer. Wheat with low test weight may be more difficult to properly process compared to wheat with higher test weight. In general, kernel size is more variable with low test weight grains, making processing more difficult. When the roller is set properly for larger kernels, many small kernels will pass through the rollers unprocessed. When set properly for smaller kernels, many larger kernels will be processed too finely.

Kernel hardness, defined as the resistance of the kernel to fracture, is a critical factor affecting grain processing and product quality for feed wheat. The wheat industry has applied this trait for decades to differentiate grain quality and market classes. Hard wheat kernels require more force to fracture than do soft wheat grains, which is caused by differences in the endosperm starch-protein matrix [7]. Kernel hardness can be measured as a hardness index using a Single Kernel Characterization System [8], milling energy using a Comparamill [9] or by particle size analysis [10]. The particle size analysis is the measurement of particle size after feed processing, and is a commonly used method to evaluate end-use quality [11]. Soft wheat fractures easily with small particle size and limited starch damage, while hard wheat produces larger particles with increased starch damage. Kernel hardness is closely related to


**Table 1.** Variation with variety of physical characteristics of wheat; kernel hardness was estimated by particle size index, lower number indicates harder kernel.

the wheat process affecting the starch damage, particle size and process quality. The grain hardness is therefore one of the important distinguishing factors in the wheat evaluation for grain quality and plays an important role with regard to the suitability of processing. Kernel hardness is measured on a scale from 0 to 35 with durum variety being the hardest (0) and soft white spring the softest (35) (**Table 1**). The increased kernel hardness is generally associated with a decrease in the rate of starch digestion, likely because the protein protects starch granules from microbial digestion. Kernel hardness could be a particularly important property as hard kernels may be more susceptible to shattering and generating the fine particles that are often associated with rumen acidosis [12] and bloat in ruminants fed high-grain diet [13]. It has been found that ruminant performance is significantly influenced by particle size of the feed consumed, and a negative relationship between feed particle size and rumen dry matter digestibility of grains was reported [14].
