Dairy Update Issue 119, August 1995Effect of Cow Prep on Milk Flow, Quality and Parlor ThroughputJeffrey K. Reneau and John P. Chastain INTRODUCTIONPremilking cow prep is proven to be an important step in achieving maximum milk yield, quality and udder health. Several studies indicate an advantage in milk flow rates and machine on-time by optimizing teat stimulation and prep-lag (Table 1). Milk quality and udder infection are improved by good prep procedure (9, 10, 15). However, milkers, compelled by the speed of premilking cow prep rather than thoroughness, often fail to achieve either adequate teat sanitation or consistent milk letdown stimulus. In herds where there is more than one milker, there is usually a great variation in milking routine. All of these factors can contribute to lower milk quality, yield and poor udder health as well as inefficient milking.
Quite often, there are great differences in opinion by respected milk quality consultants regarding what is optimal premilking cow prep. Unfortunately, this may confuse many dairy farmers. The truth of the matter is that there is no single premilking cow prep method that is best for every farm. There are, however, proven scientific principles that should always be considered when adapting premilking cow prep to your farm. In the "real world," nothing is perfect; there will be tradeoffs between what is optimal and practical. Herd size, regional differences in weather, housing and labor force as well as whether milk quality premium programs are offered may influence what pre-milking cow prep is most appropriate for your farm. The purpose of this paper is: 1) to review the principles that govern optimal cow prep and milking efficiency, 2) to present results of a model based on literature values that demonstrates the effect of prep-lag time and milk flow rate on parlor throughput, and 3) provide regional scenarios that estimate the potential economic impact of optimizing premilking cow prep. It is hoped that this discussion will help farm managers adapt premilking cow prep procedures that best satisfy scientific principles and maximize economic returns. MILK LETDOWNAlmost everyone is familiar with the concept of milk letdown and the role of oxytocin in achieving it. However, few realize that milk letdown is more involved than the simple action of oxytocin on the mammary myoepithelial cell that surrounds each milk secreting alveolus. Studies have shown that milk ejection is not entirely dependent on the action of oxytocin and that there are also many other factors that control the effectiveness of oxytocin response (12, 13, 14, 24). The effect of teat stimulation on sympathetic tone in the mammary gland is a second milk letdown mechanism. Teat stimulation initiates a local autonomic reflex resulting in a decrease in smooth muscle tone around mammary ducts and teat sphincters. There is also an increase in blood flow to the mammary gland as well as a decrease in the response threshold of the myoepithelial cell to oxytocin (12). Although the local autonomic reflex letdown mechanism is independent of oxytocin for its effect, this mechanism potentiates oxytocin response. These two mechanisms work together to accomplish efficient milk removal. The effects of oxytocin on the mammary myoepithelial cell and the uterine smooth muscle cells are similar, and they are mediated by oxytocin receptors. Progesterone and estrogen levels regulate the availability of oxytocin receptors on uterine smooth muscle cells and are thought to have a similar effect on the mammary myoepithelial cell. Adequate levels of calcium in the diet are needed to ensure normal contraction of any smooth muscle cell including the mammary myoepithelial cell. The trace mineral, magnesium, plays a role in oxytocin receptor availability and smooth muscle contractility. It is by this direct means that dietary magnesium affects milk butterfat percent. Cobalt and manganese have also been found to influence the effectiveness of the oxytocin response (12, 13, 20, 23). Clearly, milk letdown is a complex mechanism. It has been generally observed that milk letdown response varies with stage of lactation. Late lactation cows typically require more stimulus to achieve good milk letdown than early lactation cows. It can be reasoned that, during early lactation, milk letdown is more intense because: 1) a more distended myoepithelial cell will contract with a greater force and 2) the cyclic exposure to estrogen in early lactation maintains the sensitivity of oxytocin receptor sites to oxytocin thus achieving a more powerful oxytocin response (23). After the cow is pregnant and under the hormonal influence of progesterone, the affinity of oxytocin receptor sites for oxytocin declines, and smooth muscle cells become less responsive (20). It can be theorized that the hormonal changes accompanying pregnancy shift milk letdown dependence from the oxytocin mechanism to the local autonomic reflex controlled mechanism. It is thought, but not yet proven, that teat stimulation is more critical in eliciting the local autonomic reflex milk letdown mechanism than the oxytocin milk letdown mechanism. PREP TIMEPrep time is defined as the time taken to manually clean and dry the teat surface. The object is to be sure that the teat surfaces are consistently clean and dry before the milking machine is attached and that adequate teat massage has occurred to stimulate milk letdown. Recent studies demonstrate that less than 10 seconds is inadequate stimulus for consistent milk letdown response in all cows. While 10 seconds will provide adequate milk letdown stimulus for American Holsteins in early lactation, it is not adequate for late lactation cows. European Fresians and Jersey cattle require more stimulus for a consistent milk letdown response (17). Studies show that good cleaning and drying with separate towels will reduce bacterial populations on teat surfaces by 75% (9). Predip data demonstrate that improved teat sanitation reduces intramammary infection rate (8, 10, 15). It appears that a teat cleaning and drying procedure that results in a quality stimulus of 10 to 20 seconds is adequate to consistently achieve milk letdown while effectively sanitizing teat surfaces in most cases. Cow cleanliness has a great effect on cow prep efficiency. It is estimated that dirty cows will easily double cow prep time and, thus, unnecessarily slow down parlor throughput. Design and evaluation of cow prep procedure should always be done within the context of general herd sanitation. Forestripping, to check for clinical mastitis, is a recommended premilking cow prep procedure. Forestripping is a very powerful milk letdown stimulus and, therefore, is best used early during the cow prep procedure. However, if the premilking cow prep procedure is greater than 20 seconds, the addition of forestripping will add little advantage to milking efficiency (17). Therefore, in those circumstances where a minimal cow prep (10 seconds) is considered appropriate, forestripping should be included in the cow prep procedure to ensure consistent milk letdown response. PREP-LAG TIMEPrep-lag time is the time between the beginning of teat preparation to the application of the milking machine (Figure 1). Recent U.S. and Denmark studies have determined that prep-lag timing is of critical importance in optimizing milking efficiency. These studies report the ideal prep-lag time to be 1.3 minutes, or 1 minute and 18 seconds (17). The range of 1 to 1.5 minutes is accepted as the optimal prep-lag time for all stages of lactation. Prep-lag times of greater than 3 minutes were found to result in more residual milk and lower milk yields regardless of stage of lactation (17). Exceedingly long prep-lag times are more common in stall barn milking and likely to limit herd performance. Use of end-of-milking indicators are helpful in alleviating this problem.
STANDARDIZATION OF ROUTINECows love routine. They perform best when all feeding, milking or any other management routine is done the exact same way every day. Complete lactation studies demonstrated a 5.5% increase in lactational yield when a standardized milking routine was used compared to an impulsive and variable milking routine (16). This evidence supports the recommendation that one goal of every milking routine is to milk every cow exactly the same at every milking regardless of stage of lactation or who is milking. MODEL TO DESCRIBE THE EFFECT OF PREP-LAG TIME AND MILK FLOW RATE ON THROUGHPUTA rule-based model was written to describe the effects of prep-lag time and milk flow rate on throughput in herringbone and parallel parlors. The model is simply a collection of equations and values that were developed to represent the available literature (1, 2, 3, 4, 6, 7, 11). The model was developed for herringbone and parallel parlors up to double-20 in size. Rapid-exit was included for herringbone parlors with 10 or more stalls per side. The labor efficiency of each size parlor was set to be equal to the mean of the literature values used to calibrate the model. The model does not differentiate between herringbone and parallel parlors since the data indicates that the difference in throughput is slight or nonexistent (2). A description of the model is provided in Appendix A. The most important rule used in the model is an equation that was developed to describe the impact of milk yield (lb/cow/milking) and prep-lag time (as defined in Figure 1) on milk flow rate. Milk flow rate and yield determine the unit on-time in a parlor and can have a significant impact on throughput. A graph of this equation is shown in Figure 2. The equation is given in the Appendix (Equation A4). The relationship was developed based on information provided in the literature (1, 7, 11, 17) and reflects observed trends. The graph clearly indicates that milk flow rate increases significantly with increases in milk yield and prep-lag time. Furthermore, as prep-lag time was increased from 60 to 90 seconds, the increase in milk flow rate was not as great.
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Table 2. Estimates of steady-state throughput rates (c/hr) for automated herringbone and parallel parlors assuming one operator using the rule-based model described in the Appendix. |
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Prep-lag, sec |
Milk yield, lb/cow/milking |
|||||||
20 |
25 |
27 |
30 |
33 |
35 |
37 |
40 |
|
| 0 | 58 |
52 |
50 |
48 |
46 |
44 |
43 |
41 |
| 10 | 61 |
55 |
53 |
51 |
48 |
47 |
46 |
44 |
| 20 | 63 |
57 |
55 |
53 |
50 |
49 |
48 |
46 |
| 30 | 65 |
59 |
57 |
54 |
52 |
50 |
49 |
47 |
| 40 | 65 |
60 |
58 |
55 |
53 |
51 |
50 |
48 |
| 60 | 65 |
60 |
58 |
56 |
53 |
52 |
51 |
49 |
| 90 | 62 |
57 |
56 |
54 |
52 |
51 |
49 |
48 |
| 120 | 57 |
53 |
52 |
50 |
48 |
47 |
46 |
45 |
|
||||||||
Prep-lag, sec |
Milk yield, lb/cow/milking |
|||||||
20 |
25 |
27 |
30 |
33 |
35 |
37 |
40 |
|
| 0 | 71 |
65 |
63 |
60 |
57 |
55 |
54 |
52 |
| 10 | 75 |
68 |
66 |
63 |
60 |
58 |
57 |
55 |
| 20 | 77 |
70 |
68 |
65 |
63 |
61 |
59 |
57 |
| 30 | 79 |
72 |
70 |
67 |
64 |
63 |
61 |
59 |
| 40 | 79 |
73 |
71 |
68 |
65 |
64 |
62 |
60 |
| 60 | 79 |
73 |
71 |
68 |
66 |
64 |
63 |
61 |
| 90 | 76 |
70 |
69 |
66 |
64 |
63 |
61 |
59 |
| 120 | 70 |
66 |
64 |
62 |
60 |
59 |
58 |
56 |
|
||||||||
Prep-lag, sec |
Milk yield, lb/cow/milking |
|||||||
20 |
25 |
27 |
30 |
33 |
35 |
37 |
40 |
|
| 0 | 83 |
76 |
73 |
70 |
67 |
65 |
64 |
61 |
| 10 | 87 |
79 |
77 |
74 |
71 |
69 |
67 |
65 |
| 20 | 89 |
82 |
80 |
76 |
73 |
71 |
70 |
67 |
| 30 | 91 |
84 |
81 |
78 |
75 |
73 |
72 |
69 |
| 40 | 92 |
85 |
82 |
79 |
76 |
75 |
73 |
70 |
| 60 | 91 |
85 |
83 |
80 |
77 |
75 |
74 |
71 |
| 90 | 87 |
82 |
80 |
77 |
75 |
73 |
72 |
70 |
| 120 | 82 |
77 |
75 |
73 |
71 |
69 |
68 |
66 |
|
||||||||
Prep-lag, sec |
Milk yield, lb/cow/milking |
|||||||
20 |
25 |
27 |
30 |
33 |
35 |
37 |
40 |
|
| 0 | 95 |
87 |
84 |
81 |
77 |
75 |
73 |
71 |
| 10 | 99 |
91 |
88 |
85 |
81 |
79 |
77 |
75 |
| 20 | 101 |
94 |
91 |
87 |
84 |
82 |
80 |
77 |
| 30 | 103 |
96 |
93 |
89 |
86 |
84 |
82 |
80 |
| 40 | 104 |
97 |
94 |
91 |
87 |
86 |
84 |
81 |
| 60 | 104 |
97 |
94 |
91 |
88 |
86 |
85 |
82 |
| 90 | 100 |
94 |
92 |
89 |
86 |
84 |
83 |
80 |
| 120 | 93 |
88 |
86 |
84 |
81 |
80 |
78 |
76 |
|
||||||||
Prep-lag, sec |
Milk yield, lb/cow/milking |
|||||||
20 |
25 |
27 |
30 |
33 |
35 |
37 |
40 |
|
| 0 | 120 |
110 |
107 |
103 |
99 |
96 |
94 |
91 |
| 10 | 125 |
115 |
112 |
108 |
104 |
101 |
99 |
95 |
| 20 | 128 |
119 |
116 |
111 |
107 |
105 |
102 |
99 |
| 30 | 130 |
121 |
118 |
114 |
110 |
107 |
105 |
102 |
| 40 | 131 |
122 |
119 |
115 |
111 |
109 |
107 |
103 |
| 60 | 131 |
122 |
120 |
116 |
112 |
110 |
108 |
105 |
| 90 | 126 |
119 |
116 |
113 |
109 |
107 |
105 |
103 |
| 120 | 118 |
112 |
110 |
106 |
104 |
102 |
100 |
97 |
|
||||||||
Prep-lag, sec |
Milk yield, lb/cow/milking |
|||||||
20 |
25 |
27 |
30 |
33 |
35 |
37 |
40 |
|
| 0 | 124 |
116 |
113 |
109 |
106 |
103 |
101 |
98 |
| 10 | 128 |
120 |
117 |
114 |
110 |
108 |
106 |
102 |
| 20 | 131 |
123 |
120 |
117 |
113 |
111 |
109 |
106 |
| 30 | 133 |
125 |
122 |
119 |
115 |
113 |
111 |
108 |
| 40 | 134 |
126 |
124 |
120 |
117 |
115 |
113 |
110 |
| 60 | 133 |
126 |
124 |
121 |
118 |
116 |
114 |
111 |
| 90 | 129 |
123 |
121 |
118 |
115 |
113 |
112 |
109 |
| 120 | 123 |
117 |
115 |
112 |
110 |
108 |
107 |
104 |
The potential benefits of optimizing udder preparation and throughput are: 1) increase in milk yield due to a decrease in somatic cell count (SCC) where increased SCC was caused by inadequate cow prep, 2) increase in milk price due to reduced SCC, or 3) decrease in labor costs. The benefits will be calculated for a 400-cow dairy in the Southeast, a 400-cow dairy in the Upper Midwest, and a Southwestern dairy that operates the parlor 21 hours per day. All three herds are assumed to have an initial production of 19,800 lb/cow/yr. The equations used to calculate yield increases, milking shifts and labor costs are described in the following sections.
Yield reduction due to SCC. The reduction in milk yield due to SCC is well documented and is shown in Table 3. Equations are provided in the table to facilitate calculation of yield losses based on the linear SCC score (LS) for first lactation and older cows. The yield benefits of reducing SCC depends on the percentage of heifers in the herd and the magnitude of decrease in SCC. An increase in yield due to a reduction in SCC is calculated as the average yield loss at the high SCC value minus the average yield loss at the low SCC value. In all of the scenarios that will be presented, the cull rate was assumed to be 33%. Therefore, the average yield loss at a given SCC is (0.33 × MYL for heifers) + (0.67 × MYL for older cows).
Table 3. Relationship between somatic cell count (SCC), linear SCC score (LS), and daily yield loss (18). |
Daily yield loss, lb/cow/day |
|||
Average SCC |
LS |
First lactation |
Older cows |
.... |
0 - 2 |
0 |
0 |
100,000 |
3 |
1.39 |
2.76 |
200,000 |
4 |
2.85 |
5.67 |
400,000 |
5 |
4.31 |
8.59 |
800,000 |
6 |
5.76 |
11.50 |
1,600,000 |
7 |
7.22 |
14.41 |
3,200,000 |
8 |
8.68 |
17.32 |
6,400,000 |
9 |
10.13 |
20.24 |
Regression equations to relate LS to daily milk yield loss (MYL). |
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First Lactation: |
MYL = 1.457 LS 2.98 |
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Older Cows: |
MYL = 2.913 LS 5.98 |
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Milking-time
throughput, set-up and clean-up time. The values shown in Table 3 are estimates of the steady-state throughput
(SST) and is a measure of parlor performance at full capacity. Total
milking throughput or milking-time throughput, as defined by Barry
et al. (5), includes all of the delays associated with group changes,
cows milked with a bucket, time when the milking area is not full
of cows at the end of groups, unit falloffs, and any other delay.
Milking-time throughput (MTT) is typically less than steady state
and is a better measure of what the operator experiences. Milking-time
throughput includes all delays but not the time for parlor set-up
and clean-up. Barry et al. (5) developed the following regression
equation that relates steady-state throughput to milking time throughput
for 30 herringbone parlors ranging from double-6 to double-12:
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Barry (5) also determined that the average set-up and clean-up time for the 30 parlors in the study was 0.5 hours. Calculation of labor costs. Labor costs are determined by the length of the milking shift and the labor rate. The length of a milking shift was estimated by the following equation:
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(Equation 2) |
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The labor rate was assumed to be $9/hr and includes social security tax, workers compensation insurance and unemployment tax. Annual labor cost was calculated as follows:
|
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Annual
labor cost = (Milking shift) × (No. of milkings) × |
(Equation 3) |