Milk production peaks eight weeks after calving. Feed intake increases for the first twelve weeks. With limited feed intake, quality of the ration must be high enough meet both energy (65% TDN) and protein (11%) requirements on a dry basis.
The April 26 BYOB article “Grazing Dormant Stockpiled Forage” focused on the need to provide supplemental hay or silage and grain to meet nutrient requirements of the lactating cow because the carry over forage has low quality.
In carry over crested wheatgrass and creeping red fescue, protein contents are 7 and 10% on a dry basis. Digestibility of grasses decreases by approximately 20% compared to the previous fall (Lastiwka). Thus, available protein is 5.6 and 8% in these two feeds. Energy availability could be reduced by the same amount.
For grasses, new growth at the 4 to 5 leaf stage is very high quality. For example, crested wheatgrass and creeping red fescue at the five-leaf stage contains 22% protein. As grasses mature, protein content decreases by 2 to 3% every week (Sulieman et. al., 1999).
Cows will graze between five and eight hours a day (Boland). In that time, a 1400-pound cow can consume up to 35 pounds or 2.5% of their body weight in dry feed per day. Over wintered material contains 90 to 95% dry matter whereas new growth is only 10 to 15% dry matter.
To meet the lactating cow protein requirements, 34% of daily forage intake should be new growth. The cow would need to consume roughly 80 pounds of new grass and 26 pounds of old material. Preference would be for the new growth.
If the cows are turned in when the new grasses are very short, it is unlikely that they will be able to consume 80 pounds per day. Manure shape will be similar to a pyramid or a pile of onion rings. If they are not eating enough fresh grass and protein intake is low; total feed intake will be reduced because of the effort needed to consume enough grass and slower passage rates of the ingested feed. Milk production and calf growth rates will decrease because the cow does not have sufficient protein and energy in the diet.
A good article that explains grazing behavior of cattle is https://extension.msstate.edu/sites/default/files/topic-files/cattle-business-mississippi-articles/cattle-business-mississippi-articles-landing-page/stocker_apr2011.pdf
For additional information on spring pasture forage quality, contact Barry at 403-741-6032 or firstname.lastname@example.org