Cow-calf relationship during milking and its effect on milk yield and calf live weight gain

Restricted suckling is a characteristic of dual purpose cattle systems found in tropical America and has a series of advantages over artificial rearing, such as higher milk yield and calf live weight gain, together with lower incidence of mastitis. However, an important limitation of this practice is the complicated management during milking. It implies appropriate installations for machine milking, high labour requirements, difficulties in handling large calves and health problems in the milking parlour. A series of experiments was carried out to evaluate the cow-calf relationship during milking and its effect on milk fractions, fat content and calf live weight gain. It was found that suckling before milking and calf presence, in contact or within view distance of the dam during milking, increased saleable milk yield and fat content at the expense of consumed milk and calf live weight gain. This is a topic of research which has received little attention to date and could contribute to improving dual purpose systems through the facilitation of milking management.

Key words: Cattle, cow management, milk fat, milk yield, restricted suckling
 

The purpose of cow milk, as in other mammals, is to feed calves during the early stages of life. More than 80 % of milk is stored in udder alveoli and is transferred to the cistern by a neuro-hormonal reflex initiated by the contact of the calf’s mouth with the udder that culminates in the contraction of the myoepithelial cells surrounding the alveoli by the action of oxytocin liberated from the pituitary gland. 

The main objective of dairy cow domestication was to obtain milk for human consumption, and it implied substituting calf suckling by hand or machine milking. The latter stimulus is enough in European dairy cows to allow efficient milking, perhaps as a consequence of a long process of selection against those cows that required the stimulus of the calves. But in Zebu x European crossbred animals, let down does not occur in variable proportions of cows without calf stimulation. Alvarez et al (1980) observed that half of a herd of 33 first parity Brown Swiss and Holstein cross with Zebu cows milked without the calves had short lactations of less than 100 days and very low milk yields. 

In order to milk crossbred animals more efficiently a mixed system has been developed, in which a short period of suckling is allowed, followed by hand or machine milking and culminating in calves suckling of the remaining milk. This system has a series of advantages over milking without calves, such as higher milk yields, lower mastitis incidence and higher calf live weight gains (Ryle and Ørskov 1990; Preston et al 1995; Sandoval-Castro et al 1995). However, an important limitation is the complicated management during milking, which implies appropriate installations for machine milking, high labour requirements, difficulties in handling large calves and health problems in the milking parlour. This paper present the results of a series of studies carried out to evaluate the cow-calf relationship during milking and its effect on milk fractions, their fat content and calf live weight gain. All the experiments where carried out at the Faculty of Agronomy in Maracay in a dual purpose herd composed of ⅝ to ¾ Holstein x Brahman cows. All animals were machine milked twice daily, beginning at 0630 and 1530 h. Restricted suckling was carried out during nearly 30 minutes after the morning milking, and weaning was at 17 weeks of age or 90 kg of live weight. 

An option which permits suckling without the mentioned inconveniences is to milk the cow without calf stimulation and allow restricted suckling after milking. This procedure is used in very few dual purpose farms. In Table 1 results obtained with this strategy (W) are compared with two others where suckling was allowed before milking until let down occurred either at the morning milking only (AM) or at both morning and afternoon milkings (AM+PM). Saleable milk yield obtained without the calf was 1 to 2 kg/day lower than in cows stimulated by the calf, and fat content was also lower. The lower saleable milk yield on treatment W was only partly compensated by a slight increase in live weight gain of calves, because there was a trend for animals consuming less milk to increase consumption of solid feed. The low saleable milk fat content of cows not suckled before milking is of practical importance, because it is penalised by the industry. Other milk constituents, such as protein, were not analysed, but other studies have shown that this fraction does not vary between saleable and consumed milk (Sandoval-Castro et al 1995). There is a gradient in milk fat content that varies from less than 1 % in milk initially stored in the cistern to 10 % in residual milk in the alveoli, and fat content of saleable milk is positively related to its proportion of total milk. These results should not be extrapolated to animals with a larger proportion of Zebu, where possibly the saleable milk in cows milked without the calf is lower. 

Suckling before milking has a series of inconveniences already mentioned, but as was shown before, its absence decreases saleable milk yield and fat content. An option to facilitate management is to tie the calf beside its dam, without allowing contact with the udder. To evaluate this strategy four treatments were evaluated: 

            (W)      Without the calf at milking
            (P)         Presence of the calf during milking
            (S)         Suckling before milking        
            (SP)     Suckling before milking and permanence during milking 

Suckling in Treatments S and SP was allowed until milk let-down occurred and the calves were separated at the start of machine milking (S) or remained tied beside dams until milking finished (SP). The latter procedure is the most frequent practice management on farms. 

As in the first trial, suckling before milking increased saleable milk yield by almost 2 kg/day at the expense of consumed milk (Table 2) and fat content increased by more than one percentage unit. Milk yield consumed before milking in treatment S was 0.7 kg/day with 0.4 % fat in the morning and 0.5 kg/day with 2.2 % fat in the afternoon. The fat content differences between morning and afternoon milking are probably a consequence of the presence or absence of low fat milk accumulated in the cistern, that starts to be present after 7 h after the previous milking (Bruckmaier and Blum 1998).  

The main contribution of this trial is that it shows that only the presence of the calf tied to the dam is enough to stimulate the let-down of milk to levels similar to those obtained with suckling before milking. This results in saleable milk yields and fat contents which are higher than those obtained from cows milked without their calves. The differences are higher in the afternoon, where saleable milk in Treatment W cows was almost 3 times lower than in the treatments with the calves in the milking parlour. The removal before milking of 1.2 kg/day milk with very low levels of fat in treatments S and PS resulted in slightly, but not significantly, lower saleable milk yields with higher fat contents than in Treatment P. 

Total daily milk yield in the four treatments was of 12 to 13 kg/day, equivalent to an hourly secretion of 0.5 kg. This was less than the 1.3 to 2.1 kg milk consumed by calves in a similar period between the end of morning milking and the end of restricted suckling on treatments with the calf in the milking parlour (Table 2), indicating that a significant amount of udder milk is not extracted at milking and is consumed later on by the calf. It also shows that even the presence of the calf during milking is not sufficient to allow a let-down similar to suckling, but that is an advantage in dual purpose systems, because a significant amount of milk remains in the udder to supply calf nutrients. 

Table 2. Yield and fat content of milk and calf live weight gain (LWG) of cows milked without the calf (W), with suckling before milking (S), with the calf present during milking (P) or with the calf present plus suckling  (SP)

Values in the same row with different letters are different (P<0.05)
1 Values assumed from measurements in Treatment S
Source:  M Tesorero, J Combellas and L Gabaldón (unpublished information)

From data in Table 2 it was speculated that calf stimulation at milking was not enough to allow complete let-down of milk from the alveoli. It was followed by an experiment to quantify its magnitude by injecting some of the cows with oxytocin (OT). Two groups of cows were milked with or without the presence of their calves during milking (Treatments W and P). Within each group about half of the animals received intramuscular injections of OT before milking during 10 days after days 30, 60 and 90 of lactation. Milk consumed milk the after morning milking is shown in Table 3. The results are influenced by the short period of sampling and the small number of animals used, resulting in smaller differences in consumed milk between treatments with or without calf stimulation than in the other experiments. But the main result is that consumed milk decreased on average from 2.9 to 1.0 kg/day when OT was injected. It means that OT due to calf presence during milking is not sufficient to obtain a let-down comparable to that obtained with exogenous injections of OT. From these results it can be derived that the difference of 1.9 kg/day in favor of cows not receiving exogenous OT is milk that remained in the udder alveoli and was not ejected by milking or calf stimulation. 

To overcome the inconveniences of calves in the milking parlour, an option evaluated was cow stimulation beforehand, by allowing the cow-calf contact just before milking, but separated by a fence to facilitate management (Treatment B below). An exploratory trial over periods of three weeks was carried out using a 4x4 Latin square with four recently calved cows and three weeks periods to compare the following treatments: 

(W)   Without the calf at milking
(B)   Presence of the calf with its dam before going into the milking parlour
(P)   Permanence of the calf with its dam during milking
(SP) Suckling before milking and permanence during milking 

The results are limited by the use of a small number of animals and the presence of residual effects, derived from the use of a change design in early lactation, since it has been shown that the separation of the calf at this stage of lactation can affect milk yield during the next five or more weeks (Bar-Peled et al 1995, Hernández et al 1999). However, it was observed that the cow-calf contact before going into the parlour is not sufficient to induce a level of saleable milk yield similar to that obtained in Treatments P and SP, with the calf present in the milking parlour (Table 4). Although the influence was not significant, a trend was observed towards increasing saleable milk and reducing consumed milk in comparison to Treatment W, in which there was no calf stimulus before or during milking.

Table 2 shows that similar saleable milk yields were obtained between cows suckled before milking and cows with their calves closely tied to them during milking. The latter procedure facilitates milking, but does not avoid all the inconveniences of calf presence in the milking parlour. An option to facilitate management is to increase the distance between the cow and its offspring, locating the calf within view of the cow to stimulate milk let-down. An experiment was carried out with 14 cows, half of them with the calf tied near the head during milking and the other half with the calf tied behind a fence at about 1.2 m. Results from the first three month of lactation show that there are no differences between both strategies (Table 5). Milk consumed was 19.0 and 17.5 % of total milk, slightly higher than in other trials because a shorter period with higher milk yield in early lactation was considered. This is an important advance, because the calves are kept at a short distance from the milking parlor but outside it. However, this management still requires individual handling of each cow and calf and new strategies should be evaluated, such as the handling of groups of calves to further facilitate the milking process. 

The results described above indicate that cow-calf management during milking influences milk yield fractions, and the results in Tables 1 and 2 also indicate a positive relationship between calf milk consumption and live weight gain (LWG). A more detailed study of this relationship was carried out with data from 24 calves whose dams were milked without their calves but with restricted suckling during 30 minutes after the morning milking. Half of the calves were weaned at 17 weeks and the others continued restricted suckling until 35 weeks of lactation. All calves received forage ad libitum and 1 and 1.5 kg/day concentrate before and after 17 weeks, respectively. 

Linear regressions between milk consumption (MC) and LWG in both periods are presented below and show a close correlation (Figure 1). It is interesting that the regression coefficients were similar in both periods and indicate that 6.5 kg milk are required to increase LWG by 1 kg. Khan and Preston (cited by Preston et al 1995) found a slightly higher conversion, of 4.9 kg milk per kg LWG, almost twice the value obtained with artificial rearing. This is partly explained by the higher energy content of milk suckled after milking, associated with its high fat content, but other factors could also be involved, such as the better health of suckling calves. It can also be appreciated from Figure 1 that there is high variation between cows in the amount of milk that remains in the udder after milking, with a range of 0.7 to 5.5 kg/day in the first period.

        Weeks 1 to 17:            LWG = 0.065 + 0.157 MC    r = 0.93
        Weeks 18 to 35:          LWG = 0.102 + 0.157 MC    r = 0.82 

The general relation of LWG to milk consumption could be extrapolated to other herds, although the precise magnitude of the response is influenced by other factors, such as the amount consumed of other components of the diet and their quality. Another question is the LWG which is desired to achieve, but that is outside the scope of this paper.  

Physiological studies evaluating the blood concentration of OT and other hormones in dual purpose cows with restricted suckling are not available, but recent advances in the physiology of lactation in temperate dairy cows could help to interpret the results obtained. Milk is continuously secreted into the lumen of the alveoli, where it is stored until OT action induces the contraction of the surrounding myoepythelial cells and milk is ejected into large ducts and the cistern. A few years ago it was held that there was an OT concentration threshold above which let-down occurred (Bruckmaier et al 1994; Schams et al 1994), but this simple approach could not explain the results presented here, in which different stimuli in restricted suckled cows resulted in variable proportions of saleable and consumed milk, or in other words, that variable amounts of milk are ejected from the alveoli as a consequence of the kind of stimulus applied. More recent evidence indicates that: 

• Elevated OT concentrations are necessary during milking for complete milk removal (Bruckmaier et al 1994). 

• Blood OT levels are higher with suckling than with milking when these strategies are alternated (Bar-Peled et al 1995). 

• OT release does not occur or is very low when milking takes place after suckling, but the contrary is not true and suckling after milking elevates OT to normal levels (Tacin et al 2001). 

• Much more myoepithelial cell contraction is required to press milk out of incompletely filled alveoli (Bruckmaier and Hilger 2001). 

Based on these findings the following speculative analysis was made in an attempt to understand and integrate the results obtained here. When the cow is milked without the calf and restricted suckling occurs afterwards, milking induces OT release, but as was shown by Bar-Peled et al (1995), OT level reached is much lower than with suckling and the proportion of saleable milk is small. It is more evident with udders which are not very full (Bruckmaier and Hilger 2001), such as in the afternoon milking in these trials (see Table 2), in which much more myoepithelial cell contraction is required to press milk out of incompletely filled alveoli (Bruckmaier and Hilger 2001). 

When suckling before milking is allowed until let-down is observed and the calf is separated from its dam during milking, a high OT release occurs but, based on the results of Tacin et al (2001), it ceases during milking. Milk ejection continues during milking induced by OT remaining in blood, until it stops as OT levels decrease and higher concentrations are required to contract the less full alveoli towards the end of milking (Bruckmaier and Hilger 2001). The additional stimulus of calf presence during milking possibly contributes to maintaining OT secretion and milk suckled afterwards is slightly reduced (see Table 2). However, the OT release with calf presence must be lower than with suckling, because the application of exogenous OT during milking with calf presence appreciably increases the quantity suckled afterwards (Table 3). 

Calf presence is effective in increasing saleable milk when it occurs during milking, but not before (Table 4). It must be continuous (Bruckmaier et al 1994) and combined with other factors, such as the familiar site of milking, the action of milking and the noise of the milking machine. Tying the calf near the cow’s head during milking stimulates all its senses, but as was shown in Table 5, just visual and audible stimuli when the calf is present but separated from its dam are sufficient to induce milk ejection. 

It is necessary to emphasise that this analysis is speculative. Physiological studies on dual purpose cows are required to have a better understanding of the mechanisms involved and derive more solid conclusions to serve as the basis for appropriate management.  

Cows and their calves interact in a variety of ways according to management, influencing saleable and consumed milk fractions, milk fat content and calf LWG. The results presented here permit some practical conclusions: 

• The absence of calf stimulation at milking reduces saleable milk yield and fat content. 

• Calf presence increases saleable milk yield and fat content when it occurs during milking, but not before. This stimulus is not sufficient to allow the complete removal of milk from the udder. 

• Suckling before milking until let down is observed increases saleable milk yield and fat content, but this stimulus does not continue during milking and milk is not completely removed. 

• Milk suckled before milking has a low fat content. So the procedure should slightly increase saleable milk fat, but at levels not detected in these studies. The effect is reduced if the period since the previous suckling is short. 

• In cows milked twice daily at different intervals, response to calf stimulation increases as the period from the previous suckling is reduced.  

• There is a close relationship between milk consumed and calf LWG, and consequently calves are affected by changes in management that influence the saleable:consumed milk ratio. 

• And finally, studies on the practical and physiological aspects of the cow-calf relationship in the milking parlor have not received the importance that they deserve. They could contribute to facilitate management in dual purpose systems and help to improve their efficiency. 

The authors express their gratitude to the editors of the book “Avances en la Ganadería de Doble Propósito” (González Stagnaro et al 2002) for the authorization to publish this modified translation of the paper “Relación vaca-becerro en la sala de ordeño y su influencia sobre la producción de leche y su contenido de grasa” 

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