21 March 1997


Does it pay to extend the lactations of high yielding dairy cows? Emma Leonard, of CEDAR, outlines the research which is trying to find out

ARE 305-day lactations and 365-day calving intervals still relevant for todays dairy industry? That is the question currently being tackled in a MAFF-funded study by the Hannah Research Institute, Scottish Agricultural College and CEDAR, University of Reading.

Milk records shows us that the lactations of many cows within the herd are greater than 305 days. But that is generally not from choice but due to their failure to conceive to first service. Consequently, little is known about the effect on physical or financial performance due to the deliberate extension of the lactation.

If the cows production is persistent it is her calving date and the required dry period of eight weeks which really determines the length of her lactation. But what determines persistency? That will be one of the problems tackled by the researchers at the Hannah.

"We know heifer lactations are more persistent than mature cows, probably because of the udder and the animal as a whole is still growing," says Chris Knight of the Hannah. "We also know that the season has a major effect, but how much of the spring flush, for example, is due to nutritional or environmental factors is still uncertain."

Work at the Hannah will focus on the effects of milking frequency, calving season, genetic merit and nutritional management on lactation length. They will also explore how extending the lactation affects animal health, fertility and the udder as well as milk processing properties.

At SAC Auchincruive a computer model will be produced integrating the physical and financial aspects of these changes. An important part of this will be trying to identify indicators in the heifer lactation which may assist in choosing cows for extended lactations. "It is unlikely that milk production will be the stimulus for extending the lactation but a range of health and production factors," reports SACs Tim Wassel. "If we find benefits from extending the lactation it is likely to suit only a proportion of the herd and not across all cows."

CEDAR is currently carrying out a management trial to assess the feasibility and performance of extending lactation in cows and heifers. Twenty-four cows and 12 heifers have been split into two treatment groups. Both groups are under identical management with the exception that 50% will have a delayed service with the aim of extending the lactation to 410 days.

"At the moment the system is simple to manage. It is post-turnout that things will be more challenging because we have minimal options for supplementing cows at grass," explains CEDARs Prof Beever. "Normally this is no problem but the most cows will be drying off as our grass burns off. This will not be the case for the animals which will be extended." This highlights just one of the practical difficulties farmers could face when they look to extend lactation.

This is a three-year study, but what if you want to extend lactations before the year 2000? Some help is already at hand. In 1995 the CEDAR supporters group commissioned a financial model to be produced which looked at the impact of extending lactation and the advantages and disadvantages of adopting such a policy.

Julian Parks and Heather Neil from the Department of Agriculture, the University of Reading, worked with farmer members of the CEDAR supporters group in order to include all the issues associated with extending lactation over a range of different farming systems.

"The greatest difficulty in managing extended lactations relates to being able to identify appropriate individuals," says Dr Parks. "Herds which milk record will be able to plot individual milk curves to assess persistency if their records are on a pc-based programme." He reports that heifers and high yielding cows, over 10,000 litres, showed the greatest persistency whilst lactation length becomes more unpredictable after the fifth lactation.

Based on this he recommends producers consider cows in lactation three to five if they want to deliberately prolong lactation. "Obviously the system is more easily introduced into herds calving all year round, and higher yielding cows showed a greater potential."

Extending the lactations of high yielding cows could be an advantage to their overall management, especially if it is used in combination with an embryo flushing programme. Delaying first service for these high producers would move the point of conception away from the period of energy deficit at peak lactation. The use of the embryo programme would prevent any delay in genetic gain which would otherwise occur due to the reduction in the annual calf crop. Another positive factor could be less calvings in a cows lifetime. This is considered to be the most stressful period for the cow and the time when she is most likely to experience metabolic disorders.

On the negative side are concerns about reduced conception rates due to delayed service. But research from 1950 found little evidence to support poor fertility in animals served post-100 days. It is important to distinguish the effects of delayed service and cows served for the third and fourth time.

But a real concern relating to delayed service can be the unwanted activity and reduction of yield during the several heats which will occur before service.

The reduction in calf income due to a reduced calf crop was just one of the elements which was built into the financial model. A choice of lactation extensions could be used in the model, but only small improvements were found for the lifetime gross margin of cows at either medium yield – 6500 litres – or high yields of 9300 litres. If these were averaged on an annual basis the shorter lactation showed an improved financial return, despite incorporating a dry period (see graph). In addition the month of calving for a herd selling on a variable price milk contract, is more significant for a 305-day lactation than for a 485-day extended lactation.

Another point which the model revealed was the impact of replacement rate on gross margin. Extending the lactation was shown to improve margin a cow a year by £50 when replacement rates fell below 20% and extension was beneficial where the cost of replacements rose above £1400.

But the overall conclusion was that with our current level of knowledge on the impact of extending lactation it was a management policy which would only be suitable for a few farms. It is to be hoped the MAFF study will lead to a new phase of understanding which may be able to recommend or discount extended lactation once and for all.n

Emma Leonard of CEDAR… Are 305-day lactations economical for high yielding dairy cows?



&#8226 Less calvings in cows lifetime, less stress.

&#8226 Moves service away from peak lactation.

&#8226 Savings when replacement costs high


&#8226 Reduced conception rates due to delayed service.

&#8226 Less calf income.

&#8226 Waiting several weeks before service would reduce yield.

Delaying first service of high producing cows would move point of conception away from peak energy deficit and peak lactation.