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Although much has been written about the effect of pre-calving intake on post-calving intake and lactation performance, there are still differences of opinion and variable results, especially as it affects first-calf heifers in comparison to older cows. Authors Janovick and Drackley again addressed this question in a comprehensive study reported in the Journal of Dairy Science, Number 7 of Volume 93 of 2010, the title of the paper being: Prepartum dietary management of energy intake affects postpartum intake and lactation performance by primiparous and multiparous Holstein cows.
Pre-calving intake of feed and energy is normally limited to prevent overconsumption relative to requirements. This principle which can be done by restricting feed intake, but which in practice more often is done by feeding more roughage often with lower quality in the pre-calving period, is known to improve cow health and liver function during lactation. Whereas this practice is generally accepted for older cows, first- calf heifers at this stage still have requirements for growth in addition to lactation. It is therefore conceivable that their milk yield may be impaired; in fact some studies have suggested that a high plane of nutrition pre-calving is required for first-calf heifers if lactation performance is to be optimized, whereas other studies differ. In some of the latter studies it was found that there was no relationship between pre-calving intake and productivity or longevity; rather, reproductive problems were the primary reason for young and older cows leaving the herd. The idea which the authors in this study pursued was that ad lib feeding of a diet with a greater inclusion of roughage fibre can be formulated that would provide adequate nutrients but limit energy intake to what the NRC prescribes, thereby achieving similar benefits to those observed with restricted intakes. The objectives of the study on both first-calf heifers and older cows were therefore to determine the effects of controlling pre-calf energy intake on: 1) changes in feed intake, body weight, body condition score (BCS) and energy balance pre- and post-calving, and 2) post-calving milk production and milk composition.
In the study first-calf heifers and older Holstein calves were fed one of three pre-calving energy intakes: A moderate energy diet was fed either ad lib (a), or restricted (b), to supply 150 or 80% of NRC calculated energy requirements for dry cows in late gestation. To limit energy intake to 100% of NRC requirements at ad lib feed intake (c), chopped wheat straw was included at 32% of dry matter in a control diet. The results showed that both first-calf heifers and older cows gained body condition on treatment (a), but were not over-conditioned at calving (BCS = 3.5). Older cows in this group lost more BCS than older cows in groups (b) and (c), whereas first-calf heifers lost the same amount of BCS in all treatments. The addition of wheat straw [(c) treatment] prevented a large decrease in feed intake pre-calving in both first-calf heifers and older cows. During the first three weeks after calving, feed intake as a percentage of body weight was lower for treatment (a) than for treatment (b) in the older cow groups; again no differences were found for the first-calf heifer treatments. Of significance is that pre-calving dietary effects did not carry through to the post-calving measured period of eight weeks. Because of greater mobilization of body stores, treatment (a) older cows had greater milk fat percentages and fat-corrected milk yield during the first three weeks of lactation, compared to older cows in treatments (b) and (c). The older cows in treatment (a) experienced a 55% decrease in energy balance and the first-calf heifers a 40% decrease, compared to the other treatments, and accordingly had a greater contribution of energy from body energy reserves to milk energy output.
In summary, overfeeding of energy pre-calving resulted in large changes in energy balance. Even in the absence of over-conditioning, a large change in feed intake and energy balance pre-calving influenced post-calving feed intake and BCS loss, especially in older cows. Chopped wheat straw was effective at controlling pre-calf energy intake, although first-calf heifers did not achieve the predicted feed intake. Even so, controlling or restricting energy intake in first-calf heifers was not detrimental to lactation performance over the first eight weeks of lactation. The authors concluded that even though pre-calving dietary effects did not carry over to the first eight weeks of lactation, controlling pre-calving energy intake should ease the transition to lactation. It should also reduce feeding costs and therefore is recommended, even for first-calf heifers.