There are good reasons to turn animals onto carry over or stockpiled forage after the snow has melted. Moving cow calf pairs from a calving pen into a larger area reduces the risk of sickness spreading amongst the calves. Using stockpiled forage replaces some of the feed delivered to the herd. This helps to stretch the feed supply and reduces overall cost. But this feeding system cannot replace 100% of the ration.
Not taking a second cut or letting a paddock growing until freeze up provides forage to be grazed later. Quality of stockpiled forages depends on plant maturity at freeze up. The more mature the plant, the lower the quality. Also, plant quality is lower in the spring compared to the fall. Alfalfa plants drop their leaves after a killing frost. This reduces the amount of protein and energy available to the animals. In grasses, some of the soluble protein and carbohydrates leach out over winter.
Manitoba Agriculture, Food, and Rural Initiatives (MAFRI) completed a three year study on forage quality in stockpiled forage. https://gov.mb.ca/agriculture/crops/crop-management/forages/pubs/feeding_quality_stockpiled_forage.pdf
The study measured quality loss in alfalfa and twelve common grass species. Energy (TDN) content declined over the winter. For the alfalfa, TDN dropped from 58% to 43%. Grasses ranged from 60 to 68% TDN in the fall and 52 to 58% in the spring. Not enough energy to support a cow in mid pregnancy let alone one that is lactating.
Crude protein content was more variable between the grass species tested. Amounts ranged from 8 to 13% in the fall and by March, values were between 6.5 and 11%. Only two varieties had sufficient amounts of protein to support a lactating cow requires a minimum of 11% protein. Alfalfa had the largest decline from 13% to 7.8% protein.
These results show that the stockpiled forages cannot meet the nutrient requirements of a lactating cow. Supplementing 40 to 50% of their dry matter intake with a combination of hay, silage, grain, and a protein supplement is common.
If not supplemented, the cows will lose weight and body condition. Cows will not be able to produce as much milk which reduces calf growth rates, and requires more time to start cycling. First service conception rates are impaired. Feeding a balanced ration between calving and the start of the breeding season is critical to maximize the size of next years’ calf crop.