During lactation, dairy cows have very high nutritional requirements relative to most other species. Meeting these requirements, especially for energy and protein, is challenging. Diets must have sufficient nutrient concentrations to support production and metabolic health, while also supporting rumen health and the efficiency of fermentative digestion.
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 moistu re concentrations. DMI is affected by both animal and fee d 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 non lactating, 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 week 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.
Energy requirements for lactating dairy cows are met primarily by carbohydrate fractions of the diet. These consist of fibrous and non fibrous carbohydrates. Fibrous carbohydrate proportions are generally measured as NDF and expressed as a percentage of dry matter. Non fiber carbohydrate (NFC) proportions are calculated by subtracting the proportions (as dry matter) of NDF, crude protein, fat, and ash from 100%. Non fiber 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 non forage 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. 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 ferment ability 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 individually
Dietary energy is usually measured in megacalories (Mcal) or megajoules (MJ). When the energy in a given feedstuff is expressed in terms of the Mcal or MJ actually available for metabolism, heat production, or storage in the animal, the term metabolizable energy (ME) is used. The efficiency of utilization of ME varies based on the physiologic functions supported, which include body maintenance, growth, and lactation. The net energy (NE) system takes into account the differences in efficiency of ME utilization for each of these processes and assigns a separate NE value to individual feedstuffs based on each of these energy-requiring processes, ie, body maintenance, growth, and lactation. Thus, in the USA, in which the NE system is typically used, energy values of feedstuffs for ruminants are expressed as NE for maintenance (NEM), NE for gain (NEG), and NE for lactation (NEL). This system is cumbersome and nonintuitive and has many computational disadvantages compared with alternative systems based directly on ME. However, the NE system has the major advantage of more equitably comparing the energy values of forages to concentrates when used in ruminant diets.
It has typical values for ME, NEL, NEM, and NEG, for some feedstuffs commonly fed to dairy cows. The values in these and other published tables are estimates of the energy delivered to lactating cows consuming feed at three times the maintenance consumption rate, ie, three times more feed than they would consume were they not in production. The listed values are typical averages for the feeds; the actual values for individual feeds may vary considerably, especially for forages. Laboratory analyses of feeds and forages are always advisable for both comparative evaluation and ration balancing. Values for ME and NE cannot be measured directly by typical laboratory analyses. These and any other energy values on a laboratory report are estimates, usually based on formulas with acid detergent fiber concentration as the primary independent variable. Many contemporary computer programs for ration evaluation or balancing in dairy cows do not rely on laboratory estimates of feed energy concentrations. Rather, they estimate the contributions of individual feeds to the energy supply based on feed characteristics, intake rates, and estimated rates of passage through the rumen. Such programs are frequently referred to as "models." When using programs of this type, the estimated energy values of individual feeds will diminish with increasing rates of feed intake.
Supplemental fats can be added to increase energy concentration. Fat concentrations in typical dairy diets without supplemental fat are usually low, ~2.5% of dry matter. Supplemental fats may be added to attain a total ration fat concentration of ~6% of dry matter. Fats in ruminant diets can induce undesirable metabolic effects, both within the rumen microbial population and within the animal. Ramifications of these effects include reduced fiber digestion, indigestion and poor rumen health, and suppression of milk fat concentration. The major benefit of supplemental fat in ruminant diets is that dietary energy concentration can be increased without increasing the NFC concentration.
Fats may be supplemented from vegetable sources such as oil seeds, animal sources such as tallow, and specialty fat sources that are manufactured to be rumen inert, ie, not interact with the metabolism of rumen microbes. Supplemental fats from vegetable sources generally have a relatively high proportion of unsaturated fatty acids. Unsaturated fats adversely affect rumen microbial activity. In addition, these fatty acids are extensively converted to saturated fatty acids in the rumen. When fed in excessive dietary concentration, intermediate products from the saturation process may escape the rumen and be absorbed by intestinal digestion. Some of these products are trans-fatty acids, some of which directly suppress mammary butterfat synthesis. Supplemental fats from animal sources are more saturated and thus less detrimental to microbial activity and less apt to result in suppression of butterfat synthesis. Rumen-inert fats are designed to have little or no effect on rumen microbial activity and mammary butterfat synthesis. In general, when supplementing fats to dairy diets, up to 400 g (~2% of diet dry matter) may be added as vegetable fats, particularly if the fats are added as oil seeds, which tend to be less detrimental than free oils. An additional 200–400 g may be added from highly saturated or preferably rumen-inert sources, generally not to exceed a total of 6.5% fat in the total dietary dry matter.
The protein requirements of lactating dairy cows are high because of the demand for amino acids for milk protein synthesis. Two systems of describing the dietary protein supply and requirements for dairy cows are in general use: the crude protein system and the metabolizable protein system. The crude protein system considers only the total amount of dietary protein, or protein equivalent from nonprotein nitrogen sources. Crude protein values are based on the measurement of total dietary nitrogen and the assumption that protein is 16% nitrogen. The crude protein system is relatively simple to use and has provided a traditional means of formulating dairy cow rations provides general guidelines for the required crude protein concentration of diets for large- and small-breed dairy cattle at various levels of production. It can be used for general evaluations of the protein adequacy of dairy diets. The metabolizable protein (MP) system is more complex than the crude protein system, and it was developed in recognition of the fact that not all crude protein provided to cows may be available for absorption as amino acids.
The availability of high-quality water for ad lib consumption is critical. Insufficient water intake leads immediately to reduced feed intake and milk production. Water requirements of dairy cows are related to milk production, DMI, ration dry matter concentration, salt or sodium intake, and ambient temperature. Various formulas have been devised to predict water requirements. Two formulas to estimate water consumption of lactating dairy cows are as follows:
Note: FWI is free water intake (water consumed by drinking rather than in feed), DMI is in kg/day, milk is in kg/day, Na is in g/day, and temperature is in °C. Water consumed as part of the diet contributes to the total water requirements; thus, diets with higher moisture concentrations result in lower FWI.
Providing adequate access to water is critical to encourage maximal water intake. Water should be placed near feed sources and in milking parlor return alleys, because most water is consumed in association with feeding or after milking. For water troughs, a minimum of 5 cm of length per cow at a height of 90 cm is recommended. One water cup per 10 cows is recommended when cows are housed in groups and given water via drinking cups or fountains. Individual cow water intake rates are 4–15 L/min. Many cows may drink simultaneously, especially right after milking, so trough volumes and drinking cup flow rates should be great enough that water availability is not limited during times of peak demand. Water troughs and drinking cups should be cleaned frequently and positioned to avoid fecal contamination.
Poor water quality may result in reduced water consumption, with resultant decreases in feed consumption and milk production. Several factors determine water quality. Total dissolved solids (TDSs), also referred to as total soluble salts, is a major factor that refers to the total amount of inorganic solute in the water