**3. Local regulation of the milk yield response to frequent milking**

Shortly after the report in the 1930s that frequent milking increased milk production, several studies utilized unilateral frequent milking (**UFM**) to investigate the effect. Half-udder designs are extremely powerful because they eliminate variation between animals due to environment, nutrition, and genetics. In addition, both udder halves are theoretically exposed to the same systemic factors, hence responses to frequent milking are strictly at the level of the mammary gland. A summary of the milk yield response to frequent milking in selected half-udder experiments is presented in Table 7. The early reports provided strong evidence for local regulation of milk production, and increases in milk yield from 8.4 to 32% in the frequently-milked udder half were observed (Ludwick et al., 1941; Cash & Yapp, 1950; Agarwala & Sundaresan, 1955; Claesson et al., 1959). Morag (1973b) reported that the increase in milk production in response to UFM occurs within 24 h, and the magnitude of the response was independent of previous milk production. In addition, heifers respond to UFM; Hillerton et al. (1990) milked udder halves 2X or 4X for 4 wk during mid-lactation. In both cows and heifers, milk production of 4X udder halves increased by 10.4% relative to 2X udder halves (Hillerton et al., 1990).



aReports that did not use half-udders are represented in Tables 1 and 5.

bNumbers in columns represent the increase in milk yield observed with increased milking frequency.

c 2X = twice daily milking; 3X = thrice daily milking; 4X = four times daily milking.

dDIM = days in milk.

e Acute = the increase in milk production during frequent milking.

f Pers. = the persistent increase in milk production after frequent milking, if reported.

**Table 7.** Summary of select literature reports on the effects of unilateral frequent milking (UFM) on milk yielda

As mentioned previously, an emerging theme in these experiments has been that the effects of frequent milking during early lactation on milk production persist even after a lower milking frequency is resumed (Bar-Peled et al., 1995; Hale et al., 2003; Dahl et al., 2004b). Although this persistent milk yield response has been consistently observed (Table 5), it was unknown whether the response was regulated by hormones, by local factors within the mammary gland, or by the combination of the two. To investigate this question, we used a half-udder model and assigned cows to UFM (4X of the right udder half, 2X of the left udder half) for d 1 to 21 of lactation, followed by 2X for the remainder of lactation (Wall & McFadden, 2007a). When the half-udder milk yields were adjusted to the equivalent of a whole udder basis, the acute and long-term milk yield responses to frequent milking that we observed were consistent with those reported by Hale et al. (2003). Therefore, our results indicated that both the acute and persistent effects of frequent milking during early lactation are regulated by local factors within the mammary gland. This is illustrated in Figure 3A and B. Figure 3A (re-drawn from Bar-Peled et al., 1995) shows the milk yield response of multiparous cows to 6X for the first 6 wk of lactation, followed by 3X. We observed a similar effect using a half-udder experiment (Figure 3B), and the milk yield response lasted through 270 DIM. This finding presents some intriguing questions and research opportunities. First, what are the local factors that regulate milk production capacity of the mammary gland? Once the factor(s) have been identified and pathways understood, how can we refine our approach to maximize milk production efficiency of dairy cows? Now that it is established that the factors are indeed local, the problem has become relatively simplified. Extremely powerful, within cow experiments that are less sensitive to the influence of environment, genetics and nutrition can now be designed to ask such mechanistic questions.

246 Milk Production – An Up-to-Date Overview of Animal Nutrition, Management and Health

Knight, 1992 Early 42 d ≥ 1 + 10.4

**Duration of** 

Norgaard et al., 2005 Mid 7 d ≥ 1 (+) 18% Wall & McFadden, 2007a 1 DIMd 21 d ≥ 2 (+) 3.5 kg/d

Wall & McFadden, 2007b 1 DIM 14 d ≥ 2 (+) 3.7 kg/d (acute)

Wright et al., 2011 1 DIM 21 d 1 (+) 2.8 kg/d

Shields et al., 2011 1 DIM 21 d ≥ 2 (+) 3.4 kg/d

**Table 7.** Summary of select literature reports on the effects of unilateral frequent milking (UFM) on

As mentioned previously, an emerging theme in these experiments has been that the effects of frequent milking during early lactation on milk production persist even after a lower milking frequency is resumed (Bar-Peled et al., 1995; Hale et al., 2003; Dahl et al., 2004b). Although this persistent milk yield response has been consistently observed (Table 5), it was unknown whether the response was regulated by hormones, by local factors within the mammary gland, or by the combination of the two. To investigate this question, we used a half-udder model and assigned cows to UFM (4X of the right udder half, 2X of the left udder half) for d 1 to 21 of lactation, followed by 2X for the remainder of lactation (Wall & McFadden, 2007a). When the half-udder milk yields were adjusted to the equivalent of a whole udder basis, the acute and long-term milk yield responses to frequent milking that we observed were consistent with those reported by Hale et al. (2003). Therefore, our results indicated that both the acute and persistent effects of frequent milking during early lactation are regulated by local factors within the mammary gland. This is illustrated in Figure 3A and B. Figure 3A (re-drawn from Bar-Peled et al., 1995) shows the milk yield response of multiparous cows to 6X for the first 6 wk of lactation, followed by 3X. We observed a similar effect using a half-udder experiment (Figure 3B), and the milk yield response lasted through 270 DIM. This finding presents some intriguing questions and research opportunities. First, what are the local factors that regulate milk production capacity of the mammary gland? Once the factor(s) have been identified and pathways understood, how can we refine our approach to maximize milk production efficiency of dairy cows? Now that it is established that the factors are indeed local, the problem has become relatively simplified. Extremely

bNumbers in columns represent the increase in milk yield observed with increased milking frequency.

**UFM Parity Change in milk yield**

**(%; 2X vs. 3X)** 

(+) 1.8 kg/d (pers.f

(+) 1.2 kg/d (pers.)

(+) 1.5 kg/d (pers.)

(+) 0.9 kg/d (pers.)

7 DIM 14 d ≥ 2 (+) 2.9 kg/d (acute)

**Change in milk yield (2X vs. 4X)** 

(acutee)

(acutee)

)

**Lactation** 

aReports that did not use half-udders are represented in Tables 1 and 5.

Acute = the increase in milk production during frequent milking.

2X = twice daily milking; 3X = thrice daily milking; 4X = four times daily milking.

Pers. = the persistent increase in milk production after frequent milking, if reported.

c

e

f

dDIM = days in milk.

milk yielda

**Reference Stage of** 

**Figure 3.** A. Six-times daily milking for days 1 to 42 of lactation increases milk production through late lactation (re-drawn from Bar-Peled et al., 1995). B. Unilateral four-times daily milking for days 1 to 21 of lactation increases milk production for the remainder of lactation (Wall and McFadden, 2007).

On the road to refinement, one theme that has transpired is the existence of a 'window' of time wherein the mammary gland is especially responsive to frequent milking. The duration of this window has been shortened from the first 10 wk of lactation (Moss & O'Grady, 1978; Thomas et al., 1978) to the first 6 wk of lactation (Bar-Peled et al., 1995; Sanders et al., 2000), and shortened further still to the first 3 wk of lactation (Hale et al., 2003; Dahl et al., 2004b; Wall & McFadden, 2007a). It was unknown whether a shorter duration or altered timing of frequent milking during early lactation would still elicit a persistent effect on milk production; however, since any costs associated with extra labor are increased only during frequent milking, it was of great interest to shorten the duration of frequent milking if a persistent increase in milk yield could still be observed. To answer this question, we assigned cows to UFM (4X of the right udder half, 2X of the left udder half) for d 1 to 14 or d 7 to 21 of lactation (Wall & McFadden, 2007b). We observed an acute milk yield response in both treatments; and a significant carry-over effect in the d 7 to 21 group. There was a numerical carry-over for the d 1 to 14 group; however it was not significant. Our results indicate that within the first 21 DIM, an interval of frequent milking as short as 2 wk can elicit a persistent increase in milk production. As mentioned previously, similar observations have been made on the response of cows and heifers to suckling during early lactation (Fulkerson et al., 1978; Peel et al., 1979). Further narrowing of this "window" within the first 21 of lactation, as well as characterization of the cellular response could provide insight into the mechanisms underlying the receptiveness of the mammary gland to stimulus during this time.

## **4. Endocrine response to frequent milking or suckling**

It has long been thought that the hormones released at milking may be involved in regulating the galactopoietic effects of frequent milking on milk production. Indeed, multiple hormones are released during milking including glucocorticoids, oxytocin, and prolactin (Tucker et al., 1975; Carruthers & Hafs, 1980; Akers & Lefcourt, 1982). Oxytocin is responsible for milk ejection. Cows suckling calves are thought to have more efficient milk ejection due to increased secretion of oxytocin elicited by the presence of the calf. In fact, on dairies using cross breeds and cows not bred for high milk production, the calf is often used as a facilitator of milk letdown during milkings (Little et al., 1991; Krohn, 2001; Bruckmaier & Wellnitz, 2008). In addition, treatment with exogenous oxytocin was associated with increased milk production of both dairy cows (Nostrand et al., 1991; Ballou et al., 1993; Lollivier & Marnet, 2005) and sheep (Zamiri et al., 2001). Therefore, it is possible that oxytocin is involved in regulating the increase in milk production elicited by frequent milking or suckling, perhaps by allowing for more complete milk removal and a decrease in negative feedback on the gland.

Along with enhanced milk production, Bar-Peled et al. (1995) observed increased concentrations of growth hormone, insulin-like growth factor-1, oxytocin and prolactin in circulation of cows that were frequently milked or suckled. In addition, the magnitude of milking-induced PRL release declines concomitantly with the decrease in milk production as lactation progresses (Koprowski & Tucker, 1973). Consequently, PRL has been hypothesized as a candidate regulator of the effects of frequent milking on milk production (Dahl et al., 2004a). In an attempt to determine whether milking-induced PRL release indeed mediates the effects of frequent milking on milk production, we assigned cows to 2X, 4X, or 2X + twice daily injections of PRL (Crawford et al., 2004; Wall et al., 2006). Four times daily milking or PRL injections increased milk production relative to 2X (Crawford et al., 2004); however our results indicated that PRL injection or frequent milking exerted distinct effects on mammary cell growth and gene expression, thus probably increased milk production via separate mechanisms (Wall et al., 2006). The response to unilateral frequent milking during early lactation supports this concept; frequent milking may stimulate milk production via local factors, whereas PRL injections may increase milk yield through a more systemic pathway.
