Grass Dry Matter Content and Weather Conditions

The water present in pasture grass, a combination of surface and intracellular, not only significantly affects herbage intake, it can also markedly dilute the nutrient value of the pasture.

Research has shown that when grass dry matter falls below 18%, voluntary intake of dairy cows decreases by 0.34 kg DM for each % fall in DM. Grass DM changes with climatic conditions and season. Summer grass often has a DM content of 25% compared with 15% in the spring.

Whatever the season the DM content can be dramatically affected by climate and during wet weather it can be as low as 10%. It is not surprising that such a fall in DM content and thus dry matter/energy intake (-2.7 kg, -30 MJ) can have disastrous consequences for milk production particularly from early lactation cows. Table 1 shows the changes in grass consumption required to support the milk production by high and medium genetic merit cows with changes in grass dry matter content:

Table 1. This data suggests that although the dry matter intakes look acceptable, the fresh matter intakes of "wet" grass may be almost impossible to achieve even for Medium Genetic cows.

Grazing Time, Bite Rate and Intake/bite

Grass dry matter intake during grazing can be calculated if you know the number of bites taken per minute, the intake per bite and the time spent grazing. It has recently been established that although the number of bites/minute don’t tend to alter that much, under most conditions, remaining at 65/minute, dry matter intake can be dramatically influenced by changes in the other two parameters.

Table 2. Cows can spend only a maximum of between 500 and 700 minutes grazing each day as a result of the time spent milking (away from pasture), ruminating and sleeping. On most farms this is often no more than 600 minutes and this only presents sufficient time to consume 15.6 kg DM. If grazing management can be improved to increase the grazing time to 11 hours, the above table shows the potential for a 1.5 kg improvement in dry matter intake. This could have significant implications for animal performance.

Intake/bite is a reflection of the amount of grass available in each bite and 0.4g/bite is what might be expected from the average pasture. However, it can be seen from the above table that if the quality of the sward (tall and dense) is improved by only 0.1g per bite, this could be reflected in an improved DM intake of almost 4 kg and can more than make up for reductions in grazing time. The challenge is therefore to ensure that a good sward (tall and dense) is presented throughout the grazing period and that herd management is adjusted to maximise grazing time.

Concentrate/Buffer Feeding at Grass

Early lactating cows cannot support milk production by nutrients supplied from grass alone, even during the spring, and require appropriate supplementation with concentrates if target performances for milk output and fertility are to be achieved. However, the effect of concentrate feeding on grass and total DM intake can very with grazing conditions and sward quality.

In a study at Moorepark in Ireland using spring calving cows grazing intakes of 16 – 17 kg DM were recorded during seasons said to have "good" grazing conditions. However, during less favourable grazing seasons, grass intake was severely reduced (-15%) (Table 3) and supplemental concentrate were needed to maintain target levels of milk production (Table 4).

Concentrate/Buffer Feeding at Grass

Table3. The results from this study showed that during poor grazing conditions, grass intake was poor and below target. Under such conditions supplementation with concentrates resulted in low substitution rates (0.2 kg Grass DM/kg Concentrate). Concentrate supplementation boosted total dry matter intakes.

Table 4. The results of this study showed that supplemental concentrates supported higher levels of milk production than grass alone. Grass alone, due to reduced intake, could only support a proportion of the milk production of early lactation cows during the spring i.e. @ 24 kg/day. Supplemental concentrates reduced live weight loss compared to grass alone and may even allow for cows to gain weight during this period. This may indicate improved nutrient status. Responses to concentrate supplementation by cows at grass depends upon the milk yield and sward quality. On moderate to good swards cows yielding 25 litres or less do not require supplementation. Above this, concentrates should be supplemented at 0.3 kg/litre and the milk yield response to supplementation improves with increasing yield

Table 5. Under supplementation of high yielding cows has also been shown to have a dramatic impact on cow condition if continued for prolonged periods when the ability of grass to support milk production is lower than expected.

Table 6. With this additional nutrient supply from concentrates and the apparent change in the nutrient status of early lactation cows, reflected in body weight change, it is possible that fertility parameters may have been improved.

Table 7. The Results from this study showed that Reproductive performance was poor on grass alone with a low pregnancy to 1st service rate (41%), a high number of services per conception and infertility rate. Concentrate supplementation resulted in dramatic improvements in all fertility parameters including calving to conception, pregnancy rates, services per conception and returned infertility rates closer to normality.