Feed intake of dairy animals

Under nearly all practical management conditions, dairy cows and growing dairy heifers are fed ad lib. Thus, voluntary feed intake is the major limitation to nutrient supply in dairy cattle. Feed intake is usually characterized as dry matter intake (DMI) to compare diets of variable moisture concentrations. DMI is affected by both animal and feed factors. Body size, milk production, and stage of lactation or gestation are the major animal factors. At peak DMI, daily DMI of high-producing cows may be 5% of body wt, and even higher in extremely high-producing cows. More typical peak DMI values are in the range of 3.5%–4% of body wt. In mature cows, DMI as a percentage of body weight is lowest during the nonlactating, or dry, period. In most cows, DMI declines to its lowest rate in the last 2–3 wk of gestation. Typical DMI during this period is <2% of body wt/day, with intake rates depressed more in fat cows than in thin ones. Feed intake during this period has an important relationship to postpartum health, with low DMI and associated prepartum negative energy balance increasing the risk of postpartum disease. After calving, DMI increases as milk production increases; however, the rate of increase in feed consumption is such that energy intake lags behind energy requirements for the first several weeks of lactation. Milk production and associated energy requirements generally peak around 6–10 wk into lactation, whereas DMI usually does not peak until 12–14 wk into lactation. This lag in DMI relative to energy requirements creates a period of negative energy balance in early lactation. Cows are at greater risk of metabolic disease during this period than at other times during their lactation cycle. Management and nutritional strategies should be designed to maximize DMI through the period of late gestation and early lactation.

Feed factors also affect DMI. Total ration moisture concentrations >50% generally decrease DMI, although this may be related more to fermentation characteristics than to moisture per se, because high-moisture feeds for dairy cattle are typically from fermented (ensiled) sources. Rations high (>30%) in neutral detergent fiber (NDF) may also limit feed intake, although the degree to which this occurs is related to the source of NDF. Environment also affects feed intake with temperatures above the thermal neutral zone (>20°C [68°F]), resulting in reduced DMI. Monitoring DMI, when possible, is a useful tool in diagnosing nutritional problems in diets of dairy cows.

Carbohydrates

Energy requirements for lactating dairy cows are met primarily by carbohydrate fractions of the diet. These consist of fibrous and nonfibrous carbohydrates. Fibrous carbohydrate proportions are generally measured as NDF and expressed as a percentage of dry matter. Nonfiber carbohydrate (NFC) proportions are calculated by subtracting the proportions (as dry matter) of NDF, crude protein, fat, and ash from 100%. Nonfiber carbohydrates primarily consist of sugars and fructans, starch, organic acids, and pectin. In fermented feeds, fermentation acids also contribute to the NFC fraction. The sum of sugars and starch is referred to as nonstructural carbohydrate (NSC), which should not be confused with NFC. Balancing fiber and NFC fractions to optimize energy intake and rumen health is a challenging aspect of dairy nutrition.

In general, fiber in the diet supports rumen health. Fiber in the rumen, especially fiber from forage sources that have not been finely chopped or ground, maintains rumen distention, which stimulates motility, cud chewing, and salivary flow. These actions affect the rumen environment favorably by stimulating the endogenous production of salivary buffers and a high rate of fluid movement through the rumen. Salivary buffers maintain rumen pH in a desirable range, while high fluid flow rates increase the efficiency of microbial energy and protein yield. Fiber, however, delivers less dietary energy than NFC. Fiber is generally less fermentable in the rumen than NFC, and rumen fermentation is the major mechanism by which energy is provided, both for the animal and the rumen microbes. Therefore, diets with high NDF concentrations promote rumen health but provide relatively less energy than diets high in NFC.

To increase the energy supply, dietary NDF concentrations are usually reduced by adding starch and other sources of NFC. This increases the rate and extent of rumen fermentation, which leads to greater energy availability. Increased ruminal fermentation also leads to the increased production of volatile fatty acids, which tends to lower rumen pH. At rumen pH values <6.2, fiber digestion is reduced; at values ≤5.5, fiber digestion is severely diminished, feed intake may be reduced, and rumen health is generally compromised. There is a reciprocal relationship between NFC and NDF proportions, so the adverse effects of high dietary NFC may be especially evident as cud chewing and salivary flow may be simultaneously diminished because of reductions in dietary NDF.

Recommended minimum NDF concentrations depend on the source and physical effectiveness of the NDF and the dietary concentration of NFC. Fiber from forage sources is, in general, more effective at stimulating salivation and cud chewing than is fiber from nonforage sources. Thus, one variable in the assessment of dietary NDF adequacy is the proportion of NDF coming from forages. Minimum NDF concentrations in the diets for high-producing cows are 25%–30%. When fiber sources from forage make up ≥75% of the NDF, then total NDF concentrations in the lower end of this range may be acceptable (see Table: Dairy Cattle:Recommended Minimum NDF Concentrations Based on Proportion of NDF Coming from Forage Sources). When a smaller portion of total NDF is derived from forage sources, then total NDF concentrations should be in the upper end of this range. Maximum recommended NFC concentrations are 38%–44%. Diets with higher NFC concentrations will benefit from higher proportions of NDF coming from forage sources. These recommendations must be viewed as broad guidelines rather than strict rules. Factors including the total fermentability of the diet as well as the fermentability of the NDF influence the NDF requirement. Diets with highly fermentable NDF sources require higher total concentrations of NDF but provide more energy per mass unit of NDF than diets with less fermentable NDF. Feeding management schemes such as totally mixed rations result in lower minimum NDF concentrations than feeding dietary components individuall