Nitrate Risk in Forage After Frost

2 days ago

4 min read

Updated: 7 hours ago

A practical guide for beef cattle producers in the Peace Region

Written by Dr. Akim Omokanye

Frost does not automatically mean your forage is unsafe.

But frost can increase nitrate levels, particularly in annual forages such as oats, barley, wheat, canola, greenfeed, cover crop cocktail blends (polycultures) and some weeds. Past PCBFA research on polycultures has shown some nitrate levels above the range where feeding management is needed. When in doubt, test before grazing or feeding.

When is the risk of frost higher?

Risk is greater when frost occurs together with: Drought followed by rain, cool, cloudy weather, hail or other crop damage, high soil nitrogen, recent manure or nitrogen fertilizer applications, immature annual forages (including regrowth's), and heavy weed growth.

After a dry spell, frost or hail, it is reasonable to wonder whether a crop is still safe to feed or graze. After a frost, plant roots may continue taking up nitrogen from the soil while damaged leaves cannot use it efficiently for growth and protein production. Nitrates can therefore build up, particularly in the lower part of the stem, so grazing plants down close to the ground can increase exposure.

What Should I Do After a Fall Frost?

1. Don't turn hungry cattle onto a frosted crop

This is one of the most important precautions.

Do not move cattle directly from an empty pasture or overnight without feed onto a suspect crop. Hungry cattle may consume large amounts quickly, increasing nitrate intake.

Instead:

Give cattle low-nitrate forage first.
Introduce the suspect forage gradually.
Watch cattle closely during the first several days.
Avoid forcing cattle to eat the lower stems.

2. Test, Don't Guess!
The only reliable way to know the nitrate level of suspect forage is to have a representative sample tested.
For forage damaged by frost, sampling about 3–4 days after damage can help capture elevated nitrate levels.

What Should Be Tested?
Labs offering forage analysis can test for Nitrates separately; this is often an add-on to other forage tests.
When taking samples, ensure you pull samples from the plant, and sections of the plants, the cattle will actually consume.
- Baled hay/greenfeed: use a hay probe and sample multiple bales from the same suspect crop.
- Silage: use an appropriate probe and sample multiple areas from the same suspect crop.
- Pasture: sampling is more difficult because cattle selectively graze different plants and plant parts. For suspect forages, take grab samples from several areas of the field.
Please Note: If you are unsure how to sample your feed, reach out to PCBFA!

Know What The Laboratory Result Means

Nitrate results may be reported as Nitrate (NO₃), Nitrate-N (NO₃-N) or Potassium Nitrate (KNO₃), as provided in Table 1 below. These are different ways of expressing nitrate concentration, so always check the units before interpreting the result!

Looking at Table 1, these ranges come from the Alberta and Saskatchewan government resources. They help interpret a test; they cannot determine a safe feeding plan without considering the rest of the diet, drinking water, how much cattle eat and whether they have gradually adjusted to that feed.

Reported As:	Generally Safe	Caution	Feeding Management Required
(NO₃) Nitrate %	0.5	0.5-1.0	1.0+
(NO₃N) Nitrate nitrogen %	<0.12	0.12 - 0.23	0.23+
(KNO₃) Potassium nitrate %	0.81	0.81 - 1.63	1.63+

If My Nitrate Result Is High, What Can I Do?

1. Mix with low-nitrate forage:

Thoroughly mix high- and low-nitrate feeds so that cattle cannot select only the high-nitrate feed.
Do not simply put a high-nitrate bale beside a low-nitrate bale. Some cattle may eat mainly from the high-nitrate bale.

2. Introduce gradually

If cattle will graze a suspect field, avoid forcing them to eat the lower stems. Do not turn hungry cattle directly onto a questionable crop; Watch them closely when they start a new pasture.
Allow rumen microbes time to adapt to the nitrate level.
Avoid sudden changes in the nitrate concentration of the diet.

3. Feed smaller amounts more frequently

Feeding two or three times per day can reduce the amount of nitrate consumed at one time compared with a single large feeding.
- If you're grazing, you can do this by rotating the cattle off the suspect crop after a few hours.

4. Provide adequate energy and a balanced ration

Adequate dietary energy helps rumen microbes convert nitrate through the rumen nitrogen pathway.
Cattle should also have continuous access to clean water.
You can use CowBytes or SheepBytes to balance your rations.

Notes Specific to Grazing A Frosted Field

Feed low-nitrate forage before turnout.
Introduce cattle gradually.
Rotate them off the suspect crop, if possible.
Monitor cattle closely.
Provide another low-nitrate feed source.
Avoid forcing cattle to graze close to the soil surface.

Key Highlights

Mix high-nitrate forage thoroughly with tested, low-nitrate forage, introducing it gradually and avoiding sudden changes in the ration.

Note: Putting out separate high- and low-nitrate bales does not give every animal the same mix, and does not ensure they won't selectively eat the high-nitrate feed.

A feed test is especially useful when forage is going into storage: hay generally retains its nitrate, and ensiling is not a dependable way to make a high-nitrate crop safe. Test stored feed before feeding it.