Three general types of nutritional management systems are typically used in dairy production: confinement systems with totally mixed rations (TMRs), confinement systems in which concentrates and forages are fed separately, and pasture-based systems.
Totally Mixed Rations
TMRs have all dietary components included in a single uniform mixture. TMRs have the nutritional advantage over other types of feeding systems, in that fiber and nonfiber ration components are delivered in uniform proportions throughout the feeding period. This minimizes fluctuations in rumen pH and promotes healthy rumen conditions, even at relatively high rates of energy intake.
Adequate management of TMR systems requires a means of accurately weighing each component of the mixture and a mixer capable of incorporating forages and concentrates into a uniform product. Several types of suitable mixers with self-contained weighing devices are available commercially. Many mixers are limited in their capacity to mix long-stemmed forage into the diet, thus limiting the use of dry hay in TMR diets. This means that maintaining particle size adequate to deliver sufficient effective fiber can be a challenge with TMR diets. Limiting the duration of the mixing process helps maintain sufficient effective fiber if silages have adequate particle length initially and the particles are not further comminuted during the mixing process. Sieves to monitor particle size in TMR mixes are available commercially. See table: Particle Size Recommendations for Totally Mixed Rations a Particle Size Recommendations for Totally Mixed Rations a for recommended particle size distribution in TMR mixes, based on a popular sieving system.
Frequent monitoring of dry-matter concentrations of moist feeds is particularly important in the management of TMR diets. This is because diets are formulated based on the nutrient concentrations of the feeds on a dry-matter basis, but the ingredients are mixed based on their moist weight. Therefore, accurate dry-matter concentration values are critical to ensure the nutrient profile of the final diet is as intended. Routine analysis of the TMR mixture for major nutrient fractions, such as moisture or fiber, is useful in assuring that the ingredient nutrient profiles and proportions are consistent with the intended diet formulation and are stable over time.
Management of feed delivery and availability is another important aspect in the use of TMR mixes. Cattle should have continuous or nearly continuous access to feed. Adequate bunk space is important, with recommendations for optimal linear space usually 45–60 cm/animal. Bunks should be cleaned daily, and the orts weighed so that daily feed intake for the group may be calculated. To assure optimal feed availability, orts should be 2%–4% of the total amount fed. Particle length of the orts should be measured to assure that sorting of the feed, which usually results in large particles being left behind, is not occurring.
For effective use of TMR diets, cows must be separated into feeding groups: minimally, a lactating and a nonlactating group, and optimally, two or more lactating cow groups and two dry cow groups. One dry cow group should comprise those cows in the first 4–6 wk of the dry period, and the other those in the 2–4 wk before calving. Diets for lactating cow groups should be formulated with a lead factor, meaning the diet should be balanced for more milk production than the average of the group. This is to ensure that the nutrient requirements of the higher-producing cows in the group are met. Optimal lead factors depend on the number of feeding groups and their stage of lactation. When two groups of lactating cows are used, feeding for >20% of the average production is frequently recommended.
Unmixed or Component-fed Rations
Confinement feeding systems in which concentrates are fed separately from forages are traditional in many areas of the world. Advantages of this system relative to the TMR system include the lack of need for specialized mixing and feed delivery equipment and the ability to adjust concentrate feeding amounts to the needs of individual animals. A major disadvantage is that starch and other nonfiber carbohydrates are generally provided in a small number of meals per day, typically at milking time. Thus, there is the possibility for large fluctuations in rumen pH, which may impair fiber digestion and contribute to poor rumen health. Systems have been developed to avoid large concentrate meals. These include computer-delivered diets in which cows are electronically identified and a computerized feeding apparatus delivers small, individually programmed meals throughout the day.
In addition to the potential for fluctuations in rumen pH, another disadvantage of component feeding systems is the inability to monitor forage intake. This is because there is no direct need to weigh the amount of forage offered if it is not being mixed with other diet components. Thus, there is no means to accurately adjust concentrate intake to match fluctuations in forage consumption. This becomes a particular problem in hot weather during which forage consumption may be reduced substantially while concentrate consumption is less affected. This leads to alterations in the intended proportions of fiber and nonfiber carbohydrates in the diets.
Pasture-based Feeding Systems
Modern means of pasture management have evolved to optimize use of forage resources in pasture systems. Such systems require intensive management of pasture for optimal dry-matter and nutrient yields and for optimal feeding and nutrition of high-producing modern dairy cows. To achieve these objectives, paddocks must be rotated frequently, so forages are consumed at an optimal stage of growth and overgrazing does not occur. Pastures are typically divided into paddocks via the use of easily moved electric fences. Cattle are rotated through paddocks as forages reach stages of growth optimal for both dry-matter yields and nutrient composition.
From a nutritional standpoint, the three major challenges of pasture-based dairy systems are maintaining favorable rumen fermentation conditions, maintaining adequate dry matter intake, and meeting energy and protein requirements. Managing rumen health and dietary fiber adequacy can be as challenging with pasture-based systems as with other dairy feeding systems. Lush, rapidly growing pasture grasses with high energy and protein density typically have low NDF concentrations. Thus, rumen fermentation conditions, particularly pH, may be a problem. Supplementing the pasture with dry forages to maintain adequate effective fiber concentrations is frequently necessary.
Dry matter consumption in pasture systems may limit nutrient intake because maximal consumption rates are lower than in confinement systems. This will limit energy intake, thus requiring supplemental energy sources to be fed to achieve high milk production. Milk production rates in unsupplemented pasture feeding situations are seldom >25 kg/day. Energy supplements may include starches such as cereal grains or highly fermentable fiber sources such as grain byproducts. Protein, and particularly sources of RUP, may also need to be supplemented. Protein concentrations in pasture grasses may be high but are generally highly rumen degradable.