Lush Spring Forages and Beef Cow Reproduction

After a long winter, most of us relish the first few warm and sunny days of spring.  For our beef farmers, those first spring days also signal that our pastures will be greening up soon.  Making the transition from winter feeding to spring pasture often represents our most economical and labor efficient feeding of the year.  While both farmers and their cows may be excited to get on the first grass of the year, recent research shows that easing the transition from stored feeds to pasture can impact the herd’s reproductive performance.

At the 2017 Driftless Beef Conference, Travis Meteer of the University of Illinois-Extension outlined three challenges with lush spring pastures for the beef cow herd.

Low dry matter.  Lush spring growth can be as low as 20% dry matter.  This makes it challenging for a cow to physically consume enough pasture forage to meet her nutritional needs.  The rumen simply runs out of physical capacity.

  • High protein. Lush spring pasture can be high in protein and moderate in energy.  When protein intake exceeds nutritional need, it requires energy for the body to excrete it.  If ration energy is only low or moderate to begin with, the additional energy required to excrete excess protein can cause a negative energy balance.
  • Low fiber. This results in high passages rates through the digestive tract.  Low fiber also results in an unsatisfied appetite.

In a University of Illinois experiment, cow / calf pairs were divided into supplemented and non-supplemented groups at the beginning of the grazing season.  The supplement was a mix of soybean hulls, ground corn cobs, and dry molasses.  The feeding rate for the supplement averaged 4 pounds per head per day for 70 days.  These feedstuffs were chosen because they are high in energy or fiber, while low in protein, and were economically accessible in the local area.

At the end of the 70 days supplementation period, there was no significant difference in cow body weight or body condition score.  There was a numerical difference in reproductive performance.  In year one of the experiment, the supplemented cows averaged a 67% conception rate to timed AI while non-supplemented cows averaged 45%.  In year two, the supplemented group’s timed AI conception rate averaged 52%, while the non-supplemented group averaged 38%.

Further work is needed to replicate these results regarding conception rates before broader recommendations can be made, especially in regards to timing of pasture turnout in relation to timing of AI.  However, the experiment serves as a reminder that while we may consider first pasture growth to be the lushest and nutritious, it often times is not balanced to the cow’s needs.

This experiment was not able to conclusively identify the biology behind the difference in conception rates.  Possible explanations include excessive protein in combination with low dry matter intakes causes a temporary negative energy balance, which has been linked to poor reproductive performance.   Also, excessive protein levels in the diet can cause physical stress.  When excessively high, ammonia from protein will attached to red blood cells instead of oxygen.  Cows will pant, similar to heat stress, to rid the ammonia and breathe more oxygen.

While not tested, it can be hypothesized a shorter supplementation period could be effective if timed around the peak in pasture forage protein content.  Practical options that were not tested in the experiment, but may produce similar results, include allowing pastures to mature a little longer before initial grazing, providing access to dry hay that is low in protein (i.e. not prime alfalfa), or supplementing with a high energy grain.  While the focus of this project was lush spring growth, in rotational grazing systems allowing adequate regrowth between grazing periods is recommended to avoid similar situations throughout the season.

More details from Mr. Meteer’s presentation can be found in the conference proceedings available from the Iowa State Extension Store: 2017 Driftless Region Beef Conference Proceedings

Written by: Ryan Sterry, UW Extension Agent- St. Croix County Agriculture Agent, and recently appeared in the Wisconsin Agriculturist

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