FEED AND ANIMAL MANAGEMENT FOR DAIRY CATTLE

Dairy operations typically include the milking cowherd with some of the cow population in the non-lactation stage (dry cows) and with growing heifers.  Distinctly different diets are required for each of these three stages of the life cycle resulting in great differences in the volumes and nutrient compositions of manure.  This fact sheet briefly highlights some factors affecting nutrient excretion with potential dietary adjustments to minimize excess nutrient excretion.  Selected nutrient requirements for dairy that could have environmental impact are listed in Table 1 (NRC, 2001).  The example below is with only a mature Holstein cow.  Reference to the NRC (2001) is imperative for other examples (such as for smaller cows like Jersey) and a more extensive and thorough evaluation of dairy diets on a commercial operation.

 

Table 1. Selected nutrient requirements of dairy cows as determined using sample diets¹

Holstein, 1500 lb., average body condition, 65 mo. age	90 Days in Milk				Early Lactation		Dry, Pregnant 270 Days in Gestation BW 1656 lb.
Milk yield, lb/d	55	77	99	120	55	77
Dry matter intake, lb/d	44.7	51.9	59.2	66	29.7	34.3	30.1
Net energy, Mcal/lb	0.62	0.67	0.7	0.73	0.94	1.01	.48
Diet % RDP	9.5	9.7	9.8	9.8	10.5	10.5	8.7
Diet % RUP	4.6	5.5	6.2	6.9	7	9	2.1
Crude Protein, a %	14.1	15.2	16.0	16.7	17.5	19.5	10.8
Calcium, %	0.62	0.61	0.67	0.60	0.74	0.79	0.45
Phosphorus, %	0.32	0.35	0.36	0.38	0.38	0.42	0.23
Potassiumb, %	1.00	1.04	1.06	1.07	1.19	1.24	0.52
Sodium, %	0.22	0.23	0.22	0.22	0.34	0.34	0.10
Copperc, ppm	11	11	11	11	16	16	13
Zinc, ppm	43	48	52	55	65	73	22

^a Equivalent to the sum of rumen degradable protein (RDP) and rumen undegradable protein (RUP) only when they are perfectly balanced.

^bHeat stress may increase the need for potassium.

^cHigh dietary molybdenum, sulfur, and iron can interfere with copper absorption increasing the requirement.

¹Adapted from Table 14-7, 14-8 and 14-9, Nutrient Requirements of Dairy Cattle. 7^th Revised Edition, 2001. National Research Council, National Academy of Sciences, National Academy Press, 2101 Constitution Ave, Washington, DC 20418 (J. H. Clark, Chair, Subcommittee on Dairy Cattle Nutrition).

Diet Formulation

Many producers still do not routinely formulate and evaluate diets.  Thus, overfeeding or underfeeding of protein and minerals can be common.  Diets should be formulated by certified animal nutritionists.  Proper diet formulation requires routine (monthly or quarterly) forage and byproduct analysis because these ingredients are highly variable and tabular values and the previous sample analysis are not reliable.  Cows should be evaluated for their body condition routinely so that the proper energy level of the diet can be determined.  Producers who do not routinely have diets formulated by qualified professionals should be advised to do so.

A 50% variation in manure production might result from differences in feed wastage, ration formulation, type of feeding program (e.g., dry lot versus pasture feeding), and/or animal grouping systems.  Since dairy cattle are ruminants, they can utilize forages (generally lower in digestibility) as well as concentrates (generally higher is digestibility) in their diets.  Depending upon the life cycle stage, the roughage to concentrate levels can vary tremendously.  As a result, volumes of manure produced are much higher with poorly digestible forages (fiber) as compared to concentrates and composition is significantly different with these two scenarios.  

Most nitrogen consumed by cattle is a part of the protein the animal consumes.  When cows consume excess protein, an increased amount of N is excreted in the urine as urea.  Small amounts of urea can also be diffused into the milk.  The concentration of urea in milk is proportional to the amount of N excreted in urine for cows of a given body weight.  Cows consuming excess protein will have higher milk urea nitrogen (MUN) concentration than cows consuming less protein.  It is recommended that MUN be routinely analyzed to help producers determine if too much or too little protein is being fed.  A general rule is that an average herd MUN should fall between 9 and 14 mg/dl of milk.  Previous target recommendations for MUN were higher, but DHIA labs that analyze milk have since improved their methodology for measuring MUN, and reduced MUN values are now being reported.

In some cases, phosphorus (P) has not been utilized efficiently on livestock farms and can accumulate in excess when applied to the land. For years, mineral P supplements such as dicalcium phosphate or monocalcium phosphate have been added to dairy cow diets at levels exceeding recommendations.  As a result, diets typically contain 25 to 35% more P than recommended by NRC (2001)

Previously, the recommendation for feeding P was about 0.40% of diet dry matter for a high-producing cow, yet most producers feed P closer to 0.50% of the diet.  Recent research (NRC 2001) has shown that less than 0.40% phosphorus is needed. Scientists now recommend that 0.32 to 0.38% P is sufficient for normal milk production assuming adequate daily dry matter feed intake.  Farmers often over feed P with the thought that they will: (1) improve reproductive efficiency and (2) that the feed ingredient tables typically underestimate the amount of P in most ingredients.  Overfeeding for reproductive performance has no scientific basis.  Often using tabular feed values make farmers believe they are feeding less than they really are. Forages in particular can be highly variable in P content and may contain more P than NRC tables indicate. To minimize intake, feeds should be analyzed for minerals using wet chemistry procedures and diets should be formulated to reduce the overfeeding of P.

Production Management

Several new technologies have the potential to increase milk production.  One such technology is the manipulation of photoperiod by the provision of artificial lighting.  It has been shown that increasing day length can increase milk production in dairy cattle by up to 8%. Nutrient intake required by such light stimulated herds increased by only 4.1% and N and P excretion increased by only 2.8% when compared with similar herds under natural day length.

Another new technology that may impact nutrient utilization and excretion is the administration of bovine growth hormone (BGH) or somatotropin.  This peptide hormone can increase milk production by as much as 30% in certain cows within the herd, although the entire herd would be increased by only 14%. The nutrient requirements of a herd treated with BGH would increase by roughly 7 to 8%, and manure P may increased by 5%.  The nutrient losses from the farm per unit of milk produced would, therefore, decrease by 8 to 10% per unit of milk produced.  Research has also demonstrated that milking three times per day instead of twice per day can increase production per cow by an average of 11% and reduce stress on a herd. This greater production per cow results in the consumption of 5% more protein, with 3.5% more nutrients excreted in manure.  The extra milking per day reduces the amount of manure nutrient losses per unit of milk produced by 7%.

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