Increased global competition, rising production costs, and relatively stagnant milk prices have forced dairy producers to search for ways to increase efficiency in order to sustain profitability. Improving feed efficiency (FE; milk produced per pound of dry matter) of dairy cattle can have a significant impact on profitability.

In the example given in Table 1, improving FE from 1.4 to 1.5, while maintaining milk production, increases profitability of a 1,000-cow dairy by $91,980/year.

Table 1  Effect of Feed Efficiency on Dairy Profitability
Herd Size, cows	Milk Yield, lb/day	Dry Matter Intake, lb/day	Feed Cost, $/lb DM	Feed Efficiency	Feed Cost, $/Year	Profitability Advantage
1000	75	53.6	$0.07	1.40	$1,369,480
1000	75	50.0	$0.07	1.50	$1,277,500	$91, 980

The energetic efficiency of milk production in relation to feed consumption is affected by a number of non-feed factors including:

• Milk composition

• Increased maintenance requirements due to cold and heat stress

• Energy expenditure for walking long distances to and from the milking facility,
  pasture, and/or pens

• Change in body weight due to growth and body weight loss and gain
  with lactation progression

 Adjusting for these factors allows nutritionists to obtain a better estimate of the true conversion of feed to productive purposes, such as tissue accretion, activity, and milk production.

Fat and protein content of milk are the primary determinants of milk energy content. A cow producing 77.1 lb/day of milk containing 3.5% fat and 3% true protein produces as much milk energy as a cow producing 72.0 lb/day of milk containing 3.9% fat and 3.2% protein.

Energy Expenditure for Temperature Stress
Heat stress has been reported to increase maintenance requirements by up to 29%. For a 1,400 lb cow this equates to 2.96 Mcal of additional NEL/day. Increased maintenance requirements result from cows panting to dissipate heat.

Cold stress also appears to affect feed efficiency by both reducing dry matter digestibility and diverting nutrients to heat generation. It has been reported that cold stress reduces dry matter digestibility by 1.8% for each 50º F reduction in temperature below 68º F. Much of the cold stress reduction in digestibility is attributed to increased passage rate of feed through the digestive tract. In addition, maintenance requirements have been estimated to be 51% higher at -4° F as compared to 64° F for a 1,323 lb cow producing 60 lb of milk containing 3.7% fat (NRC, 1981). However, cold stress adjustments for dairy cattle appear to be based upon limited data. There is little doubt that cold stress increases maintenance requirements of lactating dairy cattle. To what extent cold stress increases maintenance is unclear.

Energy Expenditures for Excess Walking
On some dairies, cows walk considerable distance from their pen or paddock to the milking center and energy expended for walking needs to be accounted. For a 1,400 lb cow, milked three times a day and housed in a pen that is 1,000 ft from the middle of the pen to the milking center, maintenance requirement increases by 5.2%. Therefore, the energy equivalent of 1.7 lb of energy-corrected milk (3.5% fat and 3% protein) per day is utilized for walking to and from the milking center.

Energy Expenditure for Growth and Adjustment for Stage of Lactation
From a large data of over 17,000 data points, it was estimated that first lactation heifer and mature cow are deriving the energy equivalent of 3.8 and 10.7 lb/day of milk (3.5% fat and 3% protein) from body stores prior to 41 days in milk (DIM). In late lactation (after 105 DIM), the first-calf heifer and mature cow are diverting the energy equivalent of 4.6 lb/day of milk (3.5% fat and 3% protein) towards body weight gain. Between 41 and 105 DIM, first-calf heifers are diverting the energy equivalent of 4.6 lb/day of milk towards growth, while body weight of mature cows remains relatively constant during this time period. It is noteworthy that the first-calf heifer derives less energy from tissue reserves in early lactation than mature cows. Once cows begin regaining body weight, first-calf heifers divert the same amount of energy towards tissue accretion on a daily basis as mature cows, but have more days of tissue accretion reflecting continued growth of first-calf heifers.

Two hypothetical herds scenarios are given in Table 2. At first glance, Herd One is more efficient at converting dietary nutrients towards productive purposes. However, Herd One is producing milk with a lower milk fat and protein content, is producing milk under thermal-neutral conditions, is walking less distance to and from the milking center, and 50% of the herd consists of mature cows less than 41 DIM. In addition, 60% of Herd Two are first-calf heifers greater than 40 DIM, and they are producing milk under heat stress conditions. After correcting for milk composition, energy expenditure for walking distance, temperature stress, growth and body weight loss and gain with progression of lactation, Herd Two is more efficient at converting dietary nutrients for activity, tissue accretion, and milk production than Herd One (Table 2).

Table 2  Effect of Correcting Observed FE for Milk Composition, Excess Walking, Body Weight Loss and Gain, and Increased Maintenance Costs Due to Heat Stress
Item	Herd One	Herd Two
Milk production, lb/day	77.1	72.0
Milk fat, %	3.5	3.9
Milk protein, %	3.0	3.2
Dry matter intake, lb/day	50.0	50.0
Observed feed efficiency	1.54	1.44
Energy corrected milk (ECM): 3.5% fat, 3.0% true protein	77.1	77.1
Cow body weight, lb	1400	1400
Walking distance, ft	150	1000
Milkings/day	3	3
Average daily high temperature, degrees F	40	90
Relative humidity, %	40	70
Wind speed, mph	5	5
Hours in direct sunlight	6	6
% first calf heifers < 40 days in milk	20	30
% first calf heifers > 40 days in milk	5	60
% cows < 40 days in milk	50	0
% cows 41 to 105 days in milk	5	10
% cows > 105 days in milk	20	0
ECM lost due to growth, lb/day	-0.5	1.7
ECM lost due to temperature stress, lb/day	0.00	9.5
ECM lost due to excess walking, lb/day	0.3	1.7
ECM adjustment for stage of lactation and parity	-4.9	1.7
Adjusted ECM, lb/day	72.5	90.0
Adjusted feed efficiency (milk produced per lb of dry matter)	1.45	1.80

Other Factors Affecting FE
A number of factors can affect FE including acidosis, excess shrink due to inaccurate feed mixing and delivery, and bird and pest infestations. Obtaining a more accurate estimate of the true FE on a dairy can help producers find ways to get more milk out of the feed provided to cows. In addition, obtaining a more accurate FE value can help producers determine the cost effectiveness of various technologies, such as balancing diets for amino acids, formulating diets to more closely meet nutrient needs of cows, feeding more digestible feedstuffs, using rbST, feeding monensin, extended day lighting, and various grain processing methods.

Generally, increasing milk production increases FE as maintenance requirements comprise a smaller portion of nutrient requirements. Table 3 provides an example comparing the FE of a cow producing 70 lb of milk versus a cow producing 95 lb of milk. Assuming that body weights of both cows are static, 30.8% of dietary energy is devoted for maintenance requirements in a the cow producing 70 lb of milk versus 24.7% of dietary energy for maintenance for the cow producing 95 lb of milk. Using NRC (2001) predicted dry matter intake (DMI), FE for the cow producing 70 lb of milk is 1.41, while the FE of the cow producing 95 lb of milk is 1.63. Thus, increasing milk production should improve FE, and producers are discouraged from restricting dry matter intake as a means of improving FE.

Table 3  Portion of Dietary Nutrients Utilized for Maintenance in Cows Producing 70 lb of Milk Versus 95 lb of Milk (NRC, 2001)
Body Weight, lb	Milk Yield, lb/daya	Maintenance Requirement, Mcal	Production Requirement, Mcal	% of Energy Requirement Used  For Maintenance	NRC (2001) Estimated DMI, lb/day	Feed Efficiency
1400	70	10.1	22.7	30.8%	49.8	1.41
1400	95	10.1	30.8	24.7%	58.4	1.63
a Contains 3.7% fat and 3.1% protein

Lameness and disease also affect FE. Most immune responses to pathogens are accompanied by a systemic acute phase protein response, which is characterized by decreased appetite and a shift in nutrient use away from growth and milk production toward production of acute phase proteins and other immune components. For example, under normal conditions, 1.17% of lysine consumed by young chicks is utilized for immune processes, while under a disease challenge, 6.71% of lysine consumed by young chicks is used for immune processes.

Trace minerals also affect FE as they are essential for maintaining optimal health and performance of animals. The impact of trace minerals and trace mineral status on feed efficiency is clearly illustrated in a trial conducted by Colorado researchers. In this trial, calves that received a diet with no supplemental zinc (diet contained 17 ppm zinc) for 28 days had 50% decrease in FE as compared to calves that received a diet containing 40 ppm zinc (23 ppm supplemental zinc from zinc sulfate; see Table 4). The decrease in FE for the calves receiving no supplemental zinc was attributed to both a 46% decrease in average daily gain and a 6.7% increase in dry matter intake. During the 14-day repletion phase, calves receiving zinc methionine (ZINPRO®*) returned to control FE levels three times faster than calves receiving zinc sulfate. Similarly, in a 22-trial feedlot summary, supplementing feedlot cattle with 360 mg of zinc from zinc methionine (ZINPRO) improved feed efficiency by 4.05%.

Item	Controla	No Added Znb
	(40 ppm Zn, ZnSO4)	(17 ppm Zn)
ADG, lb	1.30y	0.70z
DMI, lb/day	13.4	14.5
Gain/feed	0.10y	0.05z
Plasma Zn, mg/L	0.97	0.84
Liver Zn, ppm DM	106	101

Increasing levels of cobalt in lactating dairy diets have also been shown to improve FE. In a summary of two trials, supplementing lactating dairy cows with approximately 10 mg of cobalt from cobalt glucoheptonate (CoPRO®*) improved FE by 2.56%, while supplementing lactating dairy cows with approximately 20 mg of cobalt improved FE 7.37%. It should be noted that cobalt content of the control diet in both experiments was in excess of NRC (2001) requirements.

Conclusion
Feed efficiency is becoming an increasingly important production measure as dairy management becomes more refined. However, to effectively evaluate FE, it must be standardized for milk composition, changes in body condition, environmental factors, and exercise. Numerous factors, including rBST, monensin, extended day lighting, and trace mineral nutrition affect FE. *Trademarks of Zinpro Corporation.

*Trademarks of Zinpro Corporation.