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The Effect of Late Nitrogenous Top-Dressing
[Vol. XVIII, Part IV
The rapid rate at which the absorbed nitrogen is converted into ' true ' (copper precipitable)
protein is of considerable interest, but direct evidence is needed as to the nutritional value to farm
animals of such protein. A direct comparison of the amino-acids present in late top-dressed hay and
hay not top-dressed might yield circumstantial evidence on the nutritive value of the added protein,
but no work on this line has been attempted. A comparative feeding trial using dairy cows or young
stock is obviously desirable, but in view of the relatively small difference to be measured, a high or
practically unattainable degree of accuracy would be necessary. The obvious necessity of a ration
of minimal protein content adds to the difficulties of such a feeding trial.
Since the major part of the added nitrogen is present in a form precipitable by alkaline copper
hydroxide and therefore might be expected to be protein, polypeptides or certain amino-acids, it
is thought that analysis supported by information on the digestibility of the protein should give a
reasonable estimate of nutritive value of the protein in top-dressed hay. Such information is pre-
sented in this paper.
EXPERIMENTAL
The hay was from a ley in its third year. It was a good crop consisting mainly of vigorous
tye-grass with some leafy cocksfoot. The clover content was fairly low. A block of about 2 acres
was top-dressed with 2 cwt. sulphate of ammonia, per acre on 31 May and cut on 13 June. Rain
fell on the 11th day after the dressing, the total rainfall being 1.33 in. The weather was cool with
bright intervals. No rain fell after the cutting, and the top-dressed and control hays were baled in
the field on 19 and 20 June. The crop, and also the stubble after cutting, were noticeably greener
on the top-dressed area. The digestibility of these hays was determined using three sheep for
each hay.
In 1946 the hay was made from old permanent grass, the herbage consisting mainly of meadow
grasses with a little wild white clover. The botanical composition varied considerably over the
field.
The lay-out consisted of eight strips, each about 33 ft. wide, running the length of the field.
Alternate strips were top-dressed with 3 cwt. ' Nitro-Chalk ' per acre. Yields of hay were measured,
but as expected the top-dressing resulted in no increase in yield. Top-dressing was applied on 13
June and the crop was cut on 21 June. The hay was cocked on 27 June and baled on 1 July.
Slight rain fell each day between 13 and 21 June, the total rainfall being 0.8 in. A further 0.9
in. of rain was recorded in 5 days between 21 and 28 June.
The aftermath on the treated strips was markedly greener than that on the control strips.
Digestibility trials were carried out on the two hays using four sheep for each hay.
RESULTS
Chemical composition of the hays
The hays were sampled by selecting 10-12 bales at random from the main crops. These bales
were opened and 7 lb. hay removed from two places in each bale. The samples were then chaffed,
thoroughly mixed and used for the digestibility trials. Daily sub-samples were taken during the
10-day digestibility trials and these were composited for analysis.
The compositions of the hays are given in Table II.