Energy supply is the most limiting factor in most grazing situations

1 October 2009

Energy supply is the most limiting factor in most grazing situations and failure to feed cows for their level of production will result in reduced milk yield, low body condition score and poor fertility throughout the season. However, appropriate supplementation of cows at grass will yield benefits and can help achieve milk compositional goals, says Volac’s Dr Richard Kirkland. For example, diet modifications leading to an increase in milk protein of 0.2% could be worth up to 0.8 ppl for producers supplying particular cheese contracts. Conversely, many milk pricing schemes impose substantial penalties for milk with fat % below particular minimum standards.

In well-managed early-season conditions with high grass availability, cows have potential to yield over 20 litres/day from grass alone. However, high levels of production are difficult to achieve without risking major pasture under-utilisation and poor sward quality later in the season, and in practice production from grass is often much less than the potential performance. A plethora of concentrate supplements is available to plug the energy gap and enable cows to not only achieve their yield potential and maintain milk fat and protein levels, but also help to ensure they get in calf and stay in calf. It’s a case of being aware of what to feed and why.

Spring grass is typically highly digestible and contains a high proportion of water soluble carbohydrates (sugars), while fibre content increases and sugar content decreases as the season progresses. This suggests that higher fibre supplements may be more appropriate in early season while higher starch supplements would best balance late season grass.

This strategy was evaluated in a recent study at the Agri Food and Biosciences Institute (AFBI), Hillsborough. Spring-calving, high genetic merit dairy cows were allocated to either high starch or high fibre concentrate supplements offered at 10 kg dry matter (DM) per cow per day, with measurements being recorded during 10-week periods in both early-mid season (1 May to 10 July) and mid-late season (17 July to 25 September). Results from the study are presented in Table 1.

Table 1    Effect of high fibre vs high starch supplementation on animal performance

Sayers et al. (2003); AFBI, Hillsborough

The Hillsborough data demonstrate the potential of concentrate type to alter milk composition; cows fed starchy concentrates recorded reduced milk fat % and increased milk protein % relative to cows offered fibrous concentrates. In contrast, the different concentrate diets had little impact on either DM intake or milk yield in either the early or late season periods of the study.

Similar results were recorded from a study undertaken at the University of Nottingham in which cows offered a high fibre concentrate produced milk with a strong trend for higher milk fat % than cows offered a high starch concentrate. However, supplementing either concentrate type with Megalac protected fat increased milk fat by an average of 0.45% compared to the control concentrates.

Concentrate supplementation not only offers potential to alter milk production, but reducing the energy shortfall during lactation will help to reduce body condition loss which in itself will act to improve conception rates and overall fertility. Ongoing work, also at the University of Nottingham, has further demonstrated the benefits of Megalac supplementation on cow fertility with increased progesterone (the ‘hormone of pregnancy’) levels and improved egg quality.

Milk production response to supplementation is a key factor determining if the feeding regime will be cost-effective in the current marketplace and an average response of 1.15 kg milk/kg concentrate DM has been recorded in recent studies (range 1.00 to 1.36). As presented in Table 2, this level of response will return positive margins when considered in terms of milk yield, though the calculations take no account of potential bonuses for improved compositional quality of milk or improvements in fertility resulting from supplementation.

Table 2       Marginal economic responses to concentrate supplementation

 
While grazed grass is recognised as the cheapest available feed, producers must be conscious of the limitations and variability of milk production potential from grass, particularly with high yielding cows and variable weather conditions which can have major effects on DM intake. Concentrate supplementation will help alleviate negative energy balance resulting from inadequate intake of energy from grass and allow cows to reach their genetic potential for production. Use of appropriate supplements can also help meet target compositional goals while the longer term benefits of improved body condition and fertility are vital factors that must also be considered in the equation.

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