Out The Gap – Issue 2

Spring Grazing

Spring has sprung, soil temperatures are rising and if you haven’t already done so, getting out and walking your farm to complete a grass walk is crucial in assessing opening covers for the 2023 grazing season. PastureBase Ireland is Ireland’s leading database to help Irish farmers utilise and manage their grass production.

Lack of nitrogen (N) supply in soils can limit spring grass growth. However, soil type, weather conditions, and slurry applications can influence the timing and rate of early N applications. An average grass growth response of 10kg DM per 1kg N/ha in spring makes early N crucial. But before deciding on the application of chemical or organic fertiliser, a field assessment should be carried out. Using the Spring Rotation Planner (SRP) will ensure sufficient early grass is grazed to allow time for regrowth for second rotation. Simple grazing rules are as below:

Kicking off the first rotation; knowing and identifying where that percentage needs to be grazed will allow not only reduced concentrate costs but maximised grass growth required for second rotation covers of 1100 to 1200kg DM/ha/day. Every 1% grazed in early spring increases grass production by 14kg DM/ha.

Achieving 60% grazed by Mid-March will allow farms to have sufficient regrowth in March before the onset of ‘Magic day’ where grass growth exceeds demand. Targeting lighter covers with freshly calved cows to begin with, to better suit the cow based on intake over heavier covers. Once more cows have calved, grazing heavier covers later into March will allow faster grazing, while still achieving desired residuals.

Early spring grazing is not only good for the cows but it also ‘awakens’ the grass and kicks off that important spring grass growth.  A good graze out will remove dead material that has built up over the winter, allowing light into the base of the sward, producing a new leafy vegetative growth, the tiller plant. Hitting a residual of 3.5–4cm in the first round depending on the farm and soil type is hard to achieve but will prevent shading and dying of the tillering plants beneath. This lack of tillering and dead material will reduce growth in subsequent rotations.

With the second rotation starting early April, quality is most important for maximising milk solids, BCS and fertility performance before the start of the crucial breeding season. Maximising the time we have for regrowth to occur and understanding the impact of spring grass feed quality is invaluable. Aim for a few hours per day initially, allowing for 4-6kg DM intake form grazed grass. Avoiding rapid dietary transition onto grass, can reduce susceptibility to sub-acute rumen acidosis (SARA), poor rumen function and prevent loss of production.

After calving there is a restriction on Dry Matter Intake (DMI). The average DMI of 11kg DM/ cow/ day outdoors is typical for the first week post calving. This DMI begins to rise steadily over a 6-7 week period where it increases by 1kg DM/cow/ week. Turning cows out to grass creates a big change in diet, as well as imposing a stress event on the cow. It takes around 3 weeks for the bacteria in the rumen to adapt to the new diet, so it is important to manage the transition to grazing gradually, to avoid loss of performance and digestive upsets.

As conditions have been favourable, grazing a few hours a day will mean that that rumen bacteria can adapt to fresh grass gradually. Lush spring grass can often have a high crude protein content (20-25%) particularly after fertiliser application, mainly rumen degradable protein (RDP). Excess RDP is broken down into ammonia in the rumen, absorbed into the blood stream and converted to urea in the liver. This elevated blood urea N level (BUN) from excessive crude protein in the diet can decrease BCS, reduce fertility and impact lameness of the cow. During the transition to grazing, it is important to supplement cows with forages with a high energy content and digestibility to maximise intake. High DM grass silage will help to maintain milk solids as well as ensuring sufficient DMI, which is critical in early lactation.

Leafy spring grass tends to have a high proportion of leaf to stem, resulting in low structural fibre levels in the overall diet. This causes a lack of the ‘scratch factor’, impacting on cudding rates and saliva production. While the nutrient analysis of grass can vary widely, this lack of structural fibre can be accompanied by high sugar levels. High sugars levels are great for rumen fermentation, promoting good milk proteins and strong yield. However, they can challenge rumen function when supplied in excess in combination with low structural fibre, leading to SARA.

There are major benefits to getting energy-dense and protein rich spring grass into dairy cows diets where possible. You won’t find a feed with the same protein, energy or feed value like it given the current cost of concentrates. Remember, each extra day at grass in spring is approximately worth €4 /cow/day.

Happy grazing folks.