Delayed harvest and weathering damage will definitely reduce the nutrient and feeding value of your hay crop this year. Decreased calcium - due to maturity and heavier grass content. Obviously, poor quality forage does not have the same milk producing potential. Changes in rations, feeding and management are essential to cope with quality differences. Increased leaf loss due to physical damage.
Leaves are high in protein and low in fibre.
You can't change the weather, but here are some tips to help you make the best out of a bad situation. Test all forages when they enter storage, and retest at feed-out. Balance rations more frequently. Make sure new rations are in place before the feed is fed and make alterations when ensiled feed analysis results are available. Test forages for ligning to get a better handle on available energy.