Influence of early weaning on yield and fat content of milk from dual purpose cows

Restricted suckling during lactation is a widespread practice in cattle managed in dual purpose systems, appreciably reducing saleable milk and its fat content. Early weaning might solve these problems, but it could reduce milk yield after weaning. An experiment was carried out to evaluate the influence of early weaning on milk yield and fat content of crossbred Brahman x Holstein cows with restricted suckling. Twenty four animals were used, machine milked twice daily and fed 2.5 kg/day of concentrate at milking. The rest of the day they grazed paddocks of Brachiaria mutica, Cynodon nlemfuensis and Digitaria swazilandensis. Calves were suckled by their dams for 30 minutes after the morning milking. Two weaning ages were compared: at 17 weeks (W17) or at 35 weeks of lactation (W35). Milk yields after 18 weeks were corrected for yields up to 17 weeks and results presented are adjusted mean yields of milk corrected to 4 % fat (FCM) between weeks 18 and 35 of lactation.

Early weaning increased saleable FCM from 4.5 to 5.9 kg/day (P<0.05) and milk fat content from 2.4 to 3.1 % in the morning and from 3.3 to 4.4 % in the afternoon (P<0.01), but total FCM was reduced from 7.1 to 5.9 kg/day (P<0.05) and live weight gain of the calves decreased from 0.39 to 0.29 kg/day compared to animals that were not weaned during the experimental period.  All these aspects must be considered in making decisions about the appropriate weaning time.

Key words: cattle, restricted suckling, weaning, milk yield, milk fat

Introduction

Restricted suckling is a generalized practice in tropical dual purpose cattle systems and has a series of advantages over artificial rearing, such as a higher milk production, lower mastitis incidence and higher liveweight gain of the calves (see Ryle and Ørskov 1992; Preston et al 1995; Sahn et al 1997). However, an important disadvantage is the high proportion of total milk consumed by the calf, which is approximately 42 % according to the review by Sandoval-Castro et al (1995), and appreciably reduces saleable milk and its fat content.

In dual purpose systems the calf usually sucks its dam throughout the lactation, resulting in a high consumption of milk by the calf. An option for decreasing the amount of milk consumed by the calf, is to wean the animals in early or mid lactation. Hernández et al (1999) have shown that reducing the length of the suckling period increases the amount of saleable milk and that calf weight gain is not affected if solid feed is available. However, no information is available to compare yield and composition of milk from cows weaned in mid lactation or suckling their calves to the end of lactation. The objective of this experiment was to evaluate the influence of early weaning on yield and fat content of milk during the second part of lactation.

Materials and methods

A completely randomized experimental design was used to compare two treatments:

• W35:  weaning at the end of the trial at 35 weeks of lactation
  
• W17: early weaning at 17 weeks of lactation.

The experiment was carried out in Maracay, Venezuela, with 24 Brahman x Holstein cows with 5/8 to ¾ Holstein influence, 8 first calving heifers and 16 cows of 2 or more parities. The cows were kept with their calves during the first 3 days after calving and were balanced in the two treatments according to calving date and parity. The animals were machine milked twice daily at 06:30 and 16:00 h, without suckling before milking. After the morning milking they were taken to the calves’ individual pens to suckle them for approximately 30 minutes. The cows received 2.5 kg/day of concentrate with 21.7 % crude protein and 45.7 % neutral detergent fibre (NDF) at milking and grazed during the rest of the day in paddocks of Cynodon nlemfuensis, Brachiaria mutica and Digitaria swazilandensis. Calves were kept in individual pens during the first 17 weeks and received up to 1 kg/day of concentrate and chopped forage ad libitum. From week 18, the calves were supplemented with 1.5 kg/day concentrate on average, about 1.5 kg/day of fresh Gliricidia sepium foliage and were turned out to graze from 10:00 to 16:00 h in Cynodon nlemfuensis paddocks.

The cows were weighed weekly after calving and saleable milk was measured daily at each milking. LW change of cows and calves from weeks 18 to 35 of lactation was calculated as the regression coefficient of LW on time. A weekly sample of morning and afternoon saleable milk was taken for fat analysis, from a bucket where all milk from the machine milking bottle was transferred and thoroughly mixed. Milk consumed by calves was estimated weekly, weighing them before and after suckling, and a milk sample was taken just after suckling started. Fat content of saleable and consumed milk samples was analyzed by the Babcock method (AOAC 1984). Milk corrected to 4 % fat (FCM) was also calculated using the equation described in NRC (1989). Mastitis incidence was evaluated monthly with the California Mastitis Test.

To carry out the statistical analysis the results were divided in two periods, Period I between weeks 1 and 17 and Period II between weeks 18 and 35. An analysis of variance was carried out on milk and FCM yields and their saleable and consumed fractions in Period I, fat percentages in Periods I and II and LW changes of cows and calves in Period II. Milk and FCM yields in Period II were corrected by milk yield and FCM in Period I and adjusted means calculated (SAS 1996).

Results and Discussion

Feeding and management in Period I were similar in all animals (Table 1) and no differences between treatments were observed (P>0.10).  Average milk consumption by the calves was 2.1 kg/day and highly variable, accounting for 18.9 % of total milk. The fat content of the consumed milk by the calves was 6.8 % and higher than the fat content of the saleable milk (2.3 and 2.4 %, for am and pm samples. This difference has been pointed out by several authors (see Sandoval-Castro et al 1995), but in this trial saleable milk fat was lower than usual, possibly as a consequence of the absence of calf stimulus before milking and its negative effect on the fat content of this fraction (Tesorero et al 2000). After correcting milk fractions for their fat content (FCM), yields were modified, decreasing the saleable fraction and increasing the amount consumed daily, which rose to 31 % of total FCM, even through calves were suckled only once daily. Average milk consumption by the calves decreased from about 4 kg/day at the beginning of lactation to near 2.5 kg/day at 17 weeks (Figure 1). This trend towards a more rapid decline in consumed than in saleable milk at the start of lactation was observed previously (Hernández et al 1999), however, it stabilized later and remained fairly constant during Period II, in which saleable milk decreased at a higher rate. The average liveweight gain of the calves in Period I was 0.40±0.20 kg/day. 


Table 1.Yield and fat content of milk and live weight of cows in Period I (means and standard deviations)

Early weaning increased saleable milk fat percentage (P<0.01) and saleable FCM yield (P<0.05) in comparison to cows still suckling their calves (Table 2), and the same trend was observed in saleable milk (P<0.10). This effect on FCM can also be observed in Figure 1, but differences between treatments tended to be reduced as the lactation advanced and yields were very similar towards the end of lactation. The resulting total FCM yields in this period (7.1 for W35 versus 5.9 kg/day for W17), were higher for the W35 treatment (P<0.05) because of the inclusion of the milk consumed by the calf, which had a high fat content. 



Figure 1.Daily yield of saleable milk  and milk consumed by the calves weaned after 17 weeks or until the end of the experiment at 35 weeks (as FCM)






Table 2.  Mean values for milk fat content and yields in Period II





An increase in milk yield when restricted suckling is used has been observed in several studies (see Preston et al 1995; Sahn et al 1997), where this system has been compared with artificial rearing, but these trials have been limited to the period when milk is offered to calves in buckets. The results of this study have shown that restricted suckling continues stimulating milk production at advanced stages of lactation. Bar-Peled et al (1995) observed that after weaning there was a fall in milk yield, attributed by them to a psychological disturbance of milk ejection, and pointed out that there were no long term effects because cows became accustomed to the absence of the calf. Similar results were obtained by Hernández et al (1999), but in both studies all animals were weaned in early or mid lactation and there was no restricted suckling treatment continued until late lactation.

The incidence of positive mastitis was low in the trial, with only one case in a cow on the W35 treatment in Period II that required medication. Growth rates of calves in Period II were 0.29 and 0.39 kg/day in treatments W17 and W35 (P = 0.08), and the differences appear to be related to the greater milk consumption in the late weaned animals.

The results of this research can be useful for decision making about the appropriate moment to wean the animals. Early weaning increases saleable milk fat content and FCM yield, increasing the income derived from milk sale, and reducing the calf management required by restricted suckling. But it also reduces total milk yield and deprives the calf of milk in the latter part of the lactation,  with the consequent reduction in  calf growth rate. The effect of the treatments on the duration of lactation was not evaluated, because it was terminated in all the cows after 35 weeks, but from the trend in Figure 2 it appears that it would be shorter with the early weaning system, which is another important aspect to be considered.


Figure 2.Trends in total milk yield by dual-purpose cows whose calves were weaned after 17 weeks or at the end of the experimental period of 35 weeks






Conclusions

In a dual purpose cattle production system, early weaning of the calves at 17 weeks of lactation results in:

• Higher fat content in the saleable milk and higher FCM yield
  
• Reduced total FCM yield over the whole lactation
  
• A slower growth rate of the calves.
  

Acknowledgements

The authors express their gratitude to CONICIT for the financial support (Project Nº S1-99000047)

References


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