Different nutritional factors affect reproduction, growth, lactation, immunity and adaptation to thermal stresses. Such relationships should provide useful tools to adapt nutritional regimens to meet a specific production target and/or solve a specific production problem.
Experiments have shown that the heifers that are adequately fed show an earlier oestrus cycle than those with poor nutrition. Other experiments have shown that the level of nutrition also affects the age of sexual maturity of the females. In these experiments, one group of heifers was fed a poor diet and reached puberty at 710.7 days of age, while another group was fed an adequate diet with proper protein-energy balance and reached puberty at 440.1 days.
The level of mineral salts in the diet also affects the fertility of female animals. Phosphorus deficiency, for example, leads to delayed oestrus due to the low secretion of oestrogen and may also lead to dystocia, which sometimes results in the death of the dam or the fetus.
Nutrition also effects the fertility of male animals. Suboptimal nutrition suppresses the production of gonadotropins by the pituitary gland and the secondary sex hormones so that atrophy of the prostate and seminal vesicles occurs, thereby affecting semen quality in terms of fluid volume and concentration.
The intake of solid feed is vital to the calf making the transition from a pre-ruminant to a functioning ruminant. The 3 most essential nutrients for calf growth and development are water, energy and protein. Water is essential for all living animals, and it is good husbandry to provide calves with as much fresh, clean water as they want. Fibre, minerals and vitamins are also needed but play a smaller role.
The use of feed flavours may also help to achieve optimal rumen development and reduce weaning duration and costs. A study was conducted to evaluate the effects of a vanilla flavour in starter feeds on pre-weaning and post-weaning calf performance. Results indicated that calves that were fed flavoured starters weighed more at weaning. Further, the pre-weaning average daily gains increased significantly (+21%) compared with calves fed unflavoured starters. The starter consumption was also higher (+19%) and met the weaning criteria at a younger age – that is, the calves had a pre-weaning period that was shorter by 2-3 days.
Diets with high levels of concentrates and low fibre help maintain high lactation yields because of the relative increase in intake of digestible energy, and also because metabolisable energy from high-concentrate diets is used more efficiently than that from high-forage diets due to reduced heat production. Feeding a high-concentrate diet is particularly important during the first 2 months of lactation when cows have the greatest need for nutrients. If a cow is well fed in the beginning, any decrease in milk production due to poor nutrition at later stages can quickly be offset by proper supplementation. This will not be the case for a cow that is poorly fed early in lactation.
Although feeding a high-concentrate diet in early lactation increases milk yield, it also reduces milk fat percentage due to increased production of propionic acid in the rumen with high-concentrate diets. Fat-depressing effects can, however, be minimised by adding buffers to the diet (e.g. 454 g of sodium bicarbonate for 4 weeks).
Dairy cows may experience a transient decrease in dry matter intake by about 32% around 3 weeks before parturition, which exacerbates the imbalance between energy needs and energy supply. This is often associated with increased blood non-esterified fatty acids and liver triglyceride accumulation during early lactation. Such changes in the blood and liver profiles are immunosuppressive, thereby increasing the risk of infectious diseases. Feeding strategies that avoid the negative energy balance and the associated physiological changes may improve immune response and disease resistance.
Temperate dairy cows are particularly susceptible to heat stress and often show marked decreases in productivity in such conditions. The following feeding strategies help them adapt to hot climates:
In cows fed a high-protein diet during heat stress, dry matter intake was 11% greater and fat-corrected milk was 4.3% higher compared to those fed a low-protein diet.
Added fat is important in combatting heat stress in hot climates and should be at the maximum amount that can reasonably be fed (i.e., 3-8% of the total diet). This reduces the heat increment associated with alimentary tract fermentation and tissue metabolism which, in turn, reduces total body heat load.
Supplementation of minerals during heat stress is a recommended practice, particularly with high-producing dairy cows. Of greatest importance is potassium (K) since more K is required to counteract losses through milk and sweat. Urinary losses of sodium also increase during heat stress due to decreased levels of blood aldosterone. Such losses should, therefore, be balanced through dietary supplements.
As a further strategy, increasing feeding frequency should reduce heat production because this would promote a uniform rate of absorption of nutrients and spread the total heat increment due to feeding over a longer period.
Chilled water should be used under heat-stress conditions. It was found that lactating cows drinking water chilled to 10°C had lower respiration rates (70/min vs. 81/min), lower rectal temperatures (39°C vs. 40°C) and greater milk production (26 kg/d vs. 24 kg/d) than cows drinking water given at 27°C.
Cold weather is known to increase the susceptibility to pregnancy ketosis and hypomagnesaemia in ruminants due to changes in endocrine and metabolic functions. It was also found that the energy requirement of cattle increases by 0.9% for each degree below 20°C, thereby reducing the efficiency of energy utilisation for growth, lactation and other biological functions.
The following are useful tips for alleviating the effects of cold stress via feeding strategies: