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Supplementation of Lactating Cows on Pasture

Supplementing the nutritional attributes of pasture to maximize rumen fermentation and microbial protein synthesis is a challenge to efficiently utilizing pasture for high producing dairy cows. The basic supplemental nutrients needed are energy from NFC in grains, RUP and NFC to maximize rumen microbial protein synthesis and rumen bypass protein, effective fiber from forages or high fiber feed ingredients, and minerals that are deficient in the pasture.

The use of intensive grazing presents challenges to dairy nutritionists and producers. The nutrient quality of the pasture in intensively managed systems is usually higher (higher crude protein and lower fiber content) than the same plant material harvested as silage or hay. Well-managed spring pastures may have 25% crude protein (CP) with neutral detergent fiber (NDF) concentrations of less than 40%. Nitrogen fertilization may increase crude protein and the solubility of the protein in pastures. The inclusion of legumes with grasses will usually lower the NDF content compared to grass alone. Generally, the quality of the pasture decreases during the warmer summer months. These changes in pasture composition and nutrient composition make supplemental feeding with pasture-based systems more difficult than with confinement systems. We have less control over the forage component with a grazing system, which reduces the consistency of nutrient intake from day to day. Consequently, milk yield per day can be quite variable between days and milk yield per cow is usually lower. Some of the more common nutritionally related questions from nutritionists and producers with pasture systems include:

  • What is the nutrient quality of pasture?
  • What is the intake of pasture?
  • How much supplemental grain or energy should be fed, and when should it be fed?
  • What is the profitability of each additional unit of grain that is fed?
  • How do we balance the readily available protein and carbohydrates in the rumen?
  • What should be the feed ingredients and nutrient composition of the grain mixtures?
  • Should additional forages or fiber be fed?
  • Should we feed a TMR?
  • How do we monitor and manage the program to maintain milk production?

Nutritional Imbalances/Deficiencies in Pasture

High producing dairy cows require a nutritionally balanced diet to optimize production and profitability, whether a TMR or pasture-based system is used. The basics of dairy nutrition based on NRC (2001) are the guide for developing rations to complement pasture. Application of our knowledge of nutrient utilization in the rumen and by the animal will help ensure optimal animal performance. While some graziers believe that feeding cows on pasture is an “art,” the science is still needed to develop the most profitable total feeding program. High quality pasture has many positive nutritional attributes for the lactating cow, but has some nutritional imbalances and deficiencies for dairy cattle that include:

  • Total protein is high, but is highly degradable in the rumen and not efficiently utilized in the rumen unless adequate fermentable carbohydrates are provided to “capture” this protein in the rumen. Rumen undegradable protein (RUP) may be inadequate for high producing cows in early lactation.
  • The fermentable carbohydrate content, which is the major source of energy for the rumen microbes and the cow, is low in most pastures compared to the needs of the cow.
  • The fiber content of high quality pasture may be too low, particularly in the spring and fall, in its ability to stimulate adequate cud chewing and rumination, which is often called “effective fiber.” This may result in reduced milk fat content. The rate and extent of digestion of fiber may be altered with grazing due to the high quality and the high moisture of pasture.
  • The amount of several minerals including Ca, P, Mg, S, Ca, Zn, and salt are usually inadequate, whereas potassium may be too high. In general, these minerals should be added to the supplemental feeding program.
  • Vitamins and A and E are high, and supplementation is not needed in most cases.

The consequence of these imbalances and deficiencies include:

  • fast degradation in the rumen
  • fast nutrient passage and loose manure
  • reduced milk fat percent
  • reduced milk yield
  • low body condition

Nutrient utilization by the rumen microorganisms and, thus by the cow, is not optimal with pasture alone. Inefficient utilization of the high protein content of pasture and the energy cost to the cow to excrete this excess protein can lead to losses in milk production. The energy needed to excrete this urea, which is often referred to as urea cost, may be equal to a loss of 3 to 6 lb of milk per day. These limitations suggest the need for strategic supplementation of the correct nutrients to maximize the utilization of this high quality, low cost forage in order to optimize milk production.

A goal of many dairy producers who are grazing should be to first maximize the intake and utilization of pasture and to supplement with the proper amount and type of concentrate to optimize production and profit.  Maintaining intake of high quality pasture through proper grazing management offers the best opportunity to maximize pasture intake and to reduce total feed cost. A system with several paddocks and a well-planned rotation with feed budgeting and planning, provides the opportunity to have high quality forage available to cows at the correct time.

Energy Supplementation

Energy is the most limiting nutrient for dairy cows on pasture-based systems. The major sources of supplemental energy are carbohydrates from grains and concentrates, which are often referred to as non-fiber carbohydrates (NFC). The NFC content of most spring pastures, which is a measure of ruminal available carbohydrate, is between 15 to 22% of the DM. In addition, grazing cows require more energy over maintenance than non-grazing cows because of higher levels of activity. This may require 3 to 6 lb of concentrate, which is a “fixed” cost for activity without a return in milk production. The amount of NFC and concentrate fed to increase the total energy intake on a pasture-based system can have long term positive effects on energy balance, milk production, body weight and body condition changes, and reproductive performance. The utilization of pasture protein and synthesis of rumen microbial protein can be increased by supplying more NFC and ruminally available carbohydrates. The grain feeding guidelines in Table 1 were developed based on research, producer experiences, and economics.

Table 1. Grain (DM) feeding guidelines for a grass-based pasture system.ab
aAssume 1300 lb body weight.
bThese guidelines are based on high quality grass pasture available in adequate quantities assuming the approximate DMI. Lower quality forages may require more grain. Maximum grain DM fed should be equivalent to about 20 lb per day. Some adjustment of grain should be made based on body condition scores and stage of lactation. Lower amounts can likely be fed when the pasture contains legumes.
cGrain fed (DM basis) to milk yield on a lb to lb basis.
4% Fat Corrected
Milk Production (lb/day)
Spring Summer Fall
lb G:Mc lb G:Mc lb G:Mc
> 80 20 1:4 to 1:5 21-24 1:3.5 20 1:4 to 1:5
70 16 to 18 1:4 to 1:5 18-20 1:3.5 to 1:4 16 to 18 1:4 to 1:5
60 12 to 14 1:5 15-18 1:3.5 to 1:4 12 to 14 1:5
50 8 to 10 1:5 to 1:6 10-12 1:4 to 1:5 8 to 10 1:4 to 1:5
< 40 6 to 8 1:6 to 1:7 8-10 1:5 to 1:6 6 to 8 1:6 to 1:7

The recommendation is to feed about 1 lb of concentrate per 4 lb milk production with a maximum of 16 to 20 lb/cow/day. For late lactation cows, we suggest a 1:5 or 1:6 grain to milk ratio. The relative price of milk in relation to concentrates and body condition should indicate the amount of concentrate to feed. Feeding 1 lb concentrate per 5 lb milk for early lactation cows, and a 1:6 to 1:7 ratio in later lactation may be more profitable with lower milk prices.

In addition to the amounts of energy and concentrate to feed, the type of supplemental carbohydrate (grain) fed and the method of providing energy need to be considered. Feeding grains that are finely ground increases rumen availability of carbohydrate, will improve carbohydrate and protein utilization, and may improve milk protein yield. Synchronizing the supply of N and energy-yielding substrates to ruminal microbes has been suggested as a means to improve the capture of rumen degradable protein and, in turn, improve milk production.  Feeding a TMR to cows under confinement housing allows rations to be formulated that synchronize the supply of N and NFC to the rumen. With grazing conditions, the largest time spent grazing during a day is usually after milking.

A concern with grazing is that grain may often be fed twice daily at milking time. We recommend to avoid feeding over 7 to 8 lb of grain at one feeding, particularly if the cow is hungry and has not consumed forage for sometime before grain feeding. Feeding grain after cows have grazed or been fed stored forage just prior to going to pasture should minimize the concern of “slug feeding.” Feeding grain in a partial TMR will also minimize this risk.

The types of carbohydrates and grains fed may influence the response.  Feeding corn that has been steamed flaked to increase rumen availability of carbohydrate appears to improve carbohydrate and protein utilization, and improve milk protein yield. A grain mixture that includes starch and non-starch ingredients that is finely ground to improve carbohydrate utilization will likely improve the milk response.

The addition of non-forage fiber or fermentable fiber sources to the concentrate ration, such as cottonseed hulls, soy hulls, beet pulp, distillers grains, citrus pulp, wheat mids, whole cottonseeds, and some other by-products, may be beneficial in providing fermentable fiber to the rumen and should be considered if economical. A research study in Northern Ireland reported a 0.6% milk fat increase (2.9 to 3.5%) with cows producing 80 lb of milk when a concentrate supplement containing half the ingredients as fermentable fiber sources and half as starch-based sources was compared to an all starch-based concentrate supplement. Our research at Penn State had a 0.2% increase in milk fat increase with non-forage fiber sources, and further illustrates the lack of adequate effective fiber in some high quality pastures. A concentrate mixture that includes primarily starch with some non-forage fiber sources that are finely ground will provide a blend of rapidly and slowly fermentable carbohydrate and will likely improve the milk response.

Protein Supplementation

Protein content can be too high in high quality pasture. With high quality pasture that is high in protein, a grain ration with 12% crude protein is usually adequate. The high ruminally degraded protein in pasture may suggest that cows may be deficient in protein and amino acids available for absorption from the small intestine and for milk synthesis. The need for sources of ruminal undegraded protein (RUP) or bypass protein for high producing cows on pasture is questionable. A study at Penn State (Hongerholt and Muller, 1998) with cows producing 80 lb of milk, found that multiparous cows produced more milk protein (2.33 vs. 2.15 lb/d) when fed the high RUP diet. Some unpublished research indicated a benefit with the addition of rumen RUP to provide amino acids post-ruminally with high producing cows. Addition of 0.5 to 1.0 lb/cow/day of a high RUP source is likely beneficial when cows are producing greater than 70 lb of milk per day. Examples of feedstuffs high in RUP are roasted soybeans, corn distillers, fish meal, and specially processed soy protein.

Milk urea nitrogen (MUN) is being used as a diagnostic tool to monitor the dietary protein and carbohydrate balance. With only pasture, the diet will be high in rumen degradable protein in relation to dietary carbohydrates, and the MUN will likely be high. The few studies in the USA do not indicate that MUN values are excessive with pasture-based systems with averages of about 14 to 16 mg/dl. However, cows receiving pasture as the only feedstuff do have higher MUN than when cows are fed concentrate supplements. In some studies, we have found the MUN values in cows fed only pasture to be 3 to 5 mg/dl higher than when concentrate is fed. We have observed MUN values up to 20 mg/dl when milk samples were obtained within two weeks of nitrogen fertilization of pasture.

Supplemental Forages

As previously mentioned, spring pasture is often low in total fiber and low in “effective fiber” compared to stored forages. Lush pasture is 80 to 85% moisture, which can lead to a fast passage of feedstuffs through the digestive tract. Feeding 2 to 4 lb of long hay will add some effective fiber, will likely slow the rate of feed passage, and may help maintain feed intake and milk fat percentage.

Corn silage can be an excellent supplemental forage to complement pasture because it adds needed NFC as a source of energy for the rumen microbes and also “dilutes” out the high protein in spring pasture. Corn silage is a highly palatable feed, is an excellent carrier for the supplemental grain, and can allow for lower amounts of grain to be fed because corn silage is typically 40% or more corn grain by weight. Several research studies from Australia report that marginal return to the additional feeding of corn silage is 0.7 to 0.9 lb of milk per lb of corn silage dry matter when feeding up to 10 lb of corn silage DM/cow/day. One management problem is that adequate amounts must be removed from the silo daily to maintain good quality silage. Stored forages are needed to maintain DMI when pasture availability is low, particularly during the summer.

As discussed previously, the addition of fermentable fiber or non-forage fiber sources to the grain ration, such as soy hulls, citrus pulp, wheat mids, cottonseed hulls, distillers grains, and some other by-products may be beneficial in providing needed fiber in the rumen and should be considered if economical and readily available. The recent research reports benefits in milk fat percent with grain rations containing fermentable starch sources such as corn plus these fermentable fiber sources, and further illustrates the lack of adequate effective fiber in some high quality pastures in the spring.

Supplementing lactating cows on pasture with an additional source of forage is a common practice among dairy producers. There are advantages and disadvantages associated with forage supplementation with adequate pasture, and the manager’s goal should be used to guide decisions about supplementation.

Summary

Well-managed pastures that maximize production and quality are of comparable or better quality than most forages harvested with mechanical harvesting systems. A big challenge of efficiently utilizing pasture for high producing dairy cows is proper and profitable supplementation to complement the nutritional attributes of pasture and to maximize rumen fermentation and microbial protein synthesis, which in turn contributes to optimum milk production and profit throughout the grazing season. The continual changing of pasture quantity and quality during the grazing season and the potentially low utilization of protein in pasture by the lactating cow are major challenges when supplementing pastures. The basic supplemental nutrients needed to complement pasture are energy from the NFC in grains, RUP and NFC to maximize the rumen microbial protein synthesis and rumen bypass protein, fiber from forages and/or high fiber feed ingredients to increase the effective fiber in the total ration, and deficient minerals. This is no one magic grain mixture for pasture.  Feeding strategy is important and a “partial” TMR is likely to improve nutrient utilization and rumen health. To successfully manage the feeding program with grazing herds is challenging and requires even greater management skills in evaluation and monitoring than in total confinement systems.

Published as pages 23-27 in proceedings from "Nutrition of Dairy Cows on Pasture-Based Systems" held March 31, 2003 in Grantville, PA.

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Supplementation of Lactating Cows on Pasture

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Larry Muller
  • Professor Emeritus, Penn State