Milk protein content of cows in subtropical Queensland declines from the beginning of winter to the end summer and is a repeatable consistent observation across years. The genesis of such an event is hypothesised to lie in the nutritional and environmental changes that occur over this period with a change in forage type from the temperate species to the tropical species, a change in cereal grain supplementation and a reduction in intake associated with heat stress causing a change in the nutritional supply to the animal. A series of experiments examined the effect of high levels of ammonia absorption on milk production and milk components in dairy cows. The experiments used extensive, grazing trials with supplements of urea to increase ammonia absorption, and intensive, infusion of ammonia into cows indoors. Feeding urea in the supplement of cows grazing ryegrass increased milk protein significantly by 0.05 percentage units but had no effect on milk volume. Infusing ammonia intra-venously reduced milk volume (~ 20%) and milk protein content (<0.1 percentage units). The primary finding of these experiments was that infusion of high ammonia levels into dairy cows will depress milk protein production and content. However, the levels of ammonia absorption experienced under usual strip grazing conditions in Queensland over winter are not high enough to affect milk protein production and content. The effect of altering substrate supply and pattern associated with the shift in feed supply from winter to summer was examined by infusing different amino acid patterns and acetate into cows by a close arterial infusion method. Infusate was delivered to the external iliac artery, allowing the mammary gland first pass use of the nutrients. Supplying amino acids by infusion increased milk protein yield 18.7 vs 19.7 g/h (5.3 %) for infused half of the udder. Milk protein yield was increased more (19.2 vs 20.2 g/h for infused half of the udder) by infusion of a milk amino acid profile compared to a microbial amino acid profile. Infusing acetate has no effect on milk protein yield but decreased milk yield for infused half of the udder (560 vs 510 g/h). It may be concluded, that under the practical levels of crude protein and ammonia supply which arise from different agronomic practices (N fertiliser) and grazing management (stage of growth, strip grazing), that any practical increase in ammonia supply per se to the liver does not explain the depression in milk protein percentage seen in Queensland dairy herds.
| Identifer | oai:union.ndltd.org:ADTP/254221 |
| Creators | Norman Purdie |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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