Calculating the amount of hay that is needed on both a short term and over the course of an entire winter are mathematical exercises that are required on any cow calf enterprise. There are scientifically based "rules-of-thumb" that can make these mathematical exercises more precise. The size of the cow, the lactation status of the cow, and the quality of the forage are key elements in determining the average daily intake of the cow. Standing forage in pastures or as crop residue can provide much of the forage needs early in the winter and when not covered by snow. As standing pasture forages diminish in both quality and quantity during the course of the winter months and most of the forage consumption comes from stored hay, then how much hay will the cow eat voluntarily? How much hay do I need to plan to feed this winter? How much hay do I need to put out for the next few days? The following forage intake table can provide a good way to estimate forage needs.
Table 1. Forage capacity of beef cows
*DM: Based on 100% Dry Matter; Intake estimates assume that adequate protein is in the diet whether from forage or supplementation.
Many grass hay samples will test 7 to 10% moisture. Therefore, when calculating daily hay intake on an "as-fed" basis, divide the pounds of hay intake (from Table 1) by 0.93 to 0.90 to determine actual hay intake by the cow.
Intake in forage fed to cattle is generally limited by the forage capacity of the digestive tract. Forage intake is correlated with forage quality as shown in the table above. The more rapid rate of digestion and passage of higher quality forage results in considerably higher dry matter intake compared to lower quality forage that is lower in digestibility.
Lactation represents the greatest need for additional energy beyond that needed for maintenance. An average milking beef cow requires 50% more TDN or energy than she does when dry. It should be noted that lactating cows consume more forage compared to gestating cows due to the increased energy demand.
Large cows will require more energy than will small cows. Therefore the hay or forage requirements are calculated based on a percentage of the body weight of the cow. Be honest with yourself as you estimate cow size and therefore hay amounts that are needed.
These estimates do not include hay wastage. Feeding method and hay quality will affect the percentage of the hay that is wasted. Estimates of about 6% - 11% hay wastage been reported when large round bales were fed in bale ring feeders.
Source: Glenn Selk, Oklahoma State University Emeritus Extension Animal Scientist