Growth and glucose intolerance: What effect does the amount of milk replacer fed pre-weaning have?

In the last decade, several studies have evaluated the short-term effects of feeding calves higher amounts of milk during the pre-weaning stage. Among other benefits, these studies have found that larger milk allowances (compared to conventional amounts aimed primarily at cost efficiency and quick rumen development) are beneficial for pre-weaning average daily gain (ADG)[1] and for reducing the age at first calving.[2] Given these results, it seems that dairy herds would benefit by being fed higher levels of pre-weaning nutrition. However, calves given higher milk and milk replacer (MR) allowances than those fed conventionally have been reported to have increased blood insulin to glucose ratios[3] and have become insulin resistant at early ages.[4] Both of these issues could be aggravated if feeding frequency is low, as it is in most commercial herds with twice a day feeding, as the metabolic capacity could be overwhelmed. Is upping the level of pre-weaning nutrition good for growth and reproductive performance, but bad for glucose metabolism and insulin resistance? In her 2015 Ph.D. thesis for the Universitat Autonoma de Barcelona, Dr. Christina Yunta set out to find the answer.[5]

Methods

In Dr. Yunta's study, 120 female Holstein calves were fed 6L of good quality colostrum in the first two meals after birth and were then randomly assigned to one of three groups: 2L of MR twice daily for a daily total of 4L, 3L of MR twice daily for a total of 6L, or 4L of MR twice daily for a daily total of 8L. Calves had access to starter and water ad libitum, and MR and starter feed intake were recorded daily from days 2 to 70. Calves were weighed at days 0, 35, 63 and 300 and ADG was calculated as well. All calves were weaned at day 63, using step-down methods which varied in amount and timing depending on milk level in treatment. Glucose tolerance tests (GTT) were performed at days 42, 86 and 300 on 15 randomly selected heifers per group.

Growth and reproductive performance

As expected, during the first month of life, total dry matter intake (DMI) was greatest for 8L calves but, of course, the 6L and 8L calves consumed less starter feed than 4L calves (as they sated their hunger with their higher milk allowances). At 35 days, the 8L calves had the highest ADG (752 g/d) followed by the 6L calves (564 g/d) and the 4L calves (484 g/d). And, while the 4L calves caught up a little bit during weaning in terms of ADG (as they began consuming more starter earlier), at day 63, 8L calves were still 4 kg heavier than calves in both the 6L and 4L groups and at day 300, the 8L calves were 12 kg heavier than the 4L calves.

While there were no differences found between treatments in terms of rate of conception, age at conception was indeed greater for the 4L calves. While Dr. Yunta feels that larger sample sizes may be in order to more clearly see the effects that MR allowances have on reproductive performance, she feels it does seem to follow that calves fed large milk or MR allowances reach puberty and pregnancy at earlier ages, especially in light of results of previous studies.

Glucose metabolism and insulin sensitivity

Interestingly, the amount of MR offered had different effects on glucose metabolism in the short-term compared to the medium-term. While at 42 days the 8L calves released a greater amount of insulin to control glycemia than the 6L and 4L calves and thus this could seem to be detrimental for their metabolic performance, at 84 days, these differences disappeared. At 300 days, the situation quite actually reversed, with the 8L calves needing lesser amounts of insulin than both the 4L and 6L calves to control glycemia during a GTT. It seems that for the 8L calves, the negative effects on glucose metabolism were indeed transitory. And, in fact, insulin sensitivity was simply not a factor for the 8L calves by the time they were 300 days old.

It seems that the answer to Dr. Yunta's question is yes, upping the level of milk allowance pre-weaning is indeed good for growth and reproductive performance. And, while the higher milk allowance does seem to affect glucose metabolism and insulin sensitivity in the short-term, in the medium-term it does not seem to do so. While more studies of this type may be needed going forward, these results seem promising for those who espouse the idea of feeding higher planes of pre-weaning nutrition.

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