An investigation into the effects of feed composition on lactational performance was
carried out using rats and cows. A graphical representation of the feed as a triangle was
used to aid the interpretation of results. The first rat experiment showed that, on high
protein feeds, the lactational performance of rats is not depressed when offered feeds
of very low carbohydrate content. This was substantiated by the other rat experiments.
When carbohydrate in the feed was replaced by fat at low protein content (rat
experiment 2) there was a large depression in lactational performance, effectively a cessation of milk production. The interaction between the three feed components
protein, carbohydrate, and fat was highly significant. The hypothesis that maternal heat
production was limiting food intake was advanced. The third rat experiment used feeds
whose composition was marginal in relation to lactational success. The feeds also
allowed comparison between feeds of constant nutrient:energy ratio. The results of this
experiment indicated that there is an extremely abrupt threshold in feed composition
for adequate lactation. This effect could not be attributed to any one nutrient:energy
ratio. This experiment also showed the importance of maternal body reserves in
support of lactation. A model was developed to explore the hypothesis that maternal
heat production was limiting performance, however this model failed. An experiment
using sheep was conducted in order to permit prediction of the volatile fatty acid
proportions arising from a range of feeds. This experiment was designed to allow
application of the rat work to dairy cows. A dairy cow trial was conducted, to compare
different feed types and feeding levels. The results of this trial showed no effect of feed
type on lactational performance. A linear relationship between food intake and level of
milk production was found. This included an effect of feeding level on rate of decline in
milk yield. All these findings are discussed in detail
