As genetic selection within the dairy herd establishes milking machines with higher yields and apparently greater efficiency, their ability to consume additional feed has not kept pace with their ability to produce milk. This poses an increasing challenge to farmers and their advisors.
Spring and the introduction of grazing further complicates the above problem since it is difficult to control and accurately predict the quantity of grass consumed. Variations in feed intake, particularly if inadequate concentrates or buffer feeds are available, could present a serious challenge resulting in depressed milk yield and quality, increased mobilisation of body reserves and impaired fertility.
Grass Consumption – How much will cows eat?
Although recent research suggest that cows can consume up to 20.7 kg of grass dry matter per day during early lactation, such intakes may not be “common” on most farms since:
They were recorded during June/July when grass dry matters are normally significantly higher than April or May.
They were also recorded with mature cows in their 2nd, 3rd and 4th lactations. A typical UK herd will include around 25% first lactation heifers that possess a lower intake capacity.
Grass consumption is more typically between 12 – 17 kg DM per day. In addition, the cow herself is influenced by a number of factors associated with sward characteristics, grazing time and buffer feeding.
Zero grazed, fresh weight grass, fed to cows each day at a trial carried out by the Kingshay Trust established that in wet weather, cows had to consume 100 kgs each to achieve a grass dry matter intake of 15 kgs. On drier days this was reduced to only 75 ks per head.
Sward / Pasture Characteristics
The grass leaf has a higher digestibility than the stem thus the greater the proportion of leaf the higher the digestibility
Generally a 5% increase in leaf content would be equivalent to a 1% improvement in digestibility. Good sward management is essential in order to maximise digestibility since grass above 6 cm will be predominantly leaf whereas below 6 cm there are higher levels of dead and low digestibility material. At turnout target grass cover should be around 10 – 12 cm in order to provide satisfactory supplies of readily available material.
A recent series of trials looked at the impact of different sward digestibility’s (Organic Matter Digestibility OMD) on dairy cow performance using spring calved cows in late summer and autumn.
Daily milk yields and grass dry matter intakes were directly related to grass digestibility. Cows allocated the 76% OMD sward were able to consume 0.6 kg more grass and this produced an extra 1 kg more milk than those offered the 73% sward.
Comparison between the 73% and 71% OMD groups shows a similar pattern of intake and milk production.
The milk yield improvements were however, less dramatic (+1.3 kg milk from 1.8 kg dry matter) than that with the highest OMD sward due to the poorer quality.
Other studies have extended these results to show that when pasture OMD exceeds 74% each extra kg of DM intake can result in a 1 kg increase in milk yield.
Most farmers are keen to commence grazing in the spring as early as conditions allow. This date is influenced by a number of factors including grass supply, stocking rate, spring nitrogen application and calving pattern.
Some key points for consideration are:
Target grass cover at turnout should be around 3500 kg DM/ha or a grass height of 10 – 12 cm.
A post grazing height of 6 cm and a grazing cycle of 21 days has been recommended. This combination will allow acceptable herbage re-growth combined with grass intake without any loss of herbage quality.
Grazing to a grass height below 4.5 cm during this period will slow down future regrowth.
If ground conditions are not suitable, farmers if possible should not be forced into turning cows out early otherwise grass growth will suffer in the long term.